path: root/linux-2.6-utrace.patch

utrace core

This adds the utrace facility, a new modular interface in the kernel for
implementing user thread tracing and debugging.  This fits on top of the
tracehook_* layer, so the new code is well-isolated.

The new interface is in <linux/utrace.h> and the DocBook utrace book
describes it.  It allows for multiple separate tracing engines to work in
parallel without interfering with each other.  Higher-level tracing
facilities can be implemented as loadable kernel modules using this layer.

The new facility is made optional under CONFIG_UTRACE.
When this is not enabled, no new code is added.
It can only be enabled on machines that have all the
prerequisites and select CONFIG_HAVE_ARCH_TRACEHOOK.

In this initial version, utrace and ptrace do not play together at all.
If ptrace is attached to a thread, the attach calls in the utrace kernel
API return -EBUSY.  If utrace is attached to a thread, the PTRACE_ATTACH
or PTRACE_TRACEME request will return EBUSY to userland.  The old ptrace
code is otherwise unchanged and nothing using ptrace should be affected
by this patch as long as utrace is not used at the same time.  In the
future we can clean up the ptrace implementation and rework it to use
the utrace API.

Signed-off-by: Roland McGrath <roland@redhat.com>
---
 Documentation/DocBook/Makefile    |    2 +-
 Documentation/DocBook/utrace.tmpl |  590 +++++++++
 fs/proc/array.c                   |    3 +
 include/linux/sched.h             |    5 +
 include/linux/tracehook.h         |   87 ++-
 include/linux/utrace.h            |  692 +++++++++++
 init/Kconfig                      |    9 +
 kernel/Makefile                   |    1 +
 kernel/fork.c                     |    3 +
 kernel/ptrace.c                   |   14 +
 kernel/utrace.c                   | 2436 +++++++++++++++++++++++++++++++++++++
 11 files changed, 3840 insertions(+), 2 deletions(-)

diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile
index ab8300f..95f59e4 100644  
--- a/Documentation/DocBook/Makefile
+++ b/Documentation/DocBook/Makefile
@@ -9,7 +9,7 @@
 DOCBOOKS := z8530book.xml mcabook.xml device-drivers.xml \
 	    kernel-hacking.xml kernel-locking.xml deviceiobook.xml \
 	    procfs-guide.xml writing_usb_driver.xml networking.xml \
-	    kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml \
+	    kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml utrace.xml \
 	    gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
 	    mac80211.xml debugobjects.xml sh.xml regulator.xml \
diff --git a/Documentation/DocBook/utrace.tmpl b/Documentation/DocBook/utrace.tmpl
new file mode 100644
index ...e149f49 100644  
--- /dev/null
+++ b/Documentation/DocBook/utrace.tmpl
@@ -0,0 +1,590 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="utrace">
+  <bookinfo>
+    <title>The utrace User Debugging Infrastructure</title>
+  </bookinfo>
+
+  <toc></toc>
+
+  <chapter id="concepts"><title>utrace concepts</title>
+
+  <sect1 id="intro"><title>Introduction</title>
+
+  <para>
+    <application>utrace</application> is infrastructure code for tracing
+    and controlling user threads.  This is the foundation for writing
+    tracing engines, which can be loadable kernel modules.
+  </para>
+
+  <para>
+    The basic actors in <application>utrace</application> are the thread
+    and the tracing engine.  A tracing engine is some body of code that
+    calls into the <filename>&lt;linux/utrace.h&gt;</filename>
+    interfaces, represented by a <structname>struct
+    utrace_engine_ops</structname>.  (Usually it's a kernel module,
+    though the legacy <function>ptrace</function> support is a tracing
+    engine that is not in a kernel module.)  The interface operates on
+    individual threads (<structname>struct task_struct</structname>).
+    If an engine wants to treat several threads as a group, that is up
+    to its higher-level code.
+  </para>
+
+  <para>
+    Tracing begins by attaching an engine to a thread, using
+    <function>utrace_attach_task</function> or
+    <function>utrace_attach_pid</function>.  If successful, it returns a
+    pointer that is the handle used in all other calls.
+  </para>
+
+  </sect1>
+
+  <sect1 id="callbacks"><title>Events and Callbacks</title>
+
+  <para>
+    An attached engine does nothing by default.  An engine makes something
+    happen by requesting callbacks via <function>utrace_set_events</function>
+    and poking the thread with <function>utrace_control</function>.
+    The synchronization issues related to these two calls
+    are discussed further below in <xref linkend="teardown"/>.
+  </para>
+
+  <para>
+    Events are specified using the macro
+    <constant>UTRACE_EVENT(<replaceable>type</replaceable>)</constant>.
+    Each event type is associated with a callback in <structname>struct
+    utrace_engine_ops</structname>.  A tracing engine can leave unused
+    callbacks <constant>NULL</constant>.  The only callbacks required
+    are those used by the event flags it sets.
+  </para>
+
+  <para>
+    Many engines can be attached to each thread.  When a thread has an
+    event, each engine gets a callback if it has set the event flag for
+    that event type.  For most events, engines are called in the order they
+    attached.  Engines that attach after the event has occurred do not get
+    callbacks for that event.  This includes any new engines just attached
+    by an existing engine's callback function.  Once the sequence of
+    callbacks for that one event has completed, such new engines are then
+    eligible in the next sequence that starts when there is another event.
+  </para>
+
+  <para>
+    Event reporting callbacks have details particular to the event type,
+    but are all called in similar environments and have the same
+    constraints.  Callbacks are made from safe points, where no locks
+    are held, no special resources are pinned (usually), and the
+    user-mode state of the thread is accessible.  So, callback code has
+    a pretty free hand.  But to be a good citizen, callback code should
+    never block for long periods.  It is fine to block in
+    <function>kmalloc</function> and the like, but never wait for i/o or
+    for user mode to do something.  If you need the thread to wait, use
+    <constant>UTRACE_STOP</constant> and return from the callback
+    quickly.  When your i/o finishes or whatever, you can use
+    <function>utrace_control</function> to resume the thread.
+  </para>
+
+  <para>
+    The <constant>UTRACE_EVENT(SYSCALL_ENTRY)</constant> event is a special
+    case.  While other events happen in the kernel when it will return to
+    user mode soon, this event happens when entering the kernel before it
+    will proceed with the work requested from user mode.  Because of this
+    difference, the <function>report_syscall_entry</function> callback is
+    special in two ways.  For this event, engines are called in reverse of
+    the normal order (this includes the <function>report_quiesce</function>
+    call that precedes a <function>report_syscall_entry</function> call).
+    This preserves the semantics that the last engine to attach is called
+    "closest to user mode"--the engine that is first to see a thread's user
+    state when it enters the kernel is also the last to see that state when
+    the thread returns to user mode.  For the same reason, if these
+    callbacks use <constant>UTRACE_STOP</constant> (see the next section),
+    the thread stops immediately after callbacks rather than only when it's
+    ready to return to user mode; when allowed to resume, it will actually
+    attempt the system call indicated by the register values at that time.
+  </para>
+
+  </sect1>
+
+  <sect1 id="safely"><title>Stopping Safely</title>
+
+  <sect2 id="well-behaved"><title>Writing well-behaved callbacks</title>
+
+  <para>
+    Well-behaved callbacks are important to maintain two essential
+    properties of the interface.  The first of these is that unrelated
+    tracing engines should not interfere with each other.  If your engine's
+    event callback does not return quickly, then another engine won't get
+    the event notification in a timely manner.  The second important
+    property is that tracing should be as noninvasive as possible to the
+    normal operation of the system overall and of the traced thread in
+    particular.  That is, attached tracing engines should not perturb a
+    thread's behavior, except to the extent that changing its user-visible
+    state is explicitly what you want to do.  (Obviously some perturbation
+    is unavoidable, primarily timing changes, ranging from small delays due
+    to the overhead of tracing, to arbitrary pauses in user code execution
+    when a user stops a thread with a debugger for examination.)  Even when
+    you explicitly want the perturbation of making the traced thread block,
+    just blocking directly in your callback has more unwanted effects.  For
+    example, the <constant>CLONE</constant> event callbacks are called when
+    the new child thread has been created but not yet started running; the
+    child can never be scheduled until the <constant>CLONE</constant>
+    tracing callbacks return.  (This allows engines tracing the parent to
+    attach to the child.)  If a <constant>CLONE</constant> event callback
+    blocks the parent thread, it also prevents the child thread from
+    running (even to process a <constant>SIGKILL</constant>).  If what you
+    want is to make both the parent and child block, then use
+    <function>utrace_attach_task</function> on the child and then use
+    <constant>UTRACE_STOP</constant> on both threads.  A more crucial
+    problem with blocking in callbacks is that it can prevent
+    <constant>SIGKILL</constant> from working.  A thread that is blocking
+    due to <constant>UTRACE_STOP</constant> will still wake up and die
+    immediately when sent a <constant>SIGKILL</constant>, as all threads
+    should.  Relying on the <application>utrace</application>
+    infrastructure rather than on private synchronization calls in event
+    callbacks is an important way to help keep tracing robustly
+    noninvasive.
+  </para>
+
+  </sect2>
+
+  <sect2 id="UTRACE_STOP"><title>Using <constant>UTRACE_STOP</constant></title>
+
+  <para>
+    To control another thread and access its state, it must be stopped
+    with <constant>UTRACE_STOP</constant>.  This means that it is
+    stopped and won't start running again while we access it.  When a
+    thread is not already stopped, <function>utrace_control</function>
+    returns <constant>-EINPROGRESS</constant> and an engine must wait
+    for an event callback when the thread is ready to stop.  The thread
+    may be running on another CPU or may be blocked.  When it is ready
+    to be examined, it will make callbacks to engines that set the
+    <constant>UTRACE_EVENT(QUIESCE)</constant> event bit.  To wake up an
+    interruptible wait, use <constant>UTRACE_INTERRUPT</constant>.
+  </para>
+
+  <para>
+    As long as some engine has used <constant>UTRACE_STOP</constant> and
+    not called <function>utrace_control</function> to resume the thread,
+    then the thread will remain stopped.  <constant>SIGKILL</constant>
+    will wake it up, but it will not run user code.  When the stop is
+    cleared with <function>utrace_control</function> or a callback
+    return value, the thread starts running again.
+    (See also <xref linkend="teardown"/>.)
+  </para>
+
+  </sect2>
+
+  </sect1>
+
+  <sect1 id="teardown"><title>Tear-down Races</title>
+
+  <sect2 id="SIGKILL"><title>Primacy of <constant>SIGKILL</constant></title>
+  <para>
+    Ordinarily synchronization issues for tracing engines are kept fairly
+    straightforward by using <constant>UTRACE_STOP</constant>.  You ask a
+    thread to stop, and then once it makes the
+    <function>report_quiesce</function> callback it cannot do anything else
+    that would result in another callback, until you let it with a
+    <function>utrace_control</function> call.  This simple arrangement
+    avoids complex and error-prone code in each one of a tracing engine's
+    event callbacks to keep them serialized with the engine's other
+    operations done on that thread from another thread of control.
+    However, giving tracing engines complete power to keep a traced thread
+    stuck in place runs afoul of a more important kind of simplicity that
+    the kernel overall guarantees: nothing can prevent or delay
+    <constant>SIGKILL</constant> from making a thread die and release its
+    resources.  To preserve this important property of
+    <constant>SIGKILL</constant>, it as a special case can break
+    <constant>UTRACE_STOP</constant> like nothing else normally can.  This
+    includes both explicit <constant>SIGKILL</constant> signals and the
+    implicit <constant>SIGKILL</constant> sent to each other thread in the
+    same thread group by a thread doing an exec, or processing a fatal
+    signal, or making an <function>exit_group</function> system call.  A
+    tracing engine can prevent a thread from beginning the exit or exec or
+    dying by signal (other than <constant>SIGKILL</constant>) if it is
+    attached to that thread, but once the operation begins, no tracing
+    engine can prevent or delay all other threads in the same thread group
+    dying.
+  </para>
+  </sect2>
+
+  <sect2 id="reap"><title>Final callbacks</title>
+  <para>
+    The <function>report_reap</function> callback is always the final event
+    in the life cycle of a traced thread.  Tracing engines can use this as
+    the trigger to clean up their own data structures.  The
+    <function>report_death</function> callback is always the penultimate
+    event a tracing engine might see; it's seen unless the thread was
+    already in the midst of dying when the engine attached.  Many tracing
+    engines will have no interest in when a parent reaps a dead process,
+    and nothing they want to do with a zombie thread once it dies; for
+    them, the <function>report_death</function> callback is the natural
+    place to clean up data structures and detach.  To facilitate writing
+    such engines robustly, given the asynchrony of
+    <constant>SIGKILL</constant>, and without error-prone manual
+    implementation of synchronization schemes, the
+    <application>utrace</application> infrastructure provides some special
+    guarantees about the <function>report_death</function> and
+    <function>report_reap</function> callbacks.  It still takes some care
+    to be sure your tracing engine is robust to tear-down races, but these
+    rules make it reasonably straightforward and concise to handle a lot of
+    corner cases correctly.
+  </para>
+  </sect2>
+
+  <sect2 id="refcount"><title>Engine and task pointers</title>
+  <para>
+    The first sort of guarantee concerns the core data structures
+    themselves.  <structname>struct utrace_engine</structname> is
+    a reference-counted data structure.  While you hold a reference, an
+    engine pointer will always stay valid so that you can safely pass it to
+    any <application>utrace</application> call.  Each call to
+    <function>utrace_attach_task</function> or
+    <function>utrace_attach_pid</function> returns an engine pointer with a
+    reference belonging to the caller.  You own that reference until you
+    drop it using <function>utrace_engine_put</function>.  There is an
+    implicit reference on the engine while it is attached.  So if you drop
+    your only reference, and then use
+    <function>utrace_attach_task</function> without
+    <constant>UTRACE_ATTACH_CREATE</constant> to look up that same engine,
+    you will get the same pointer with a new reference to replace the one
+    you dropped, just like calling <function>utrace_engine_get</function>.
+    When an engine has been detached, either explicitly with
+    <constant>UTRACE_DETACH</constant> or implicitly after
+    <function>report_reap</function>, then any references you hold are all
+    that keep the old engine pointer alive.
+  </para>
+
+  <para>
+    There is nothing a kernel module can do to keep a <structname>struct
+    task_struct</structname> alive outside of
+    <function>rcu_read_lock</function>.  When the task dies and is reaped
+    by its parent (or itself), that structure can be freed so that any
+    dangling pointers you have stored become invalid.
+    <application>utrace</application> will not prevent this, but it can
+    help you detect it safely.  By definition, a task that has been reaped
+    has had all its engines detached.  All
+    <application>utrace</application> calls can be safely called on a
+    detached engine if the caller holds a reference on that engine pointer,
+    even if the task pointer passed in the call is invalid.  All calls
+    return <constant>-ESRCH</constant> for a detached engine, which tells
+    you that the task pointer you passed could be invalid now.  Since
+    <function>utrace_control</function> and
+    <function>utrace_set_events</function> do not block, you can call those
+    inside a <function>rcu_read_lock</function> section and be sure after
+    they don't return <constant>-ESRCH</constant> that the task pointer is
+    still valid until <function>rcu_read_unlock</function>.  The
+    infrastructure never holds task references of its own.  Though neither
+    <function>rcu_read_lock</function> nor any other lock is held while
+    making a callback, it's always guaranteed that the <structname>struct
+    task_struct</structname> and the <structname>struct
+    utrace_engine</structname> passed as arguments remain valid
+    until the callback function returns.
+  </para>
+
+  <para>
+    The common means for safely holding task pointers that is available to
+    kernel modules is to use <structname>struct pid</structname>, which
+    permits <function>put_pid</function> from kernel modules.  When using
+    that, the calls <function>utrace_attach_pid</function>,
+    <function>utrace_control_pid</function>,
+    <function>utrace_set_events_pid</function>, and
+    <function>utrace_barrier_pid</function> are available.
+  </para>
+  </sect2>
+
+  <sect2 id="reap-after-death">
+    <title>
+      Serialization of <constant>DEATH</constant> and <constant>REAP</constant>
+    </title>
+    <para>
+      The second guarantee is the serialization of
+      <constant>DEATH</constant> and <constant>REAP</constant> event
+      callbacks for a given thread.  The actual reaping by the parent
+      (<function>release_task</function> call) can occur simultaneously
+      while the thread is still doing the final steps of dying, including
+      the <function>report_death</function> callback.  If a tracing engine
+      has requested both <constant>DEATH</constant> and
+      <constant>REAP</constant> event reports, it's guaranteed that the
+      <function>report_reap</function> callback will not be made until
+      after the <function>report_death</function> callback has returned.
+      If the <function>report_death</function> callback itself detaches
+      from the thread, then the <function>report_reap</function> callback
+      will never be made.  Thus it is safe for a
+      <function>report_death</function> callback to clean up data
+      structures and detach.
+    </para>
+  </sect2>
+
+  <sect2 id="interlock"><title>Interlock with final callbacks</title>
+  <para>
+    The final sort of guarantee is that a tracing engine will know for sure
+    whether or not the <function>report_death</function> and/or
+    <function>report_reap</function> callbacks will be made for a certain
+    thread.  These tear-down races are disambiguated by the error return
+    values of <function>utrace_set_events</function> and
+    <function>utrace_control</function>.  Normally
+    <function>utrace_control</function> called with
+    <constant>UTRACE_DETACH</constant> returns zero, and this means that no
+    more callbacks will be made.  If the thread is in the midst of dying,
+    it returns <constant>-EALREADY</constant> to indicate that the
+    <constant>report_death</constant> callback may already be in progress;
+    when you get this error, you know that any cleanup your
+    <function>report_death</function> callback does is about to happen or
+    has just happened--note that if the <function>report_death</function>
+    callback does not detach, the engine remains attached until the thread
+    gets reaped.  If the thread is in the midst of being reaped,
+    <function>utrace_control</function> returns <constant>-ESRCH</constant>
+    to indicate that the <function>report_reap</function> callback may
+    already be in progress; this means the engine is implicitly detached
+    when the callback completes.  This makes it possible for a tracing
+    engine that has decided asynchronously to detach from a thread to
+    safely clean up its data structures, knowing that no
+    <function>report_death</function> or <function>report_reap</function>
+    callback will try to do the same.  <constant>utrace_detach</constant>
+    returns <constant>-ESRCH</constant> when the <structname>struct
+    utrace_engine</structname> has already been detached, but is
+    still a valid pointer because of its reference count.  A tracing engine
+    can use this to safely synchronize its own independent multiple threads
+    of control with each other and with its event callbacks that detach.
+  </para>
+
+  <para>
+    In the same vein, <function>utrace_set_events</function> normally
+    returns zero; if the target thread was stopped before the call, then
+    after a successful call, no event callbacks not requested in the new
+    flags will be made.  It fails with <constant>-EALREADY</constant> if
+    you try to clear <constant>UTRACE_EVENT(DEATH)</constant> when the
+    <function>report_death</function> callback may already have begun, if
+    you try to clear <constant>UTRACE_EVENT(REAP)</constant> when the
+    <function>report_reap</function> callback may already have begun, or if
+    you try to newly set <constant>UTRACE_EVENT(DEATH)</constant> or
+    <constant>UTRACE_EVENT(QUIESCE)</constant> when the target is already
+    dead or dying.  Like <function>utrace_control</function>, it returns
+    <constant>-ESRCH</constant> when the thread has already been detached
+    (including forcible detach on reaping).  This lets the tracing engine
+    know for sure which event callbacks it will or won't see after
+    <function>utrace_set_events</function> has returned.  By checking for
+    errors, it can know whether to clean up its data structures immediately
+    or to let its callbacks do the work.
+  </para>
+  </sect2>
+
+  <sect2 id="barrier"><title>Using <function>utrace_barrier</function></title>
+  <para>
+    When a thread is safely stopped, calling
+    <function>utrace_control</function> with <constant>UTRACE_DETACH</constant>
+    or calling <function>utrace_set_events</function> to disable some events
+    ensures synchronously that your engine won't get any more of the callbacks
+    that have been disabled (none at all when detaching).  But these can also
+    be used while the thread is not stopped, when it might be simultaneously
+    making a callback to your engine.  For this situation, these calls return
+    <constant>-EINPROGRESS</constant> when it's possible a callback is in
+    progress.  If you are not prepared to have your old callbacks still run,
+    then you can synchronize to be sure all the old callbacks are finished,
+    using <function>utrace_barrier</function>.  This is necessary if the
+    kernel module containing your callback code is going to be unloaded.
+  </para>
+  <para>
+    After using <constant>UTRACE_DETACH</constant> once, further calls to
+    <function>utrace_control</function> with the same engine pointer will
+    return <constant>-ESRCH</constant>.  In contrast, after getting
+    <constant>-EINPROGRESS</constant> from
+    <function>utrace_set_events</function>, you can call
+    <function>utrace_set_events</function> again later and if it returns zero
+    then know the old callbacks have finished.
+  </para>
+  <para>
+    Unlike all other calls, <function>utrace_barrier</function> (and
+    <function>utrace_barrier_pid</function>) will accept any engine pointer you
+    hold a reference on, even if <constant>UTRACE_DETACH</constant> has already
+    been used.  After any <function>utrace_control</function> or
+    <function>utrace_set_events</function> call (these do not block), you can
+    call <function>utrace_barrier</function> to block until callbacks have
+    finished.  This returns <constant>-ESRCH</constant> only if the engine is
+    completely detached (finished all callbacks).  Otherwise it waits
+    until the thread is definitely not in the midst of a callback to this
+    engine and then returns zero, but can return
+    <constant>-ERESTARTSYS</constant> if its wait is interrupted.
+  </para>
+  </sect2>
+
+</sect1>
+
+</chapter>
+
+<chapter id="core"><title>utrace core API</title>
+
+<para>
+  The utrace API is declared in <filename>&lt;linux/utrace.h&gt;</filename>.
+</para>
+
+!Iinclude/linux/utrace.h
+!Ekernel/utrace.c
+
+</chapter>
+
+<chapter id="machine"><title>Machine State</title>
+
+<para>
+  The <function>task_current_syscall</function> function can be used on any
+  valid <structname>struct task_struct</structname> at any time, and does
+  not even require that <function>utrace_attach_task</function> was used at all.
+</para>
+
+<para>
+  The other ways to access the registers and other machine-dependent state of
+  a task can only be used on a task that is at a known safe point.  The safe
+  points are all the places where <function>utrace_set_events</function> can
+  request callbacks (except for the <constant>DEATH</constant> and
+  <constant>REAP</constant> events).  So at any event callback, it is safe to
+  examine <varname>current</varname>.
+</para>
+
+<para>
+  One task can examine another only after a callback in the target task that
+  returns <constant>UTRACE_STOP</constant> so that task will not return to user
+  mode after the safe point.  This guarantees that the task will not resume
+  until the same engine uses <function>utrace_control</function>, unless the
+  task dies suddenly.  To examine safely, one must use a pair of calls to
+  <function>utrace_prepare_examine</function> and
+  <function>utrace_finish_examine</function> surrounding the calls to
+  <structname>struct user_regset</structname> functions or direct examination
+  of task data structures.  <function>utrace_prepare_examine</function> returns
+  an error if the task is not properly stopped, or is dead.  After a
+  successful examination, the paired <function>utrace_finish_examine</function>
+  call returns an error if the task ever woke up during the examination.  If
+  so, any data gathered may be scrambled and should be discarded.  This means
+  there was a spurious wake-up (which should not happen), or a sudden death.
+</para>
+
+<sect1 id="regset"><title><structname>struct user_regset</structname></title>
+
+<para>
+  The <structname>struct user_regset</structname> API
+  is declared in <filename>&lt;linux/regset.h&gt;</filename>.
+</para>
+
+!Finclude/linux/regset.h
+
+</sect1>
+
+<sect1 id="task_current_syscall">
+  <title><filename>System Call Information</filename></title>
+
+<para>
+  This function is declared in <filename>&lt;linux/ptrace.h&gt;</filename>.
+</para>
+
+!Elib/syscall.c
+
+</sect1>
+
+<sect1 id="syscall"><title><filename>System Call Tracing</filename></title>
+
+<para>
+  The arch API for system call information is declared in
+  <filename>&lt;asm/syscall.h&gt;</filename>.
+  Each of these calls can be used only at system call entry tracing,
+  or can be used only at system call exit and the subsequent safe points
+  before returning to user mode.
+  At system call entry tracing means either during a
+  <structfield>report_syscall_entry</structfield> callback,
+  or any time after that callback has returned <constant>UTRACE_STOP</constant>.
+</para>
+
+!Finclude/asm-generic/syscall.h
+
+</sect1>
+
+</chapter>
+
+<chapter id="internals"><title>Kernel Internals</title>
+
+<para>
+  This chapter covers the interface to the tracing infrastructure
+  from the core of the kernel and the architecture-specific code.
+  This is for maintainers of the kernel and arch code, and not relevant
+  to using the tracing facilities described in preceding chapters.
+</para>
+
+<sect1 id="tracehook"><title>Core Calls In</title>
+
+<para>
+  These calls are declared in <filename>&lt;linux/tracehook.h&gt;</filename>.
+  The core kernel calls these functions at various important places.
+</para>
+
+!Finclude/linux/tracehook.h
+
+</sect1>
+
+<sect1 id="arch"><title>Architecture Calls Out</title>
+
+<para>
+  An arch that has done all these things sets
+  <constant>CONFIG_HAVE_ARCH_TRACEHOOK</constant>.
+  This is required to enable the <application>utrace</application> code.
+</para>
+
+<sect2 id="arch-ptrace"><title><filename>&lt;asm/ptrace.h&gt;</filename></title>
+
+<para>
+  An arch defines these in <filename>&lt;asm/ptrace.h&gt;</filename>
+  if it supports hardware single-step or block-step features.
+</para>
+
+!Finclude/linux/ptrace.h arch_has_single_step arch_has_block_step
+!Finclude/linux/ptrace.h user_enable_single_step user_enable_block_step
+!Finclude/linux/ptrace.h user_disable_single_step
+
+</sect2>
+
+<sect2 id="arch-syscall">
+  <title><filename>&lt;asm/syscall.h&gt;</filename></title>
+
+  <para>
+    An arch provides <filename>&lt;asm/syscall.h&gt;</filename> that
+    defines these as inlines, or declares them as exported functions.
+    These interfaces are described in <xref linkend="syscall"/>.
+  </para>
+
+</sect2>
+
+<sect2 id="arch-tracehook">
+  <title><filename>&lt;linux/tracehook.h&gt;</filename></title>
+
+  <para>
+    An arch must define <constant>TIF_NOTIFY_RESUME</constant>
+    and <constant>TIF_SYSCALL_TRACE</constant>
+    in its <filename>&lt;asm/thread_info.h&gt;</filename>.
+    The arch code must call the following functions, all declared
+    in <filename>&lt;linux/tracehook.h&gt;</filename> and
+    described in <xref linkend="tracehook"/>:
+
+    <itemizedlist>
+      <listitem>
+	<para><function>tracehook_notify_resume</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_report_syscall_entry</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_report_syscall_exit</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_signal_handler</function></para>
+      </listitem>
+    </itemizedlist>
+
+  </para>
+
+</sect2>
+
+</sect1>
+
+</chapter>
+
+</book>
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 822c2d5..9069c91 100644  
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -82,6 +82,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/ptrace.h>
 #include <linux/tracehook.h>
+#include <linux/utrace.h>
 
 #include <asm/pgtable.h>
 #include <asm/processor.h>
@@ -189,6 +190,8 @@ static inline void task_state(struct seq
 		cred->uid, cred->euid, cred->suid, cred->fsuid,
 		cred->gid, cred->egid, cred->sgid, cred->fsgid);
 
+	task_utrace_proc_status(m, p);
+
 	task_lock(p);
 	if (p->files)
 		fdt = files_fdtable(p->files);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6c8928b..139d300 100644  
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1393,6 +1393,11 @@ struct task_struct {
 #endif
 	seccomp_t seccomp;
 
+#ifdef CONFIG_UTRACE
+	struct utrace *utrace;
+	unsigned long utrace_flags;
+#endif
+
 /* Thread group tracking */
    	u32 parent_exec_id;
    	u32 self_exec_id;
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index c78b2f4..71fa250 100644  
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -49,6 +49,7 @@
 #include <linux/sched.h>
 #include <linux/ptrace.h>
 #include <linux/security.h>
+#include <linux/utrace.h>
 struct linux_binprm;
 
 /**
@@ -63,6 +64,8 @@ struct linux_binprm;
  */
 static inline int tracehook_expect_breakpoints(struct task_struct *task)
 {
+	if (unlikely(task_utrace_flags(task) & UTRACE_EVENT(SIGNAL_CORE)))
+		return 1;
 	return (task_ptrace(task) & PT_PTRACED) != 0;
 }
 
@@ -111,6 +114,9 @@ static inline void ptrace_report_syscall
 static inline __must_check int tracehook_report_syscall_entry(
 	struct pt_regs *regs)
 {
+	if ((task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_ENTRY)) &&
+	    utrace_report_syscall_entry(regs))
+		return 1;
 	ptrace_report_syscall(regs);
 	return 0;
 }
@@ -134,6 +140,9 @@ static inline __must_check int tracehook
  */
 static inline void tracehook_report_syscall_exit(struct pt_regs *regs, int step)
 {
+	if (task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_EXIT))
+		utrace_report_syscall_exit(regs);
+
 	if (step && (task_ptrace(current) & PT_PTRACED)) {
 		siginfo_t info;
 		user_single_step_siginfo(current, regs, &info);
@@ -201,6 +210,8 @@ static inline void tracehook_report_exec
 					 struct linux_binprm *bprm,
 					 struct pt_regs *regs)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(EXEC)))
+		utrace_report_exec(fmt, bprm, regs);
 	if (!ptrace_event(PT_TRACE_EXEC, PTRACE_EVENT_EXEC, 0) &&
 	    unlikely(task_ptrace(current) & PT_PTRACED))
 		send_sig(SIGTRAP, current, 0);
@@ -218,10 +229,37 @@ static inline void tracehook_report_exec
  */
 static inline void tracehook_report_exit(long *exit_code)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(EXIT)))
+		utrace_report_exit(exit_code);
 	ptrace_event(PT_TRACE_EXIT, PTRACE_EVENT_EXIT, *exit_code);
 }
 
 /**
+ * tracehook_init_task - task_struct has just been copied
+ * @task:		new &struct task_struct just copied from parent
+ *
+ * Called from do_fork() when @task has just been duplicated.
+ * After this, @task will be passed to tracehook_free_task()
+ * even if the rest of its setup fails before it is fully created.
+ */
+static inline void tracehook_init_task(struct task_struct *task)
+{
+	utrace_init_task(task);
+}
+
+/**
+ * tracehook_free_task - task_struct is being freed
+ * @task:		dead &struct task_struct being freed
+ *
+ * Called from free_task() when @task is no longer in use.
+ */
+static inline void tracehook_free_task(struct task_struct *task)
+{
+	if (task_utrace_struct(task))
+		utrace_free_task(task);
+}
+
+/**
  * tracehook_prepare_clone - prepare for new child to be cloned
  * @clone_flags:	%CLONE_* flags from clone/fork/vfork system call
  *
@@ -285,6 +323,8 @@ static inline void tracehook_report_clon
 					  unsigned long clone_flags,
 					  pid_t pid, struct task_struct *child)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(CLONE)))
+		utrace_report_clone(clone_flags, child);
 	if (unlikely(task_ptrace(child))) {
 		/*
 		 * It doesn't matter who attached/attaching to this
@@ -317,6 +357,9 @@ static inline void tracehook_report_clon
 						   pid_t pid,
 						   struct task_struct *child)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(CLONE)) &&
+	    (clone_flags & CLONE_VFORK))
+		utrace_finish_vfork(current);
 	if (unlikely(trace))
 		ptrace_event(0, trace, pid);
 }
@@ -351,6 +394,10 @@ static inline void tracehook_report_vfor
  */
 static inline void tracehook_prepare_release_task(struct task_struct *task)
 {
+	/* see utrace_add_engine() about this barrier */
+	smp_mb();
+	if (task_utrace_flags(task))
+		utrace_maybe_reap(task, task_utrace_struct(task), true);
 }
 
 /**
@@ -365,6 +412,7 @@ static inline void tracehook_prepare_rel
 static inline void tracehook_finish_release_task(struct task_struct *task)
 {
 	ptrace_release_task(task);
+	BUG_ON(task->exit_state != EXIT_DEAD);
 }
 
 /**
@@ -386,6 +434,8 @@ static inline void tracehook_signal_hand
 					    const struct k_sigaction *ka,
 					    struct pt_regs *regs, int stepping)
 {
+	if (task_utrace_flags(current))
+		utrace_signal_handler(current, stepping);
 	if (stepping && (task_ptrace(current) & PT_PTRACED))
 		ptrace_notify(SIGTRAP);
 }
@@ -403,6 +453,8 @@ static inline void tracehook_signal_hand
 static inline int tracehook_consider_ignored_signal(struct task_struct *task,
 						    int sig)
 {
+	if (unlikely(task_utrace_flags(task) & UTRACE_EVENT(SIGNAL_IGN)))
+		return 1;
 	return (task_ptrace(task) & PT_PTRACED) != 0;
 }
 
@@ -422,6 +474,9 @@ static inline int tracehook_consider_ign
 static inline int tracehook_consider_fatal_signal(struct task_struct *task,
 						  int sig)
 {
+	if (unlikely(task_utrace_flags(task) & (UTRACE_EVENT(SIGNAL_TERM) |
+						UTRACE_EVENT(SIGNAL_CORE))))
+		return 1;
 	return (task_ptrace(task) & PT_PTRACED) != 0;
 }
 
@@ -436,6 +491,8 @@ static inline int tracehook_consider_fat
  */
 static inline int tracehook_force_sigpending(void)
 {
+	if (unlikely(task_utrace_flags(current)))
+		return utrace_interrupt_pending();
 	return 0;
 }
 
@@ -465,6 +522,8 @@ static inline int tracehook_get_signal(s
 				       siginfo_t *info,
 				       struct k_sigaction *return_ka)
 {
+	if (unlikely(task_utrace_flags(task)))
+		return utrace_get_signal(task, regs, info, return_ka);
 	return 0;
 }
 
@@ -492,6 +551,8 @@ static inline int tracehook_get_signal(s
  */
 static inline int tracehook_notify_jctl(int notify, int why)
 {
+	if (task_utrace_flags(current) & UTRACE_EVENT(JCTL))
+		utrace_report_jctl(notify, why);
 	return notify ?: task_ptrace(current) ? why : 0;
 }
 
@@ -502,6 +563,8 @@ static inline int tracehook_notify_jctl(
  */
 static inline void tracehook_finish_jctl(void)
 {
+	if (task_utrace_flags(current))
+		utrace_finish_stop();
 }
 
 #define DEATH_REAP			-1
@@ -524,6 +587,8 @@ static inline void tracehook_finish_jctl
 static inline int tracehook_notify_death(struct task_struct *task,
 					 void **death_cookie, int group_dead)
 {
+	*death_cookie = task_utrace_struct(task);
+
 	if (task_detached(task))
 		return task->ptrace ? SIGCHLD : DEATH_REAP;
 
@@ -560,6 +625,15 @@ static inline void tracehook_report_deat
 					  int signal, void *death_cookie,
 					  int group_dead)
 {
+	/*
+	 * If utrace_set_events() was just called to enable
+	 * UTRACE_EVENT(DEATH), then we are obliged to call
+	 * utrace_report_death() and not miss it.  utrace_set_events()
+	 * checks @task->exit_state under tasklist_lock to synchronize
+	 * with exit_notify(), the caller.
+	 */
+	if (task_utrace_flags(task) & _UTRACE_DEATH_EVENTS)
+		utrace_report_death(task, death_cookie, group_dead, signal);
 }
 
 #ifdef TIF_NOTIFY_RESUME
@@ -589,10 +663,21 @@ static inline void set_notify_resume(str
  * asynchronously, this will be called again before we return to
  * user mode.
  *
- * Called without locks.
+ * Called without locks.  However, on some machines this may be
+ * called with interrupts disabled.
  */
 static inline void tracehook_notify_resume(struct pt_regs *regs)
 {
+	struct task_struct *task = current;
+	/*
+	 * Prevent the following store/load from getting ahead of the
+	 * caller which clears TIF_NOTIFY_RESUME. This pairs with the
+	 * implicit mb() before setting TIF_NOTIFY_RESUME in
+	 * set_notify_resume().
+	 */
+	smp_mb();
+	if (task_utrace_flags(task))
+		utrace_resume(task, regs);
 }
 #endif	/* TIF_NOTIFY_RESUME */
 
diff --git a/include/linux/utrace.h b/include/linux/utrace.h
new file mode 100644
index ...f251efe 100644  
--- /dev/null
+++ b/include/linux/utrace.h
@@ -0,0 +1,692 @@
+/*
+ * utrace infrastructure interface for debugging user processes
+ *
+ * Copyright (C) 2006-2009 Red Hat, Inc.  All rights reserved.
+ *
+ * This copyrighted material is made available to anyone wishing to use,
+ * modify, copy, or redistribute it subject to the terms and conditions
+ * of the GNU General Public License v.2.
+ *
+ * Red Hat Author: Roland McGrath.
+ *
+ * This interface allows for notification of interesting events in a
+ * thread.  It also mediates access to thread state such as registers.
+ * Multiple unrelated users can be associated with a single thread.
+ * We call each of these a tracing engine.
+ *
+ * A tracing engine starts by calling utrace_attach_task() or
+ * utrace_attach_pid() on the chosen thread, passing in a set of hooks
+ * (&struct utrace_engine_ops), and some associated data.  This produces a
+ * &struct utrace_engine, which is the handle used for all other
+ * operations.  An attached engine has its ops vector, its data, and an
+ * event mask controlled by utrace_set_events().
+ *
+ * For each event bit that is set, that engine will get the
+ * appropriate ops->report_*() callback when the event occurs.  The
+ * &struct utrace_engine_ops need not provide callbacks for an event
+ * unless the engine sets one of the associated event bits.
+ */
+
+#ifndef _LINUX_UTRACE_H
+#define _LINUX_UTRACE_H	1
+
+#include <linux/list.h>
+#include <linux/kref.h>
+#include <linux/signal.h>
+#include <linux/sched.h>
+
+struct linux_binprm;
+struct pt_regs;
+struct utrace;
+struct user_regset;
+struct user_regset_view;
+
+/*
+ * Event bits passed to utrace_set_events().
+ * These appear in &struct task_struct.@utrace_flags
+ * and &struct utrace_engine.@flags.
+ */
+enum utrace_events {
+	_UTRACE_EVENT_QUIESCE,	/* Thread is available for examination.  */
+	_UTRACE_EVENT_REAP,  	/* Zombie reaped, no more tracing possible.  */
+	_UTRACE_EVENT_CLONE,	/* Successful clone/fork/vfork just done.  */
+	_UTRACE_EVENT_EXEC,	/* Successful execve just completed.  */
+	_UTRACE_EVENT_EXIT,	/* Thread exit in progress.  */
+	_UTRACE_EVENT_DEATH,	/* Thread has died.  */
+	_UTRACE_EVENT_SYSCALL_ENTRY, /* User entered kernel for system call. */
+	_UTRACE_EVENT_SYSCALL_EXIT, /* Returning to user after system call.  */
+	_UTRACE_EVENT_SIGNAL,	/* Signal delivery will run a user handler.  */
+	_UTRACE_EVENT_SIGNAL_IGN, /* No-op signal to be delivered.  */
+	_UTRACE_EVENT_SIGNAL_STOP, /* Signal delivery will suspend.  */
+	_UTRACE_EVENT_SIGNAL_TERM, /* Signal delivery will terminate.  */
+	_UTRACE_EVENT_SIGNAL_CORE, /* Signal delivery will dump core.  */
+	_UTRACE_EVENT_JCTL,	/* Job control stop or continue completed.  */
+	_UTRACE_NEVENTS
+};
+#define UTRACE_EVENT(type)	(1UL << _UTRACE_EVENT_##type)
+
+/*
+ * All the kinds of signal events.
+ * These all use the @report_signal() callback.
+ */
+#define UTRACE_EVENT_SIGNAL_ALL	(UTRACE_EVENT(SIGNAL) \
+				 | UTRACE_EVENT(SIGNAL_IGN) \
+				 | UTRACE_EVENT(SIGNAL_STOP) \
+				 | UTRACE_EVENT(SIGNAL_TERM) \
+				 | UTRACE_EVENT(SIGNAL_CORE))
+/*
+ * Both kinds of syscall events; these call the @report_syscall_entry()
+ * and @report_syscall_exit() callbacks, respectively.
+ */
+#define UTRACE_EVENT_SYSCALL	\
+	(UTRACE_EVENT(SYSCALL_ENTRY) | UTRACE_EVENT(SYSCALL_EXIT))
+
+/*
+ * The event reports triggered synchronously by task death.
+ */
+#define _UTRACE_DEATH_EVENTS (UTRACE_EVENT(DEATH) | UTRACE_EVENT(QUIESCE))
+
+/*
+ * Hooks in <linux/tracehook.h> call these entry points to the utrace dispatch.
+ */
+void utrace_free_task(struct task_struct *);
+bool utrace_interrupt_pending(void);
+void utrace_resume(struct task_struct *, struct pt_regs *);
+void utrace_finish_stop(void);
+void utrace_maybe_reap(struct task_struct *, struct utrace *, bool);
+int utrace_get_signal(struct task_struct *, struct pt_regs *,
+		      siginfo_t *, struct k_sigaction *);
+void utrace_report_clone(unsigned long, struct task_struct *);
+void utrace_finish_vfork(struct task_struct *);
+void utrace_report_exit(long *exit_code);
+void utrace_report_death(struct task_struct *, struct utrace *, bool, int);
+void utrace_report_jctl(int notify, int type);
+void utrace_report_exec(struct linux_binfmt *, struct linux_binprm *,
+			struct pt_regs *regs);
+bool utrace_report_syscall_entry(struct pt_regs *);
+void utrace_report_syscall_exit(struct pt_regs *);
+void utrace_signal_handler(struct task_struct *, int);
+
+#ifndef CONFIG_UTRACE
+
+/*
+ * <linux/tracehook.h> uses these accessors to avoid #ifdef CONFIG_UTRACE.
+ */
+static inline unsigned long task_utrace_flags(struct task_struct *task)
+{
+	return 0;
+}
+static inline struct utrace *task_utrace_struct(struct task_struct *task)
+{
+	return NULL;
+}
+static inline void utrace_init_task(struct task_struct *child)
+{
+}
+
+static inline void task_utrace_proc_status(struct seq_file *m,
+					   struct task_struct *p)
+{
+}
+
+#else  /* CONFIG_UTRACE */
+
+static inline unsigned long task_utrace_flags(struct task_struct *task)
+{
+	return task->utrace_flags;
+}
+
+static inline struct utrace *task_utrace_struct(struct task_struct *task)
+{
+	struct utrace *utrace;
+
+	/*
+	 * This barrier ensures that any prior load of task->utrace_flags
+	 * is ordered before this load of task->utrace.  We use those
+	 * utrace_flags checks in the hot path to decide to call into
+	 * the utrace code.  The first attach installs task->utrace before
+	 * setting task->utrace_flags nonzero with implicit barrier in
+	 * between, see utrace_add_engine().
+	 */
+	smp_rmb();
+	utrace = task->utrace;
+
+	smp_read_barrier_depends(); /* See utrace_task_alloc().  */
+	return utrace;
+}
+
+static inline void utrace_init_task(struct task_struct *task)
+{
+	task->utrace_flags = 0;
+	task->utrace = NULL;
+}
+
+void task_utrace_proc_status(struct seq_file *m, struct task_struct *p);
+
+
+/*
+ * Version number of the API defined in this file.  This will change
+ * whenever a tracing engine's code would need some updates to keep
+ * working.  We maintain this here for the benefit of tracing engine code
+ * that is developed concurrently with utrace API improvements before they
+ * are merged into the kernel, making LINUX_VERSION_CODE checks unwieldy.
+ */
+#define UTRACE_API_VERSION	20091216
+
+/**
+ * enum utrace_resume_action - engine's choice of action for a traced task
+ * @UTRACE_STOP:		Stay quiescent after callbacks.
+ * @UTRACE_INTERRUPT:		Make @report_signal() callback soon.
+ * @UTRACE_REPORT:		Make some callback soon.
+ * @UTRACE_SINGLESTEP:		Resume in user mode for one instruction.
+ * @UTRACE_BLOCKSTEP:		Resume in user mode until next branch.
+ * @UTRACE_RESUME:		Resume normally in user mode.
+ * @UTRACE_DETACH:		Detach my engine (implies %UTRACE_RESUME).
+ *
+ * See utrace_control() for detailed descriptions of each action.  This is
+ * encoded in the @action argument and the return value for every callback
+ * with a &u32 return value.
+ *
+ * The order of these is important.  When there is more than one engine,
+ * each supplies its choice and the smallest value prevails.
+ */
+enum utrace_resume_action {
+	UTRACE_STOP,
+	UTRACE_INTERRUPT,
+	UTRACE_REPORT,
+	UTRACE_SINGLESTEP,
+	UTRACE_BLOCKSTEP,
+	UTRACE_RESUME,
+	UTRACE_DETACH,
+	UTRACE_RESUME_MAX
+};
+#define UTRACE_RESUME_BITS	(ilog2(UTRACE_RESUME_MAX) + 1)
+#define UTRACE_RESUME_MASK	((1 << UTRACE_RESUME_BITS) - 1)
+
+/**
+ * utrace_resume_action - &enum utrace_resume_action from callback action
+ * @action:		&u32 callback @action argument or return value
+ *
+ * This extracts the &enum utrace_resume_action from @action,
+ * which is the @action argument to a &struct utrace_engine_ops
+ * callback or the return value from one.
+ */
+static inline enum utrace_resume_action utrace_resume_action(u32 action)
+{
+	return action & UTRACE_RESUME_MASK;
+}
+
+/**
+ * enum utrace_signal_action - disposition of signal
+ * @UTRACE_SIGNAL_DELIVER:	Deliver according to sigaction.
+ * @UTRACE_SIGNAL_IGN:		Ignore the signal.
+ * @UTRACE_SIGNAL_TERM:		Terminate the process.
+ * @UTRACE_SIGNAL_CORE:		Terminate with core dump.
+ * @UTRACE_SIGNAL_STOP:		Deliver as absolute stop.
+ * @UTRACE_SIGNAL_TSTP:		Deliver as job control stop.
+ * @UTRACE_SIGNAL_REPORT:	Reporting before pending signals.
+ * @UTRACE_SIGNAL_HANDLER:	Reporting after signal handler setup.
+ *
+ * This is encoded in the @action argument and the return value for
+ * a @report_signal() callback.  It says what will happen to the
+ * signal described by the &siginfo_t parameter to the callback.
+ *
+ * The %UTRACE_SIGNAL_REPORT value is used in an @action argument when
+ * a tracing report is being made before dequeuing any pending signal.
+ * If this is immediately after a signal handler has been set up, then
+ * %UTRACE_SIGNAL_HANDLER is used instead.  A @report_signal callback
+ * that uses %UTRACE_SIGNAL_DELIVER|%UTRACE_SINGLESTEP will ensure
+ * it sees a %UTRACE_SIGNAL_HANDLER report.
+ */
+enum utrace_signal_action {
+	UTRACE_SIGNAL_DELIVER	= 0x00,
+	UTRACE_SIGNAL_IGN	= 0x10,
+	UTRACE_SIGNAL_TERM	= 0x20,
+	UTRACE_SIGNAL_CORE	= 0x30,
+	UTRACE_SIGNAL_STOP	= 0x40,
+	UTRACE_SIGNAL_TSTP	= 0x50,
+	UTRACE_SIGNAL_REPORT	= 0x60,
+	UTRACE_SIGNAL_HANDLER	= 0x70
+};
+#define	UTRACE_SIGNAL_MASK	0xf0
+#define UTRACE_SIGNAL_HOLD	0x100 /* Flag, push signal back on queue.  */
+
+/**
+ * utrace_signal_action - &enum utrace_signal_action from callback action
+ * @action:		@report_signal callback @action argument or return value
+ *
+ * This extracts the &enum utrace_signal_action from @action, which
+ * is the @action argument to a @report_signal callback or the
+ * return value from one.
+ */
+static inline enum utrace_signal_action utrace_signal_action(u32 action)
+{
+	return action & UTRACE_SIGNAL_MASK;
+}
+
+/**
+ * enum utrace_syscall_action - disposition of system call attempt
+ * @UTRACE_SYSCALL_RUN:		Run the system call.
+ * @UTRACE_SYSCALL_ABORT:	Don't run the system call.
+ *
+ * This is encoded in the @action argument and the return value for
+ * a @report_syscall_entry callback.
+ */
+enum utrace_syscall_action {
+	UTRACE_SYSCALL_RUN	= 0x00,
+	UTRACE_SYSCALL_ABORT	= 0x10
+};
+#define	UTRACE_SYSCALL_MASK	0xf0
+#define	UTRACE_SYSCALL_RESUMED	0x100 /* Flag, report_syscall_entry() repeats */
+
+/**
+ * utrace_syscall_action - &enum utrace_syscall_action from callback action
+ * @action:		@report_syscall_entry callback @action or return value
+ *
+ * This extracts the &enum utrace_syscall_action from @action, which
+ * is the @action argument to a @report_syscall_entry callback or the
+ * return value from one.
+ */
+static inline enum utrace_syscall_action utrace_syscall_action(u32 action)
+{
+	return action & UTRACE_SYSCALL_MASK;
+}
+
+/*
+ * Flags for utrace_attach_task() and utrace_attach_pid().
+ */
+#define UTRACE_ATTACH_MATCH_OPS		0x0001 /* Match engines on ops.  */
+#define UTRACE_ATTACH_MATCH_DATA	0x0002 /* Match engines on data.  */
+#define UTRACE_ATTACH_MATCH_MASK	0x000f
+#define UTRACE_ATTACH_CREATE		0x0010 /* Attach a new engine.  */
+#define UTRACE_ATTACH_EXCLUSIVE		0x0020 /* Refuse if existing match.  */
+
+/**
+ * struct utrace_engine - per-engine structure
+ * @ops:	&struct utrace_engine_ops pointer passed to utrace_attach_task()
+ * @data:	engine-private &void * passed to utrace_attach_task()
+ * @flags:	event mask set by utrace_set_events() plus internal flag bits
+ *
+ * The task itself never has to worry about engines detaching while
+ * it's doing event callbacks.  These structures are removed from the
+ * task's active list only when it's stopped, or by the task itself.
+ *
+ * utrace_engine_get() and utrace_engine_put() maintain a reference count.
+ * When it drops to zero, the structure is freed.  One reference is held
+ * implicitly while the engine is attached to its task.
+ */
+struct utrace_engine {
+/* private: */
+	struct kref kref;
+	void (*release)(void *);
+	struct list_head entry;
+
+/* public: */
+	const struct utrace_engine_ops *ops;
+	void *data;
+
+	unsigned long flags;
+};
+
+/**
+ * utrace_engine_get - acquire a reference on a &struct utrace_engine
+ * @engine:	&struct utrace_engine pointer
+ *
+ * You must hold a reference on @engine, and you get another.
+ */
+static inline void utrace_engine_get(struct utrace_engine *engine)
+{
+	kref_get(&engine->kref);
+}
+
+void __utrace_engine_release(struct kref *);
+
+/**
+ * utrace_engine_put - release a reference on a &struct utrace_engine
+ * @engine:	&struct utrace_engine pointer
+ *
+ * You must hold a reference on @engine, and you lose that reference.
+ * If it was the last one, @engine becomes an invalid pointer.
+ */
+static inline void utrace_engine_put(struct utrace_engine *engine)
+{
+	kref_put(&engine->kref, __utrace_engine_release);
+}
+
+/**
+ * struct utrace_engine_ops - tracing engine callbacks
+ *
+ * Each @report_*() callback corresponds to an %UTRACE_EVENT(*) bit.
+ * utrace_set_events() calls on @engine choose which callbacks will
+ * be made to @engine from @task.
+ *
+ * Most callbacks take an @action argument, giving the resume action
+ * chosen by other tracing engines.  All callbacks take an @engine
+ * argument.  The @report_reap callback takes a @task argument that
+ * might or might not be @current.  All other @report_* callbacks
+ * report an event in the @current task.
+ *
+ * For some calls, @action also includes bits specific to that event
+ * and utrace_resume_action() is used to extract the resume action.
+ * This shows what would happen if @engine wasn't there, or will if
+ * the callback's return value uses %UTRACE_RESUME.  This always
+ * starts as %UTRACE_RESUME when no other tracing is being done on
+ * this task.
+ *
+ * All return values contain &enum utrace_resume_action bits.  For
+ * some calls, other bits specific to that kind of event are added to
+ * the resume action bits with OR.  These are the same bits used in
+ * the @action argument.  The resume action returned by a callback
+ * does not override previous engines' choices, it only says what
+ * @engine wants done.  What @current actually does is the action that's
+ * most constrained among the choices made by all attached engines.
+ * See utrace_control() for more information on the actions.
+ *
+ * When %UTRACE_STOP is used in @report_syscall_entry, then @current
+ * stops before attempting the system call.  In this case, another
+ * @report_syscall_entry callback will follow after @current resumes if
+ * %UTRACE_REPORT or %UTRACE_INTERRUPT was returned by some callback
+ * or passed to utrace_control().  In a second or later callback,
+ * %UTRACE_SYSCALL_RESUMED is set in the @action argument to indicate
+ * a repeat callback still waiting to attempt the same system call
+ * invocation.  This repeat callback gives each engine an opportunity
+ * to reexamine registers another engine might have changed while
+ * @current was held in %UTRACE_STOP.
+ *
+ * In other cases, the resume action does not take effect until @current
+ * is ready to check for signals and return to user mode.  If there
+ * are more callbacks to be made, the last round of calls determines
+ * the final action.  A @report_quiesce callback with @event zero, or
+ * a @report_signal callback, will always be the last one made before
+ * @current resumes.  Only %UTRACE_STOP is "sticky"--if @engine returned
+ * %UTRACE_STOP then @current stays stopped unless @engine returns
+ * different from a following callback.
+ *
+ * The report_death() and report_reap() callbacks do not take @action
+ * arguments, and only %UTRACE_DETACH is meaningful in the return value
+ * from a report_death() callback.  None of the resume actions applies
+ * to a dead thread.
+ *
+ * All @report_*() hooks are called with no locks held, in a generally
+ * safe environment when we will be returning to user mode soon (or just
+ * entered the kernel).  It is fine to block for memory allocation and
+ * the like, but all hooks are asynchronous and must not block on
+ * external events!  If you want the thread to block, use %UTRACE_STOP
+ * in your hook's return value; then later wake it up with utrace_control().
+ *
+ * @report_quiesce:
+ *	Requested by %UTRACE_EVENT(%QUIESCE).
+ *	This does not indicate any event, but just that @current is in a
+ *	safe place for examination.  This call is made before each specific
+ *	event callback, except for @report_reap.  The @event argument gives
+ *	the %UTRACE_EVENT(@which) value for the event occurring.  This
+ *	callback might be made for events @engine has not requested, if
+ *	some other engine is tracing the event; calling utrace_set_events()
+ *	call here can request the immediate callback for this occurrence of
+ *	@event.  @event is zero when there is no other event, @current is
+ *	now ready to check for signals and return to user mode, and some
+ *	engine has used %UTRACE_REPORT or %UTRACE_INTERRUPT to request this
+ *	callback.  For this case, if @report_signal is not %NULL, the
+ *	@report_quiesce callback may be replaced with a @report_signal
+ *	callback passing %UTRACE_SIGNAL_REPORT in its @action argument,
+ *	whenever @current is entering the signal-check path anyway.
+ *
+ * @report_signal:
+ *	Requested by %UTRACE_EVENT(%SIGNAL_*) or %UTRACE_EVENT(%QUIESCE).
+ *	Use utrace_signal_action() and utrace_resume_action() on @action.
+ *	The signal action is %UTRACE_SIGNAL_REPORT when some engine has
+ *	used %UTRACE_REPORT or %UTRACE_INTERRUPT; the callback can choose
+ *	to stop or to deliver an artificial signal, before pending signals.
+ *	It's %UTRACE_SIGNAL_HANDLER instead when signal handler setup just
+ *	finished (after a previous %UTRACE_SIGNAL_DELIVER return); this
+ *	serves in lieu of any %UTRACE_SIGNAL_REPORT callback requested by
+ *	%UTRACE_REPORT or %UTRACE_INTERRUPT, and is also implicitly
+ *	requested by %UTRACE_SINGLESTEP or %UTRACE_BLOCKSTEP into the
+ *	signal delivery.  The other signal actions indicate a signal about
+ *	to be delivered; the previous engine's return value sets the signal
+ *	action seen by the the following engine's callback.  The @info data
+ *	can be changed at will, including @info->si_signo.  The settings in
+ *	@return_ka determines what %UTRACE_SIGNAL_DELIVER does.  @orig_ka
+ *	is what was in force before other tracing engines intervened, and
+ *	it's %NULL when this report began as %UTRACE_SIGNAL_REPORT or
+ *	%UTRACE_SIGNAL_HANDLER.  For a report without a new signal, @info
+ *	is left uninitialized and must be set completely by an engine that
+ *	chooses to deliver a signal; if there was a previous @report_signal
+ *	callback ending in %UTRACE_STOP and it was just resumed using
+ *	%UTRACE_REPORT or %UTRACE_INTERRUPT, then @info is left unchanged
+ *	from the previous callback.  In this way, the original signal can
+ *	be left in @info while returning %UTRACE_STOP|%UTRACE_SIGNAL_IGN
+ *	and then found again when resuming with %UTRACE_INTERRUPT.
+ *	The %UTRACE_SIGNAL_HOLD flag bit can be OR'd into the return value,
+ *	and might be in @action if the previous engine returned it.  This
+ *	flag asks that the signal in @info be pushed back on @current's queue
+ *	so that it will be seen again after whatever action is taken now.
+ *
+ * @report_clone:
+ *	Requested by %UTRACE_EVENT(%CLONE).
+ *	Event reported for parent, before the new task @child might run.
+ *	@clone_flags gives the flags used in the clone system call, or
+ *	equivalent flags for a fork() or vfork() system call.  This
+ *	function can use utrace_attach_task() on @child.  Then passing
+ *	%UTRACE_STOP to utrace_control() on @child here keeps the child
+ *	stopped before it ever runs in user mode, %UTRACE_REPORT or
+ *	%UTRACE_INTERRUPT ensures a callback from @child before it
+ *	starts in user mode.
+ *
+ * @report_jctl:
+ *	Requested by %UTRACE_EVENT(%JCTL).
+ *	Job control event; @type is %CLD_STOPPED or %CLD_CONTINUED,
+ *	indicating whether we are stopping or resuming now.  If @notify
+ *	is nonzero, @current is the last thread to stop and so will send
+ *	%SIGCHLD to its parent after this callback; @notify reflects
+ *	what the parent's %SIGCHLD has in @si_code, which can sometimes
+ *	be %CLD_STOPPED even when @type is %CLD_CONTINUED.
+ *
+ * @report_exec:
+ *	Requested by %UTRACE_EVENT(%EXEC).
+ *	An execve system call has succeeded and the new program is about to
+ *	start running.  The initial user register state is handy to be tweaked
+ *	directly in @regs.  @fmt and @bprm gives the details of this exec.
+ *
+ * @report_syscall_entry:
+ *	Requested by %UTRACE_EVENT(%SYSCALL_ENTRY).
+ *	Thread has entered the kernel to request a system call.
+ *	The user register state is handy to be tweaked directly in @regs.
+ *	The @action argument contains an &enum utrace_syscall_action,
+ *	use utrace_syscall_action() to extract it.  The return value
+ *	overrides the last engine's action for the system call.
+ *	If the final action is %UTRACE_SYSCALL_ABORT, no system call
+ *	is made.  The details of the system call being attempted can
+ *	be fetched here with syscall_get_nr() and syscall_get_arguments().
+ *	The parameter registers can be changed with syscall_set_arguments().
+ *	See above about the %UTRACE_SYSCALL_RESUMED flag in @action.
+ *	Use %UTRACE_REPORT in the return value to guarantee you get
+ *	another callback (with %UTRACE_SYSCALL_RESUMED flag) in case
+ *	@current stops with %UTRACE_STOP before attempting the system call.
+ *
+ * @report_syscall_exit:
+ *	Requested by %UTRACE_EVENT(%SYSCALL_EXIT).
+ *	Thread is about to leave the kernel after a system call request.
+ *	The user register state is handy to be tweaked directly in @regs.
+ *	The results of the system call attempt can be examined here using
+ *	syscall_get_error() and syscall_get_return_value().  It is safe
+ *	here to call syscall_set_return_value() or syscall_rollback().
+ *
+ * @report_exit:
+ *	Requested by %UTRACE_EVENT(%EXIT).
+ *	Thread is exiting and cannot be prevented from doing so,
+ *	but all its state is still live.  The @code value will be
+ *	the wait result seen by the parent, and can be changed by
+ *	this engine or others.  The @orig_code value is the real
+ *	status, not changed by any tracing engine.  Returning %UTRACE_STOP
+ *	here keeps @current stopped before it cleans up its state and dies,
+ *	so it can be examined by other processes.  When @current is allowed
+ *	to run, it will die and get to the @report_death callback.
+ *
+ * @report_death:
+ *	Requested by %UTRACE_EVENT(%DEATH).
+ *	Thread is really dead now.  It might be reaped by its parent at
+ *	any time, or self-reap immediately.  Though the actual reaping
+ *	may happen in parallel, a report_reap() callback will always be
+ *	ordered after a report_death() callback.
+ *
+ * @report_reap:
+ *	Requested by %UTRACE_EVENT(%REAP).
+ *	Called when someone reaps the dead task (parent, init, or self).
+ *	This means the parent called wait, or else this was a detached
+ *	thread or a process whose parent ignores SIGCHLD.
+ *	No more callbacks are made after this one.
+ *	The engine is always detached.
+ *	There is nothing more a tracing engine can do about this thread.
+ *	After this callback, the @engine pointer will become invalid.
+ *	The @task pointer may become invalid if get_task_struct() hasn't
+ *	been used to keep it alive.
+ *	An engine should always request this callback if it stores the
+ *	@engine pointer or stores any pointer in @engine->data, so it
+ *	can clean up its data structures.
+ *	Unlike other callbacks, this can be called from the parent's context
+ *	rather than from the traced thread itself--it must not delay the
+ *	parent by blocking.
+ *
+ * @release:
+ *	If not %NULL, this is called after the last utrace_engine_put()
+ *	call for a &struct utrace_engine, which could be implicit after
+ *	a %UTRACE_DETACH return from another callback.  Its argument is
+ *	the engine's @data member.
+ */
+struct utrace_engine_ops {
+	u32 (*report_quiesce)(u32 action, struct utrace_engine *engine,
+			      unsigned long event);
+	u32 (*report_signal)(u32 action, struct utrace_engine *engine,
+			     struct pt_regs *regs,
+			     siginfo_t *info,
+			     const struct k_sigaction *orig_ka,
+			     struct k_sigaction *return_ka);
+	u32 (*report_clone)(u32 action, struct utrace_engine *engine,
+			    unsigned long clone_flags,
+			    struct task_struct *child);
+	u32 (*report_jctl)(u32 action, struct utrace_engine *engine,
+			   int type, int notify);
+	u32 (*report_exec)(u32 action, struct utrace_engine *engine,
+			   const struct linux_binfmt *fmt,
+			   const struct linux_binprm *bprm,
+			   struct pt_regs *regs);
+	u32 (*report_syscall_entry)(u32 action, struct utrace_engine *engine,
+				    struct pt_regs *regs);
+	u32 (*report_syscall_exit)(u32 action, struct utrace_engine *engine,
+				   struct pt_regs *regs);
+	u32 (*report_exit)(u32 action, struct utrace_engine *engine,
+			   long orig_code, long *code);
+	u32 (*report_death)(struct utrace_engine *engine,
+			    bool group_dead, int signal);
+	void (*report_reap)(struct utrace_engine *engine,
+			    struct task_struct *task);
+	void (*release)(void *data);
+};
+
+/**
+ * struct utrace_examiner - private state for using utrace_prepare_examine()
+ *
+ * The members of &struct utrace_examiner are private to the implementation.
+ * This data type holds the state from a call to utrace_prepare_examine()
+ * to be used by a call to utrace_finish_examine().
+ */
+struct utrace_examiner {
+/* private: */
+	long state;
+	unsigned long ncsw;
+};
+
+/*
+ * These are the exported entry points for tracing engines to use.
+ * See kernel/utrace.c for their kerneldoc comments with interface details.
+ */
+struct utrace_engine *utrace_attach_task(struct task_struct *, int,
+					 const struct utrace_engine_ops *,
+					 void *);
+struct utrace_engine *utrace_attach_pid(struct pid *, int,
+					const struct utrace_engine_ops *,
+					void *);
+int __must_check utrace_control(struct task_struct *,
+				struct utrace_engine *,
+				enum utrace_resume_action);
+int __must_check utrace_set_events(struct task_struct *,
+				   struct utrace_engine *,
+				   unsigned long eventmask);
+int __must_check utrace_barrier(struct task_struct *,
+				struct utrace_engine *);
+int __must_check utrace_prepare_examine(struct task_struct *,
+					struct utrace_engine *,
+					struct utrace_examiner *);
+int __must_check utrace_finish_examine(struct task_struct *,
+				       struct utrace_engine *,
+				       struct utrace_examiner *);
+
+/**
+ * utrace_control_pid - control a thread being traced by a tracing engine
+ * @pid:		thread to affect
+ * @engine:		attached engine to affect
+ * @action:		&enum utrace_resume_action for thread to do
+ *
+ * This is the same as utrace_control(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_control_pid(
+	struct pid *pid, struct utrace_engine *engine,
+	enum utrace_resume_action action)
+{
+	/*
+	 * We don't bother with rcu_read_lock() here to protect the
+	 * task_struct pointer, because utrace_control will return
+	 * -ESRCH without looking at that pointer if the engine is
+	 * already detached.  A task_struct pointer can't die before
+	 * all the engines are detached in release_task() first.
+	 */
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH : utrace_control(task, engine, action);
+}
+
+/**
+ * utrace_set_events_pid - choose which event reports a tracing engine gets
+ * @pid:		thread to affect
+ * @engine:		attached engine to affect
+ * @eventmask:		new event mask
+ *
+ * This is the same as utrace_set_events(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_set_events_pid(
+	struct pid *pid, struct utrace_engine *engine, unsigned long eventmask)
+{
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH :
+		utrace_set_events(task, engine, eventmask);
+}
+
+/**
+ * utrace_barrier_pid - synchronize with simultaneous tracing callbacks
+ * @pid:		thread to affect
+ * @engine:		engine to affect (can be detached)
+ *
+ * This is the same as utrace_barrier(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_barrier_pid(struct pid *pid,
+						  struct utrace_engine *engine)
+{
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH : utrace_barrier(task, engine);
+}
+
+#endif	/* CONFIG_UTRACE */
+
+#endif	/* linux/utrace.h */
diff --git a/init/Kconfig b/init/Kconfig
index eb4b337..140e636 100644  
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -310,6 +310,15 @@ config AUDIT_TREE
 	depends on AUDITSYSCALL
 	select INOTIFY
 
+config UTRACE
+	bool "Infrastructure for tracing and debugging user processes"
+	depends on EXPERIMENTAL
+	depends on HAVE_ARCH_TRACEHOOK
+	help
+	  Enable the utrace process tracing interface.  This is an internal
+	  kernel interface exported to kernel modules, to track events in
+	  user threads, extract and change user thread state.
+
 menu "RCU Subsystem"
 
 choice
diff --git a/kernel/Makefile b/kernel/Makefile
index d7c13d2..263bb19 100644  
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -68,6 +68,7 @@ obj-$(CONFIG_IKCONFIG) += configs.o
 obj-$(CONFIG_RESOURCE_COUNTERS) += res_counter.o
 obj-$(CONFIG_STOP_MACHINE) += stop_machine.o
 obj-$(CONFIG_KPROBES_SANITY_TEST) += test_kprobes.o
+obj-$(CONFIG_UTRACE) += utrace.o
 obj-$(CONFIG_AUDIT) += audit.o auditfilter.o audit_watch.o
 obj-$(CONFIG_AUDITSYSCALL) += auditsc.o
 obj-$(CONFIG_GCOV_KERNEL) += gcov/
diff --git a/kernel/fork.c b/kernel/fork.c
index 166b8c4..3ac952e 100644  
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -152,6 +152,7 @@ void free_task(struct task_struct *tsk)
 	free_thread_info(tsk->stack);
 	rt_mutex_debug_task_free(tsk);
 	ftrace_graph_exit_task(tsk);
+	tracehook_free_task(tsk);
 	free_task_struct(tsk);
 }
 EXPORT_SYMBOL(free_task);
@@ -1018,6 +1019,8 @@ static struct task_struct *copy_process(
 	if (!p)
 		goto fork_out;
 
+	tracehook_init_task(p);
+
 	ftrace_graph_init_task(p);
 
 	rt_mutex_init_task(p);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index b7c1d32..a408bf7 100644  
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -16,6 +16,7 @@
 #include <linux/pagemap.h>
 #include <linux/smp_lock.h>
 #include <linux/ptrace.h>
+#include <linux/utrace.h>
 #include <linux/security.h>
 #include <linux/signal.h>
 #include <linux/audit.h>
@@ -164,6 +165,14 @@ bool ptrace_may_access(struct task_struc
 	return !err;
 }
 
+/*
+ * For experimental use of utrace, exclude ptrace on the same task.
+ */
+static inline bool exclude_ptrace(struct task_struct *task)
+{
+	return unlikely(!!task_utrace_flags(task));
+}
+
 int ptrace_attach(struct task_struct *task)
 {
 	int retval;
@@ -187,6 +196,8 @@ int ptrace_attach(struct task_struct *ta
 
 	task_lock(task);
 	retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH);
+	if (!retval && exclude_ptrace(task))
+		retval = -EBUSY;
 	task_unlock(task);
 	if (retval)
 		goto unlock_creds;
@@ -224,6 +235,9 @@ int ptrace_traceme(void)
 {
 	int ret = -EPERM;
 
+	if (exclude_ptrace(current)) /* XXX locking */
+		return -EBUSY;
+
 	write_lock_irq(&tasklist_lock);
 	/* Are we already being traced? */
 	if (!current->ptrace) {
diff --git a/kernel/utrace.c b/kernel/utrace.c
new file mode 100644
index ...f003e34 100644  
--- /dev/null
+++ b/kernel/utrace.c
@@ -0,0 +1,2436 @@
+/*
+ * utrace infrastructure interface for debugging user processes
+ *
+ * Copyright (C) 2006-2010 Red Hat, Inc.  All rights reserved.
+ *
+ * This copyrighted material is made available to anyone wishing to use,
+ * modify, copy, or redistribute it subject to the terms and conditions
+ * of the GNU General Public License v.2.
+ *
+ * Red Hat Author: Roland McGrath.
+ */
+
+#include <linux/utrace.h>
+#include <linux/tracehook.h>
+#include <linux/regset.h>
+#include <asm/syscall.h>
+#include <linux/ptrace.h>
+#include <linux/err.h>
+#include <linux/sched.h>
+#include <linux/freezer.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/seq_file.h>
+
+
+/*
+ * Per-thread structure private to utrace implementation.
+ * If task_struct.utrace_flags is nonzero, task_struct.utrace
+ * has always been allocated first.  Once allocated, it is
+ * never freed until free_task().
+ *
+ * The common event reporting loops are done by the task making the
+ * report without ever taking any locks.  To facilitate this, the two
+ * lists @attached and @attaching work together for smooth asynchronous
+ * attaching with low overhead.  Modifying either list requires @lock.
+ * The @attaching list can be modified any time while holding @lock.
+ * New engines being attached always go on this list.
+ *
+ * The @attached list is what the task itself uses for its reporting
+ * loops.  When the task itself is not quiescent, it can use the
+ * @attached list without taking any lock.  Nobody may modify the list
+ * when the task is not quiescent.  When it is quiescent, that means
+ * that it won't run again without taking @lock itself before using
+ * the list.
+ *
+ * At each place where we know the task is quiescent (or it's current),
+ * while holding @lock, we call splice_attaching(), below.  This moves
+ * the @attaching list members on to the end of the @attached list.
+ * Since this happens at the start of any reporting pass, any new
+ * engines attached asynchronously go on the stable @attached list
+ * in time to have their callbacks seen.
+ */
+struct utrace {
+	spinlock_t lock;
+	struct list_head attached, attaching;
+
+	struct task_struct *cloning;
+
+	struct utrace_engine *reporting;
+
+	enum utrace_resume_action resume:UTRACE_RESUME_BITS;
+	unsigned int signal_handler:1;
+	unsigned int vfork_stop:1; /* need utrace_stop() before vfork wait */
+	unsigned int death:1;	/* in utrace_report_death() now */
+	unsigned int reap:1;	/* release_task() has run */
+	unsigned int pending_attach:1; /* need splice_attaching() */
+};
+
+static struct kmem_cache *utrace_cachep;
+static struct kmem_cache *utrace_engine_cachep;
+static const struct utrace_engine_ops utrace_detached_ops; /* forward decl */
+
+static int __init utrace_init(void)
+{
+	utrace_cachep = KMEM_CACHE(utrace, SLAB_PANIC);
+	utrace_engine_cachep = KMEM_CACHE(utrace_engine, SLAB_PANIC);
+	return 0;
+}
+module_init(utrace_init);
+
+/*
+ * Set up @task.utrace for the first time.  We can have races
+ * between two utrace_attach_task() calls here.  The task_lock()
+ * governs installing the new pointer.  If another one got in first,
+ * we just punt the new one we allocated.
+ *
+ * This returns false only in case of a memory allocation failure.
+ */
+static bool utrace_task_alloc(struct task_struct *task)
+{
+	struct utrace *utrace = kmem_cache_zalloc(utrace_cachep, GFP_KERNEL);
+	if (unlikely(!utrace))
+		return false;
+	spin_lock_init(&utrace->lock);
+	INIT_LIST_HEAD(&utrace->attached);
+	INIT_LIST_HEAD(&utrace->attaching);
+	utrace->resume = UTRACE_RESUME;
+	task_lock(task);
+	if (likely(!task->utrace)) {
+		/*
+		 * This barrier makes sure the initialization of the struct
+		 * precedes the installation of the pointer.  This pairs
+		 * with smp_read_barrier_depends() in task_utrace_struct().
+		 */
+		smp_wmb();
+		task->utrace = utrace;
+	}
+	task_unlock(task);
+
+	if (unlikely(task->utrace != utrace))
+		kmem_cache_free(utrace_cachep, utrace);
+	return true;
+}
+
+/*
+ * This is called via tracehook_free_task() from free_task()
+ * when @task is being deallocated.
+ */
+void utrace_free_task(struct task_struct *task)
+{
+	kmem_cache_free(utrace_cachep, task->utrace);
+}
+
+/*
+ * This is calledwhen the task is safely quiescent, i.e. it won't consult
+ * utrace->attached without the lock.  Move any engines attached
+ * asynchronously from @utrace->attaching onto the @utrace->attached list.
+ */
+static void splice_attaching(struct utrace *utrace)
+{
+	lockdep_assert_held(&utrace->lock);
+	list_splice_tail_init(&utrace->attaching, &utrace->attached);
+	utrace->pending_attach = 0;
+}
+
+/*
+ * This is the exported function used by the utrace_engine_put() inline.
+ */
+void __utrace_engine_release(struct kref *kref)
+{
+	struct utrace_engine *engine = container_of(kref, struct utrace_engine,
+						    kref);
+	BUG_ON(!list_empty(&engine->entry));
+	if (engine->release)
+		(*engine->release)(engine->data);
+	kmem_cache_free(utrace_engine_cachep, engine);
+}
+EXPORT_SYMBOL_GPL(__utrace_engine_release);
+
+static bool engine_matches(struct utrace_engine *engine, int flags,
+			   const struct utrace_engine_ops *ops, void *data)
+{
+	if ((flags & UTRACE_ATTACH_MATCH_OPS) && engine->ops != ops)
+		return false;
+	if ((flags & UTRACE_ATTACH_MATCH_DATA) && engine->data != data)
+		return false;
+	return engine->ops && engine->ops != &utrace_detached_ops;
+}
+
+static struct utrace_engine *find_matching_engine(
+	struct utrace *utrace, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace_engine *engine;
+	list_for_each_entry(engine, &utrace->attached, entry)
+		if (engine_matches(engine, flags, ops, data))
+			return engine;
+	list_for_each_entry(engine, &utrace->attaching, entry)
+		if (engine_matches(engine, flags, ops, data))
+			return engine;
+	return NULL;
+}
+
+/*
+ * Enqueue @engine, or maybe don't if UTRACE_ATTACH_EXCLUSIVE.
+ */
+static int utrace_add_engine(struct task_struct *target,
+			     struct utrace *utrace,
+			     struct utrace_engine *engine,
+			     int flags,
+			     const struct utrace_engine_ops *ops,
+			     void *data)
+{
+	int ret;
+
+	spin_lock(&utrace->lock);
+
+	ret = -EEXIST;
+	if ((flags & UTRACE_ATTACH_EXCLUSIVE) &&
+	     unlikely(find_matching_engine(utrace, flags, ops, data)))
+		goto unlock;
+
+	/*
+	 * In case we had no engines before, make sure that
+	 * utrace_flags is not zero. Since we did unlock+lock
+	 * at least once after utrace_task_alloc() installed
+	 * ->utrace, we have the necessary barrier which pairs
+	 * with rmb() in task_utrace_struct().
+	 */
+	ret = -ESRCH;
+	if (!target->utrace_flags) {
+		target->utrace_flags = UTRACE_EVENT(REAP);
+		/*
+		 * If we race with tracehook_prepare_release_task()
+		 * make sure that either it sees utrace_flags != 0
+		 * or we see exit_state == EXIT_DEAD.
+		 */
+		smp_mb();
+		if (unlikely(target->exit_state == EXIT_DEAD)) {
+			target->utrace_flags = 0;
+			goto unlock;
+		}
+	}
+
+	/*
+	 * Put the new engine on the pending ->attaching list.
+	 * Make sure it gets onto the ->attached list by the next
+	 * time it's examined.  Setting ->pending_attach ensures
+	 * that start_report() takes the lock and splices the lists
+	 * before the next new reporting pass.
+	 *
+	 * When target == current, it would be safe just to call
+	 * splice_attaching() right here.  But if we're inside a
+	 * callback, that would mean the new engine also gets
+	 * notified about the event that precipitated its own
+	 * creation.  This is not what the user wants.
+	 */
+	list_add_tail(&engine->entry, &utrace->attaching);
+	utrace->pending_attach = 1;
+	ret = 0;
+unlock:
+	spin_unlock(&utrace->lock);
+
+	return ret;
+}
+
+/**
+ * utrace_attach_task - attach new engine, or look up an attached engine
+ * @target:	thread to attach to
+ * @flags:	flag bits combined with OR, see below
+ * @ops:	callback table for new engine
+ * @data:	engine private data pointer
+ *
+ * The caller must ensure that the @target thread does not get freed,
+ * i.e. hold a ref or be its parent.  It is always safe to call this
+ * on @current, or on the @child pointer in a @report_clone callback.
+ * For most other cases, it's easier to use utrace_attach_pid() instead.
+ *
+ * UTRACE_ATTACH_CREATE:
+ * Create a new engine.  If %UTRACE_ATTACH_CREATE is not specified, you
+ * only look up an existing engine already attached to the thread.
+ *
+ * UTRACE_ATTACH_EXCLUSIVE:
+ * Attempting to attach a second (matching) engine fails with -%EEXIST.
+ *
+ * UTRACE_ATTACH_MATCH_OPS: Only consider engines matching @ops.
+ * UTRACE_ATTACH_MATCH_DATA: Only consider engines matching @data.
+ *
+ * Calls with neither %UTRACE_ATTACH_MATCH_OPS nor %UTRACE_ATTACH_MATCH_DATA
+ * match the first among any engines attached to @target.  That means that
+ * %UTRACE_ATTACH_EXCLUSIVE in such a call fails with -%EEXIST if there
+ * are any engines on @target at all.
+ */
+struct utrace_engine *utrace_attach_task(
+	struct task_struct *target, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace *utrace = task_utrace_struct(target);
+	struct utrace_engine *engine;
+	int ret;
+
+	if (!(flags & UTRACE_ATTACH_CREATE)) {
+		if (unlikely(!utrace))
+			return ERR_PTR(-ENOENT);
+		spin_lock(&utrace->lock);
+		engine = find_matching_engine(utrace, flags, ops, data);
+		if (engine)
+			utrace_engine_get(engine);
+		spin_unlock(&utrace->lock);
+		return engine ?: ERR_PTR(-ENOENT);
+	}
+
+	if (unlikely(!ops) || unlikely(ops == &utrace_detached_ops))
+		return ERR_PTR(-EINVAL);
+
+	if (unlikely(target->flags & PF_KTHREAD))
+		/*
+		 * Silly kernel, utrace is for users!
+		 */
+		return ERR_PTR(-EPERM);
+
+	if (!utrace) {
+		if (unlikely(!utrace_task_alloc(target)))
+			return ERR_PTR(-ENOMEM);
+		utrace = task_utrace_struct(target);
+	}
+
+	engine = kmem_cache_alloc(utrace_engine_cachep, GFP_KERNEL);
+	if (unlikely(!engine))
+		return ERR_PTR(-ENOMEM);
+
+	/*
+	 * Initialize the new engine structure.  It starts out with two
+	 * refs: one ref to return, and one ref for being attached.
+	 */
+	kref_set(&engine->kref, 2);
+	engine->flags = 0;
+	engine->ops = ops;
+	engine->data = data;
+	engine->release = ops->release;
+
+	ret = utrace_add_engine(target, utrace, engine, flags, ops, data);
+
+	if (unlikely(ret)) {
+		kmem_cache_free(utrace_engine_cachep, engine);
+		engine = ERR_PTR(ret);
+	}
+
+	return engine;
+}
+EXPORT_SYMBOL_GPL(utrace_attach_task);
+
+/**
+ * utrace_attach_pid - attach new engine, or look up an attached engine
+ * @pid:	&struct pid pointer representing thread to attach to
+ * @flags:	flag bits combined with OR, see utrace_attach_task()
+ * @ops:	callback table for new engine
+ * @data:	engine private data pointer
+ *
+ * This is the same as utrace_attach_task(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+struct utrace_engine *utrace_attach_pid(
+	struct pid *pid, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace_engine *engine = ERR_PTR(-ESRCH);
+	struct task_struct *task = get_pid_task(pid, PIDTYPE_PID);
+	if (task) {
+		engine = utrace_attach_task(task, flags, ops, data);
+		put_task_struct(task);
+	}
+	return engine;
+}
+EXPORT_SYMBOL_GPL(utrace_attach_pid);
+
+/*
+ * When an engine is detached, the target thread may still see it and
+ * make callbacks until it quiesces.  We install a special ops vector
+ * with these two callbacks.  When the target thread quiesces, it can
+ * safely free the engine itself.  For any event we will always get
+ * the report_quiesce() callback first, so we only need this one
+ * pointer to be set.  The only exception is report_reap(), so we
+ * supply that callback too.
+ */
+static u32 utrace_detached_quiesce(u32 action, struct utrace_engine *engine,
+				   unsigned long event)
+{
+	return UTRACE_DETACH;
+}
+
+static void utrace_detached_reap(struct utrace_engine *engine,
+				 struct task_struct *task)
+{
+}
+
+static const struct utrace_engine_ops utrace_detached_ops = {
+	.report_quiesce = &utrace_detached_quiesce,
+	.report_reap = &utrace_detached_reap
+};
+
+/*
+ * The caller has to hold a ref on the engine.  If the attached flag is
+ * true (all but utrace_barrier() calls), the engine is supposed to be
+ * attached.  If the attached flag is false (utrace_barrier() only),
+ * then return -ERESTARTSYS for an engine marked for detach but not yet
+ * fully detached.  The task pointer can be invalid if the engine is
+ * detached.
+ *
+ * Get the utrace lock for the target task.
+ * Returns the struct if locked, or ERR_PTR(-errno).
+ *
+ * This has to be robust against races with:
+ *	utrace_control(target, UTRACE_DETACH) calls
+ *	UTRACE_DETACH after reports
+ *	utrace_report_death
+ *	utrace_release_task
+ */
+static struct utrace *get_utrace_lock(struct task_struct *target,
+				      struct utrace_engine *engine,
+				      bool attached)
+	__acquires(utrace->lock)
+{
+	struct utrace *utrace;
+
+	rcu_read_lock();
+
+	/*
+	 * If this engine was already detached, bail out before we look at
+	 * the task_struct pointer at all.  If it's detached after this
+	 * check, then RCU is still keeping this task_struct pointer valid.
+	 *
+	 * The ops pointer is NULL when the engine is fully detached.
+	 * It's &utrace_detached_ops when it's marked detached but still
+	 * on the list.  In the latter case, utrace_barrier() still works,
+	 * since the target might be in the middle of an old callback.
+	 */
+	if (unlikely(!engine->ops)) {
+		rcu_read_unlock();
+		return ERR_PTR(-ESRCH);
+	}
+
+	if (unlikely(engine->ops == &utrace_detached_ops)) {
+		rcu_read_unlock();
+		return attached ? ERR_PTR(-ESRCH) : ERR_PTR(-ERESTARTSYS);
+	}
+
+	utrace = task_utrace_struct(target);
+	spin_lock(&utrace->lock);
+	if (unlikely(!engine->ops) ||
+	    unlikely(engine->ops == &utrace_detached_ops)) {
+		/*
+		 * By the time we got the utrace lock,
+		 * it had been reaped or detached already.
+		 */
+		spin_unlock(&utrace->lock);
+		utrace = ERR_PTR(-ESRCH);
+		if (!attached && engine->ops == &utrace_detached_ops)
+			utrace = ERR_PTR(-ERESTARTSYS);
+	}
+	rcu_read_unlock();
+
+	return utrace;
+}
+
+/*
+ * Now that we don't hold any locks, run through any
+ * detached engines and free their references.  Each
+ * engine had one implicit ref while it was attached.
+ */
+static void put_detached_list(struct list_head *list)
+{
+	struct utrace_engine *engine, *next;
+	list_for_each_entry_safe(engine, next, list, entry) {
+		list_del_init(&engine->entry);
+		utrace_engine_put(engine);
+	}
+}
+
+/*
+ * We use an extra bit in utrace_engine.flags past the event bits,
+ * to record whether the engine is keeping the target thread stopped.
+ *
+ * This bit is set in task_struct.utrace_flags whenever it is set in any
+ * engine's flags.  Only utrace_reset() resets it in utrace_flags.
+ */
+#define ENGINE_STOP		(1UL << _UTRACE_NEVENTS)
+
+static void mark_engine_wants_stop(struct task_struct *task,
+				   struct utrace_engine *engine)
+{
+	engine->flags |= ENGINE_STOP;
+	task->utrace_flags |= ENGINE_STOP;
+}
+
+static void clear_engine_wants_stop(struct utrace_engine *engine)
+{
+	engine->flags &= ~ENGINE_STOP;
+}
+
+static bool engine_wants_stop(struct utrace_engine *engine)
+{
+	return (engine->flags & ENGINE_STOP) != 0;
+}
+
+/**
+ * utrace_set_events - choose which event reports a tracing engine gets
+ * @target:		thread to affect
+ * @engine:		attached engine to affect
+ * @events:		new event mask
+ *
+ * This changes the set of events for which @engine wants callbacks made.
+ *
+ * This fails with -%EALREADY and does nothing if you try to clear
+ * %UTRACE_EVENT(%DEATH) when the @report_death callback may already have
+ * begun, if you try to clear %UTRACE_EVENT(%REAP) when the @report_reap
+ * callback may already have begun, or if you try to newly set
+ * %UTRACE_EVENT(%DEATH) or %UTRACE_EVENT(%QUIESCE) when @target is
+ * already dead or dying.
+ *
+ * This can fail with -%ESRCH when @target has already been detached,
+ * including forcible detach on reaping.
+ *
+ * If @target was stopped before the call, then after a successful call,
+ * no event callbacks not requested in @events will be made; if
+ * %UTRACE_EVENT(%QUIESCE) is included in @events, then a
+ * @report_quiesce callback will be made when @target resumes.
+ *
+ * If @target was not stopped and @events excludes some bits that were
+ * set before, this can return -%EINPROGRESS to indicate that @target
+ * may have been making some callback to @engine.  When this returns
+ * zero, you can be sure that no event callbacks you've disabled in
+ * @events can be made.  If @events only sets new bits that were not set
+ * before on @engine, then -%EINPROGRESS will never be returned.
+ *
+ * To synchronize after an -%EINPROGRESS return, see utrace_barrier().
+ *
+ * When @target is @current, -%EINPROGRESS is not returned.  But note
+ * that a newly-created engine will not receive any callbacks related to
+ * an event notification already in progress.  This call enables @events
+ * callbacks to be made as soon as @engine becomes eligible for any
+ * callbacks, see utrace_attach_task().
+ *
+ * These rules provide for coherent synchronization based on %UTRACE_STOP,
+ * even when %SIGKILL is breaking its normal simple rules.
+ */
+int utrace_set_events(struct task_struct *target,
+		      struct utrace_engine *engine,
+		      unsigned long events)
+{
+	struct utrace *utrace;
+	unsigned long old_flags, old_utrace_flags;
+	int ret;
+
+	/*
+	 * We just ignore the internal bit, so callers can use
+	 * engine->flags to seed bitwise ops for our argument.
+	 */
+	events &= ~ENGINE_STOP;
+
+	utrace = get_utrace_lock(target, engine, true);
+	if (unlikely(IS_ERR(utrace)))
+		return PTR_ERR(utrace);
+
+	old_utrace_flags = target->utrace_flags;
+	old_flags = engine->flags & ~ENGINE_STOP;
+
+	if (target->exit_state &&
+	    (((events & ~old_flags) & _UTRACE_DEATH_EVENTS) ||
+	     (utrace->death &&
+	      ((old_flags & ~events) & _UTRACE_DEATH_EVENTS)) ||
+	     (utrace->reap && ((old_flags & ~events) & UTRACE_EVENT(REAP))))) {
+		spin_unlock(&utrace->lock);
+		return -EALREADY;
+	}
+
+	/*
+	 * When setting these flags, it's essential that we really
+	 * synchronize with exit_notify().  They cannot be set after
+	 * exit_notify() takes the tasklist_lock.  By holding the read
+	 * lock here while setting the flags, we ensure that the calls
+	 * to tracehook_notify_death() and tracehook_report_death() will
+	 * see the new flags.  This ensures that utrace_release_task()
+	 * knows positively that utrace_report_death() will be called or
+	 * that it won't.
+	 */
+	if ((events & ~old_utrace_flags) & _UTRACE_DEATH_EVENTS) {
+		read_lock(&tasklist_lock);
+		if (unlikely(target->exit_state)) {
+			read_unlock(&tasklist_lock);
+			spin_unlock(&utrace->lock);
+			return -EALREADY;
+		}
+		target->utrace_flags |= events;
+		read_unlock(&tasklist_lock);
+	}
+
+	engine->flags = events | (engine->flags & ENGINE_STOP);
+	target->utrace_flags |= events;
+
+	if ((events & UTRACE_EVENT_SYSCALL) &&
+	    !(old_utrace_flags & UTRACE_EVENT_SYSCALL))
+		set_tsk_thread_flag(target, TIF_SYSCALL_TRACE);
+
+	ret = 0;
+	if ((old_flags & ~events) && target != current &&
+	    !task_is_stopped_or_traced(target) && !target->exit_state) {
+		/*
+		 * This barrier ensures that our engine->flags changes
+		 * have hit before we examine utrace->reporting,
+		 * pairing with the barrier in start_callback().  If
+		 * @target has not yet hit finish_callback() to clear
+		 * utrace->reporting, we might be in the middle of a
+		 * callback to @engine.
+		 */
+		smp_mb();
+		if (utrace->reporting == engine)
+			ret = -EINPROGRESS;
+	}
+
+	spin_unlock(&utrace->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_set_events);
+
+/*
+ * Asynchronously mark an engine as being detached.
+ *
+ * This must work while the target thread races with us doing
+ * start_callback(), defined below.  It uses smp_rmb() between checking
+ * @engine->flags and using @engine->ops.  Here we change @engine->ops
+ * first, then use smp_wmb() before changing @engine->flags.  This ensures
+ * it can check the old flags before using the old ops, or check the old
+ * flags before using the new ops, or check the new flags before using the
+ * new ops, but can never check the new flags before using the old ops.
+ * Hence, utrace_detached_ops might be used with any old flags in place.
+ * It has report_quiesce() and report_reap() callbacks to handle all cases.
+ */
+static void mark_engine_detached(struct utrace_engine *engine)
+{
+	engine->ops = &utrace_detached_ops;
+	smp_wmb();
+	engine->flags = UTRACE_EVENT(QUIESCE);
+}
+
+/*
+ * Get @target to stop and return true if it is already stopped now.
+ * If we return false, it will make some event callback soonish.
+ * Called with @utrace locked.
+ */
+static bool utrace_do_stop(struct task_struct *target, struct utrace *utrace)
+{
+	if (task_is_stopped(target)) {
+		/*
+		 * Stopped is considered quiescent; when it wakes up, it will
+		 * go through utrace_finish_stop() before doing anything else.
+		 */
+		spin_lock_irq(&target->sighand->siglock);
+		if (likely(task_is_stopped(target)))
+			__set_task_state(target, TASK_TRACED);
+		spin_unlock_irq(&target->sighand->siglock);
+	} else if (utrace->resume > UTRACE_REPORT) {
+		utrace->resume = UTRACE_REPORT;
+		set_notify_resume(target);
+	}
+
+	return task_is_traced(target);
+}
+
+/*
+ * If the target is not dead it should not be in tracing
+ * stop any more.  Wake it unless it's in job control stop.
+ */
+static void utrace_wakeup(struct task_struct *target, struct utrace *utrace)
+{
+	lockdep_assert_held(&utrace->lock);
+	spin_lock_irq(&target->sighand->siglock);
+	if (target->signal->flags & SIGNAL_STOP_STOPPED ||
+	    target->signal->group_stop_count)
+		target->state = TASK_STOPPED;
+	else
+		wake_up_state(target, __TASK_TRACED);
+	spin_unlock_irq(&target->sighand->siglock);
+}
+
+/*
+ * This is called when there might be some detached engines on the list or
+ * some stale bits in @task->utrace_flags.  Clean them up and recompute the
+ * flags.  Returns true if we're now fully detached.
+ *
+ * Called with @utrace->lock held, returns with it released.
+ * After this returns, @utrace might be freed if everything detached.
+ */
+static bool utrace_reset(struct task_struct *task, struct utrace *utrace)
+	__releases(utrace->lock)
+{
+	struct utrace_engine *engine, *next;
+	unsigned long flags = 0;
+	LIST_HEAD(detached);
+
+	splice_attaching(utrace);
+
+	/*
+	 * Update the set of events of interest from the union
+	 * of the interests of the remaining tracing engines.
+	 * For any engine marked detached, remove it from the list.
+	 * We'll collect them on the detached list.
+	 */
+	list_for_each_entry_safe(engine, next, &utrace->attached, entry) {
+		if (engine->ops == &utrace_detached_ops) {
+			engine->ops = NULL;
+			list_move(&engine->entry, &detached);
+		} else {
+			flags |= engine->flags | UTRACE_EVENT(REAP);
+		}
+	}
+
+	if (task->exit_state) {
+		/*
+		 * Once it's already dead, we never install any flags
+		 * except REAP.  When ->exit_state is set and events
+		 * like DEATH are not set, then they never can be set.
+		 * This ensures that utrace_release_task() knows
+		 * positively that utrace_report_death() can never run.
+		 */
+		BUG_ON(utrace->death);
+		flags &= UTRACE_EVENT(REAP);
+	} else if (!(flags & UTRACE_EVENT_SYSCALL) &&
+		   test_tsk_thread_flag(task, TIF_SYSCALL_TRACE)) {
+		clear_tsk_thread_flag(task, TIF_SYSCALL_TRACE);
+	}
+
+	if (!flags) {
+		/*
+		 * No more engines, cleared out the utrace.
+		 */
+		utrace->resume = UTRACE_RESUME;
+		utrace->signal_handler = 0;
+	}
+
+	/*
+	 * If no more engines want it stopped, wake it up.
+	 */
+	if (task_is_traced(task) && !(flags & ENGINE_STOP))
+		utrace_wakeup(task, utrace);
+
+	/*
+	 * In theory spin_lock() doesn't imply rcu_read_lock().
+	 * Once we clear ->utrace_flags this task_struct can go away
+	 * because tracehook_prepare_release_task() path does not take
+	 * utrace->lock when ->utrace_flags == 0.
+	 */
+	rcu_read_lock();
+	task->utrace_flags = flags;
+	spin_unlock(&utrace->lock);
+	rcu_read_unlock();
+
+	put_detached_list(&detached);
+
+	return !flags;
+}
+
+void utrace_finish_stop(void)
+{
+	/*
+	 * If we were task_is_traced() and then SIGKILL'ed, make
+	 * sure we do nothing until the tracer drops utrace->lock.
+	 */
+	if (unlikely(__fatal_signal_pending(current))) {
+		struct utrace *utrace = task_utrace_struct(current);
+		spin_unlock_wait(&utrace->lock);
+	}
+}
+
+/*
+ * Perform %UTRACE_STOP, i.e. block in TASK_TRACED until woken up.
+ * @task == current, @utrace == current->utrace, which is not locked.
+ * Return true if we were woken up by SIGKILL even though some utrace
+ * engine may still want us to stay stopped.
+ */
+static void utrace_stop(struct task_struct *task, struct utrace *utrace,
+			enum utrace_resume_action action)
+{
+relock:
+	spin_lock(&utrace->lock);
+
+	if (action < utrace->resume) {
+		/*
+		 * Ensure a reporting pass when we're resumed.
+		 */
+		utrace->resume = action;
+		if (action == UTRACE_INTERRUPT)
+			set_thread_flag(TIF_SIGPENDING);
+		else
+			set_thread_flag(TIF_NOTIFY_RESUME);
+	}
+
+	/*
+	 * If the ENGINE_STOP bit is clear in utrace_flags, that means
+	 * utrace_reset() ran after we processed some UTRACE_STOP return
+	 * values from callbacks to get here.  If all engines have detached
+	 * or resumed us, we don't stop.  This check doesn't require
+	 * siglock, but it should follow the interrupt/report bookkeeping
+	 * steps (this can matter for UTRACE_RESUME but not UTRACE_DETACH).
+	 */
+	if (unlikely(!(task->utrace_flags & ENGINE_STOP))) {
+		utrace_reset(task, utrace);
+		if (task->utrace_flags & ENGINE_STOP)
+			goto relock;
+		return;
+	}
+
+	/*
+	 * The siglock protects us against signals.  As well as SIGKILL
+	 * waking us up, we must synchronize with the signal bookkeeping
+	 * for stop signals and SIGCONT.
+	 */
+	spin_lock_irq(&task->sighand->siglock);
+
+	if (unlikely(__fatal_signal_pending(task))) {
+		spin_unlock_irq(&task->sighand->siglock);
+		spin_unlock(&utrace->lock);
+		return;
+	}
+
+	__set_current_state(TASK_TRACED);
+
+	/*
+	 * If there is a group stop in progress,
+	 * we must participate in the bookkeeping.
+	 */
+	if (unlikely(task->signal->group_stop_count) &&
+			!--task->signal->group_stop_count)
+		task->signal->flags = SIGNAL_STOP_STOPPED;
+
+	spin_unlock_irq(&task->sighand->siglock);
+	spin_unlock(&utrace->lock);
+
+	schedule();
+
+	utrace_finish_stop();
+
+	/*
+	 * While in TASK_TRACED, we were considered "frozen enough".
+	 * Now that we woke up, it's crucial if we're supposed to be
+	 * frozen that we freeze now before running anything substantial.
+	 */
+	try_to_freeze();
+
+	/*
+	 * While we were in TASK_TRACED, complete_signal() considered
+	 * us "uninterested" in signal wakeups.  Now make sure our
+	 * TIF_SIGPENDING state is correct for normal running.
+	 */
+	spin_lock_irq(&task->sighand->siglock);
+	recalc_sigpending();
+	spin_unlock_irq(&task->sighand->siglock);
+}
+
+/*
+ * Called by release_task() with @reap set to true.
+ * Called by utrace_report_death() with @reap set to false.
+ * On reap, make report_reap callbacks and clean out @utrace
+ * unless still making callbacks.  On death, update bookkeeping
+ * and handle the reap work if release_task() came in first.
+ */
+void utrace_maybe_reap(struct task_struct *target, struct utrace *utrace,
+		       bool reap)
+{
+	struct utrace_engine *engine, *next;
+	struct list_head attached;
+
+	spin_lock(&utrace->lock);
+
+	if (reap) {
+		/*
+		 * If the target will do some final callbacks but hasn't
+		 * finished them yet, we know because it clears these event
+		 * bits after it's done.  Instead of cleaning up here and
+		 * requiring utrace_report_death() to cope with it, we
+		 * delay the REAP report and the teardown until after the
+		 * target finishes its death reports.
+		 */
+		utrace->reap = 1;
+
+		if (target->utrace_flags & _UTRACE_DEATH_EVENTS) {
+			spin_unlock(&utrace->lock);
+			return;
+		}
+	} else {
+		/*
+		 * After we unlock with this flag clear, any competing
+		 * utrace_control/utrace_set_events calls know that we've
+		 * finished our callbacks and any detach bookkeeping.
+		 */
+		utrace->death = 0;
+
+		if (!utrace->reap) {
+			/*
+			 * We're just dead, not reaped yet.  This will
+			 * reset @target->utrace_flags so the later call
+			 * with @reap set won't hit the check above.
+			 */
+			utrace_reset(target, utrace);
+			return;
+		}
+	}
+
+	/*
+	 * utrace_add_engine() checks ->utrace_flags != 0.  Since
+	 * @utrace->reap is set, nobody can set or clear UTRACE_EVENT(REAP)
+	 * in @engine->flags or change @engine->ops and nobody can change
+	 * @utrace->attached after we drop the lock.
+	 */
+	target->utrace_flags = 0;
+
+	/*
+	 * We clear out @utrace->attached before we drop the lock so
+	 * that find_matching_engine() can't come across any old engine
+	 * while we are busy tearing it down.
+	 */
+	list_replace_init(&utrace->attached, &attached);
+	list_splice_tail_init(&utrace->attaching, &attached);
+
+	spin_unlock(&utrace->lock);
+
+	list_for_each_entry_safe(engine, next, &attached, entry) {
+		if (engine->flags & UTRACE_EVENT(REAP))
+			engine->ops->report_reap(engine, target);
+
+		engine->ops = NULL;
+		engine->flags = 0;
+		list_del_init(&engine->entry);
+
+		utrace_engine_put(engine);
+	}
+}
+
+/*
+ * You can't do anything to a dead task but detach it.
+ * If release_task() has been called, you can't do that.
+ *
+ * On the exit path, DEATH and QUIESCE event bits are set only
+ * before utrace_report_death() has taken the lock.  At that point,
+ * the death report will come soon, so disallow detach until it's
+ * done.  This prevents us from racing with it detaching itself.
+ *
+ * Called only when @target->exit_state is nonzero.
+ */
+static inline int utrace_control_dead(struct task_struct *target,
+				      struct utrace *utrace,
+				      enum utrace_resume_action action)
+{
+	lockdep_assert_held(&utrace->lock);
+
+	if (action != UTRACE_DETACH || unlikely(utrace->reap))
+		return -ESRCH;
+
+	if (unlikely(utrace->death))
+		/*
+		 * We have already started the death report.  We can't
+		 * prevent the report_death and report_reap callbacks,
+		 * so tell the caller they will happen.
+		 */
+		return -EALREADY;
+
+	return 0;
+}
+
+/**
+ * utrace_control - control a thread being traced by a tracing engine
+ * @target:		thread to affect
+ * @engine:		attached engine to affect
+ * @action:		&enum utrace_resume_action for thread to do
+ *
+ * This is how a tracing engine asks a traced thread to do something.
+ * This call is controlled by the @action argument, which has the
+ * same meaning as the &enum utrace_resume_action value returned by
+ * event reporting callbacks.
+ *
+ * If @target is already dead (@target->exit_state nonzero),
+ * all actions except %UTRACE_DETACH fail with -%ESRCH.
+ *
+ * The following sections describe each option for the @action argument.
+ *
+ * UTRACE_DETACH:
+ *
+ * After this, the @engine data structure is no longer accessible,
+ * and the thread might be reaped.  The thread will start running
+ * again if it was stopped and no longer has any attached engines
+ * that want it stopped.
+ *
+ * If the @report_reap callback may already have begun, this fails
+ * with -%ESRCH.  If the @report_death callback may already have
+ * begun, this fails with -%EALREADY.
+ *
+ * If @target is not already stopped, then a callback to this engine
+ * might be in progress or about to start on another CPU.  If so,
+ * then this returns -%EINPROGRESS; the detach happens as soon as
+ * the pending callback is finished.  To synchronize after an
+ * -%EINPROGRESS return, see utrace_barrier().
+ *
+ * If @target is properly stopped before utrace_control() is called,
+ * then after successful return it's guaranteed that no more callbacks
+ * to the @engine->ops vector will be made.
+ *
+ * The only exception is %SIGKILL (and exec or group-exit by another
+ * thread in the group), which can cause asynchronous @report_death
+ * and/or @report_reap callbacks even when %UTRACE_STOP was used.
+ * (In that event, this fails with -%ESRCH or -%EALREADY, see above.)
+ *
+ * UTRACE_STOP:
+ *
+ * This asks that @target stop running.  This returns 0 only if
+ * @target is already stopped, either for tracing or for job
+ * control.  Then @target will remain stopped until another
+ * utrace_control() call is made on @engine; @target can be woken
+ * only by %SIGKILL (or equivalent, such as exec or termination by
+ * another thread in the same thread group).
+ *
+ * This returns -%EINPROGRESS if @target is not already stopped.
+ * Then the effect is like %UTRACE_REPORT.  A @report_quiesce or
+ * @report_signal callback will be made soon.  Your callback can
+ * then return %UTRACE_STOP to keep @target stopped.
+ *
+ * This does not interrupt system calls in progress, including ones
+ * that sleep for a long time.  For that, use %UTRACE_INTERRUPT.
+ * To interrupt system calls and then keep @target stopped, your
+ * @report_signal callback can return %UTRACE_STOP.
+ *
+ * UTRACE_RESUME:
+ *
+ * Just let @target continue running normally, reversing the effect
+ * of a previous %UTRACE_STOP.  If another engine is keeping @target
+ * stopped, then it remains stopped until all engines let it resume.
+ * If @target was not stopped, this has no effect.
+ *
+ * UTRACE_REPORT:
+ *
+ * This is like %UTRACE_RESUME, but also ensures that there will be
+ * a @report_quiesce or @report_signal callback made soon.  If
+ * @target had been stopped, then there will be a callback before it
+ * resumes running normally.  If another engine is keeping @target
+ * stopped, then there might be no callbacks until all engines let
+ * it resume.
+ *
+ * Since this is meaningless unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * UTRACE_INTERRUPT:
+ *
+ * This is like %UTRACE_REPORT, but ensures that @target will make a
+ * @report_signal callback before it resumes or delivers signals.
+ * If @target was in a system call or about to enter one, work in
+ * progress will be interrupted as if by %SIGSTOP.  If another
+ * engine is keeping @target stopped, then there might be no
+ * callbacks until all engines let it resume.
+ *
+ * This gives @engine an opportunity to introduce a forced signal
+ * disposition via its @report_signal callback.
+ *
+ * UTRACE_SINGLESTEP:
+ *
+ * It's invalid to use this unless arch_has_single_step() returned true.
+ * This is like %UTRACE_RESUME, but resumes for one user instruction only.
+ *
+ * Note that passing %UTRACE_SINGLESTEP or %UTRACE_BLOCKSTEP to
+ * utrace_control() or returning it from an event callback alone does
+ * not necessarily ensure that stepping will be enabled.  If there are
+ * more callbacks made to any engine before returning to user mode,
+ * then the resume action is chosen only by the last set of callbacks.
+ * To be sure, enable %UTRACE_EVENT(%QUIESCE) and look for the
+ * @report_quiesce callback with a zero event mask, or the
+ * @report_signal callback with %UTRACE_SIGNAL_REPORT.
+ *
+ * Since this is not robust unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * UTRACE_BLOCKSTEP:
+ *
+ * It's invalid to use this unless arch_has_block_step() returned true.
+ * This is like %UTRACE_SINGLESTEP, but resumes for one whole basic
+ * block of user instructions.
+ *
+ * Since this is not robust unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * %UTRACE_BLOCKSTEP devolves to %UTRACE_SINGLESTEP when another
+ * tracing engine is using %UTRACE_SINGLESTEP at the same time.
+ */
+int utrace_control(struct task_struct *target,
+		   struct utrace_engine *engine,
+		   enum utrace_resume_action action)
+{
+	struct utrace *utrace;
+	bool reset;
+	int ret;
+
+	if (unlikely(action >= UTRACE_RESUME_MAX)) {
+		WARN(1, "invalid action argument to utrace_control()!");
+		return -EINVAL;
+	}
+
+	/*
+	 * This is a sanity check for a programming error in the caller.
+	 * Their request can only work properly in all cases by relying on
+	 * a follow-up callback, but they didn't set one up!  This check
+	 * doesn't do locking, but it shouldn't matter.  The caller has to
+	 * be synchronously sure the callback is set up to be operating the
+	 * interface properly.
+	 */
+	if (action >= UTRACE_REPORT && action < UTRACE_RESUME &&
+	    unlikely(!(engine->flags & UTRACE_EVENT(QUIESCE)))) {
+		WARN(1, "utrace_control() with no QUIESCE callback in place!");
+		return -EINVAL;
+	}
+
+	utrace = get_utrace_lock(target, engine, true);
+	if (unlikely(IS_ERR(utrace)))
+		return PTR_ERR(utrace);
+
+	reset = task_is_traced(target);
+	ret = 0;
+
+	/*
+	 * ->exit_state can change under us, this doesn't matter.
+	 * We do not care about ->exit_state in fact, but we do
+	 * care about ->reap and ->death. If either flag is set,
+	 * we must also see ->exit_state != 0.
+	 */
+	if (unlikely(target->exit_state)) {
+		ret = utrace_control_dead(target, utrace, action);
+		if (ret) {
+			spin_unlock(&utrace->lock);
+			return ret;
+		}
+		reset = true;
+	}
+
+	switch (action) {
+	case UTRACE_STOP:
+		mark_engine_wants_stop(target, engine);
+		if (!reset && !utrace_do_stop(target, utrace))
+			ret = -EINPROGRESS;
+		reset = false;
+		break;
+
+	case UTRACE_DETACH:
+		if (engine_wants_stop(engine))
+			target->utrace_flags &= ~ENGINE_STOP;
+		mark_engine_detached(engine);
+		reset = reset || utrace_do_stop(target, utrace);
+		if (!reset) {
+			/*
+			 * As in utrace_set_events(), this barrier ensures
+			 * that our engine->flags changes have hit before we
+			 * examine utrace->reporting, pairing with the barrier
+			 * in start_callback().  If @target has not yet hit
+			 * finish_callback() to clear utrace->reporting, we
+			 * might be in the middle of a callback to @engine.
+			 */
+			smp_mb();
+			if (utrace->reporting == engine)
+				ret = -EINPROGRESS;
+		}
+		break;
+
+	case UTRACE_RESUME:
+		/*
+		 * This and all other cases imply resuming if stopped.
+		 * There might not be another report before it just
+		 * resumes, so make sure single-step is not left set.
+		 */
+		clear_engine_wants_stop(engine);
+		if (likely(reset))
+			user_disable_single_step(target);
+		break;
+
+	case UTRACE_BLOCKSTEP:
+		/*
+		 * Resume from stopped, step one block.
+		 * We fall through to treat it like UTRACE_SINGLESTEP.
+		 */
+		if (unlikely(!arch_has_block_step())) {
+			WARN(1, "UTRACE_BLOCKSTEP when !arch_has_block_step()");
+			action = UTRACE_SINGLESTEP;
+		}
+
+	case UTRACE_SINGLESTEP:
+		/*
+		 * Resume from stopped, step one instruction.
+		 * We fall through to the UTRACE_REPORT case.
+		 */
+		if (unlikely(!arch_has_single_step())) {
+			WARN(1,
+			     "UTRACE_SINGLESTEP when !arch_has_single_step()");
+			reset = false;
+			ret = -EOPNOTSUPP;
+			break;
+		}
+
+	case UTRACE_REPORT:
+		/*
+		 * Make the thread call tracehook_notify_resume() soon.
+		 * But don't bother if it's already been interrupted.
+		 * In that case, utrace_get_signal() will be reporting soon.
+		 */
+		clear_engine_wants_stop(engine);
+		if (action < utrace->resume) {
+			utrace->resume = action;
+			set_notify_resume(target);
+		}
+		break;
+
+	case UTRACE_INTERRUPT:
+		/*
+		 * Make the thread call tracehook_get_signal() soon.
+		 */
+		clear_engine_wants_stop(engine);
+		if (utrace->resume == UTRACE_INTERRUPT)
+			break;
+		utrace->resume = UTRACE_INTERRUPT;
+
+		/*
+		 * If it's not already stopped, interrupt it now.  We need
+		 * the siglock here in case it calls recalc_sigpending()
+		 * and clears its own TIF_SIGPENDING.  By taking the lock,
+		 * we've serialized any later recalc_sigpending() after our
+		 * setting of utrace->resume to force it on.
+		 */
+		if (reset) {
+			/*
+			 * This is really just to keep the invariant that
+			 * TIF_SIGPENDING is set with UTRACE_INTERRUPT.
+			 * When it's stopped, we know it's always going
+			 * through utrace_get_signal() and will recalculate.
+			 */
+			set_tsk_thread_flag(target, TIF_SIGPENDING);
+		} else {
+			struct sighand_struct *sighand;
+			unsigned long irqflags;
+			sighand = lock_task_sighand(target, &irqflags);
+			if (likely(sighand)) {
+				signal_wake_up(target, 0);
+				unlock_task_sighand(target, &irqflags);
+			}
+		}
+		break;
+
+	default:
+		BUG();		/* We checked it on entry.  */
+	}
+
+	/*
+	 * Let the thread resume running.  If it's not stopped now,
+	 * there is nothing more we need to do.
+	 */
+	if (reset)
+		utrace_reset(target, utrace);
+	else
+		spin_unlock(&utrace->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_control);
+
+/**
+ * utrace_barrier - synchronize with simultaneous tracing callbacks
+ * @target:		thread to affect
+ * @engine:		engine to affect (can be detached)
+ *
+ * This blocks while @target might be in the midst of making a callback to
+ * @engine.  It can be interrupted by signals and will return -%ERESTARTSYS.
+ * A return value of zero means no callback from @target to @engine was
+ * in progress.  Any effect of its return value (such as %UTRACE_STOP) has
+ * already been applied to @engine.
+ *
+ * It's not necessary to keep the @target pointer alive for this call.
+ * It's only necessary to hold a ref on @engine.  This will return
+ * safely even if @target has been reaped and has no task refs.
+ *
+ * A successful return from utrace_barrier() guarantees its ordering
+ * with respect to utrace_set_events() and utrace_control() calls.  If
+ * @target was not properly stopped, event callbacks just disabled might
+ * still be in progress; utrace_barrier() waits until there is no chance
+ * an unwanted callback can be in progress.
+ */
+int utrace_barrier(struct task_struct *target, struct utrace_engine *engine)
+{
+	struct utrace *utrace;
+	int ret = -ERESTARTSYS;
+
+	if (unlikely(target == current))
+		return 0;
+
+	do {
+		utrace = get_utrace_lock(target, engine, false);
+		if (unlikely(IS_ERR(utrace))) {
+			ret = PTR_ERR(utrace);
+			if (ret != -ERESTARTSYS)
+				break;
+		} else {
+			/*
+			 * All engine state changes are done while
+			 * holding the lock, i.e. before we get here.
+			 * Since we have the lock, we only need to
+			 * worry about @target making a callback.
+			 * When it has entered start_callback() but
+			 * not yet gotten to finish_callback(), we
+			 * will see utrace->reporting == @engine.
+			 * When @target doesn't take the lock, it uses
+			 * barriers to order setting utrace->reporting
+			 * before it examines the engine state.
+			 */
+			if (utrace->reporting != engine)
+				ret = 0;
+			spin_unlock(&utrace->lock);
+			if (!ret)
+				break;
+		}
+		schedule_timeout_interruptible(1);
+	} while (!signal_pending(current));
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_barrier);
+
+/*
+ * This is local state used for reporting loops, perhaps optimized away.
+ */
+struct utrace_report {
+	u32 result;
+	enum utrace_resume_action action;
+	enum utrace_resume_action resume_action;
+	bool detaches;
+	bool spurious;
+};
+
+#define INIT_REPORT(var)			\
+	struct utrace_report var = {		\
+		.action = UTRACE_RESUME,	\
+		.resume_action = UTRACE_RESUME,	\
+		.spurious = true 		\
+	}
+
+/*
+ * We are now making the report, so clear the flag saying we need one.
+ * When there is a new attach, ->pending_attach is set just so we will
+ * know to do splice_attaching() here before the callback loop.
+ */
+static enum utrace_resume_action start_report(struct utrace *utrace)
+{
+	enum utrace_resume_action resume = utrace->resume;
+	if (utrace->pending_attach ||
+	    (resume > UTRACE_INTERRUPT && resume < UTRACE_RESUME)) {
+		spin_lock(&utrace->lock);
+		splice_attaching(utrace);
+		resume = utrace->resume;
+		if (resume > UTRACE_INTERRUPT)
+			utrace->resume = UTRACE_RESUME;
+		spin_unlock(&utrace->lock);
+	}
+	return resume;
+}
+
+static inline void finish_report_reset(struct task_struct *task,
+				       struct utrace *utrace,
+				       struct utrace_report *report)
+{
+	if (unlikely(report->spurious || report->detaches)) {
+		spin_lock(&utrace->lock);
+		if (utrace_reset(task, utrace))
+			report->action = UTRACE_RESUME;
+	}
+}
+
+/*
+ * Complete a normal reporting pass, pairing with a start_report() call.
+ * This handles any UTRACE_DETACH or UTRACE_REPORT or UTRACE_INTERRUPT
+ * returns from engine callbacks.  If @will_not_stop is true and any
+ * engine's last callback used UTRACE_STOP, we do UTRACE_REPORT here to
+ * ensure we stop before user mode.  If there were no callbacks made, it
+ * will recompute @task->utrace_flags to avoid another false-positive.
+ */
+static void finish_report(struct task_struct *task, struct utrace *utrace,
+			  struct utrace_report *report, bool will_not_stop)
+{
+	enum utrace_resume_action resume = report->action;
+
+	if (resume == UTRACE_STOP)
+		resume = will_not_stop ? UTRACE_REPORT : UTRACE_RESUME;
+
+	if (resume < utrace->resume) {
+		spin_lock(&utrace->lock);
+		utrace->resume = resume;
+		if (resume == UTRACE_INTERRUPT)
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		else
+			set_tsk_thread_flag(task, TIF_NOTIFY_RESUME);
+		spin_unlock(&utrace->lock);
+	}
+
+	finish_report_reset(task, utrace, report);
+}
+
+static void finish_callback_report(struct task_struct *task,
+				   struct utrace *utrace,
+				   struct utrace_report *report,
+				   struct utrace_engine *engine,
+				   enum utrace_resume_action action)
+{
+	if (action == UTRACE_DETACH) {
+		/*
+		 * By holding the lock here, we make sure that
+		 * utrace_barrier() (really get_utrace_lock()) sees the
+		 * effect of this detach.  Otherwise utrace_barrier() could
+		 * return 0 after this callback had returned UTRACE_DETACH.
+		 * This way, a 0 return is an unambiguous indicator that any
+		 * callback returning UTRACE_DETACH has indeed caused detach.
+		 */
+		spin_lock(&utrace->lock);
+		engine->ops = &utrace_detached_ops;
+		spin_unlock(&utrace->lock);
+	}
+
+	/*
+	 * If utrace_control() was used, treat that like UTRACE_DETACH here.
+	 */
+	if (engine->ops == &utrace_detached_ops) {
+		report->detaches = true;
+		return;
+	}
+
+	if (action < report->action)
+		report->action = action;
+
+	if (action != UTRACE_STOP) {
+		if (action < report->resume_action)
+			report->resume_action = action;
+
+		if (engine_wants_stop(engine)) {
+			spin_lock(&utrace->lock);
+			clear_engine_wants_stop(engine);
+			spin_unlock(&utrace->lock);
+		}
+
+		return;
+	}
+
+	if (!engine_wants_stop(engine)) {
+		spin_lock(&utrace->lock);
+		/*
+		 * If utrace_control() came in and detached us
+		 * before we got the lock, we must not stop now.
+		 */
+		if (unlikely(engine->ops == &utrace_detached_ops))
+			report->detaches = true;
+		else
+			mark_engine_wants_stop(task, engine);
+		spin_unlock(&utrace->lock);
+	}
+}
+
+/*
+ * Apply the return value of one engine callback to @report.
+ * Returns true if @engine detached and should not get any more callbacks.
+ */
+static bool finish_callback(struct task_struct *task, struct utrace *utrace,
+			    struct utrace_report *report,
+			    struct utrace_engine *engine,
+			    u32 ret)
+{
+	report->result = ret & ~UTRACE_RESUME_MASK;
+	finish_callback_report(task, utrace, report, engine,
+			       utrace_resume_action(ret));
+
+	/*
+	 * Now that we have applied the effect of the return value,
+	 * clear this so that utrace_barrier() can stop waiting.
+	 * A subsequent utrace_control() can stop or resume @engine
+	 * and know this was ordered after its callback's action.
+	 *
+	 * We don't need any barriers here because utrace_barrier()
+	 * takes utrace->lock.  If we touched engine->flags above,
+	 * the lock guaranteed this change was before utrace_barrier()
+	 * examined utrace->reporting.
+	 */
+	utrace->reporting = NULL;
+
+	/*
+	 * We've just done an engine callback.  These are allowed to sleep,
+	 * though all well-behaved ones restrict that to blocking kalloc()
+	 * or quickly-acquired mutex_lock() and the like.  This is a good
+	 * place to make sure tracing engines don't introduce too much
+	 * latency under voluntary preemption.
+	 */
+	might_sleep();
+
+	return engine->ops == &utrace_detached_ops;
+}
+
+/*
+ * Start the callbacks for @engine to consider @event (a bit mask).
+ * This makes the report_quiesce() callback first.  If @engine wants
+ * a specific callback for @event, we return the ops vector to use.
+ * If not, we return NULL.  The return value from the ops->callback
+ * function called should be passed to finish_callback().
+ */
+static const struct utrace_engine_ops *start_callback(
+	struct utrace *utrace, struct utrace_report *report,
+	struct utrace_engine *engine, struct task_struct *task,
+	unsigned long event)
+{
+	const struct utrace_engine_ops *ops;
+	unsigned long want;
+
+	/*
+	 * This barrier ensures that we've set utrace->reporting before
+	 * we examine engine->flags or engine->ops.  utrace_barrier()
+	 * relies on this ordering to indicate that the effect of any
+	 * utrace_control() and utrace_set_events() calls is in place
+	 * by the time utrace->reporting can be seen to be NULL.
+	 */
+	utrace->reporting = engine;
+	smp_mb();
+
+	/*
+	 * This pairs with the barrier in mark_engine_detached().
+	 * It makes sure that we never see the old ops vector with
+	 * the new flags, in case the original vector had no report_quiesce.
+	 */
+	want = engine->flags;
+	smp_rmb();
+	ops = engine->ops;
+
+	if (want & UTRACE_EVENT(QUIESCE)) {
+		if (finish_callback(task, utrace, report, engine,
+				    (*ops->report_quiesce)(report->action,
+							   engine, event)))
+			return NULL;
+
+		/*
+		 * finish_callback() reset utrace->reporting after the
+		 * quiesce callback.  Now we set it again (as above)
+		 * before re-examining engine->flags, which could have
+		 * been changed synchronously by ->report_quiesce or
+		 * asynchronously by utrace_control() or utrace_set_events().
+		 */
+		utrace->reporting = engine;
+		smp_mb();
+		want = engine->flags;
+	}
+
+	if (want & ENGINE_STOP)
+		report->action = UTRACE_STOP;
+
+	if (want & event) {
+		report->spurious = false;
+		return ops;
+	}
+
+	utrace->reporting = NULL;
+	return NULL;
+}
+
+/*
+ * Do a normal reporting pass for engines interested in @event.
+ * @callback is the name of the member in the ops vector, and remaining
+ * args are the extras it takes after the standard three args.
+ */
+#define REPORT_CALLBACKS(rev, task, utrace, report, event, callback, ...)     \
+	do {								      \
+		struct utrace_engine *engine;				      \
+		const struct utrace_engine_ops *ops;			      \
+		list_for_each_entry##rev(engine, &utrace->attached, entry) {  \
+			ops = start_callback(utrace, report, engine, task,    \
+					     event);			      \
+			if (!ops)					      \
+				continue;				      \
+			finish_callback(task, utrace, report, engine,	      \
+					(*ops->callback)(__VA_ARGS__));	      \
+		}							      \
+	} while (0)
+#define REPORT(task, utrace, report, event, callback, ...)		      \
+	do {								      \
+		start_report(utrace);					      \
+		REPORT_CALLBACKS(, task, utrace, report, event, callback,     \
+				 (report)->action, engine, ## __VA_ARGS__);   \
+		finish_report(task, utrace, report, true);		      \
+	} while (0)
+
+/*
+ * Called iff UTRACE_EVENT(EXEC) flag is set.
+ */
+void utrace_report_exec(struct linux_binfmt *fmt, struct linux_binprm *bprm,
+			struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(EXEC),
+	       report_exec, fmt, bprm, regs);
+}
+
+static u32 do_report_syscall_entry(struct pt_regs *regs,
+				   struct task_struct *task,
+				   struct utrace *utrace,
+				   struct utrace_report *report,
+				   u32 resume_report)
+{
+	start_report(utrace);
+	REPORT_CALLBACKS(_reverse, task, utrace, report,
+			 UTRACE_EVENT(SYSCALL_ENTRY), report_syscall_entry,
+			 resume_report | report->result | report->action,
+			 engine, regs);
+	finish_report(task, utrace, report, false);
+
+	if (report->action != UTRACE_STOP)
+		return 0;
+
+	utrace_stop(task, utrace, report->resume_action);
+
+	if (fatal_signal_pending(task)) {
+		/*
+		 * We are continuing despite UTRACE_STOP because of a
+		 * SIGKILL.  Don't let the system call actually proceed.
+		 */
+		report->result = UTRACE_SYSCALL_ABORT;
+	} else if (utrace->resume <= UTRACE_REPORT) {
+		/*
+		 * If we've been asked for another report after our stop,
+		 * go back to report (and maybe stop) again before we run
+		 * the system call.  The second (and later) reports are
+		 * marked with the UTRACE_SYSCALL_RESUMED flag so that
+		 * engines know this is a second report at the same
+		 * entry.  This gives them the chance to examine the
+		 * registers anew after they might have been changed
+		 * while we were stopped.
+		 */
+		report->detaches = false;
+		report->spurious = true;
+		report->action = report->resume_action = UTRACE_RESUME;
+		return UTRACE_SYSCALL_RESUMED;
+	}
+
+	return 0;
+}
+
+/*
+ * Called iff UTRACE_EVENT(SYSCALL_ENTRY) flag is set.
+ * Return true to prevent the system call.
+ */
+bool utrace_report_syscall_entry(struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	u32 resume_report = 0;
+
+	do {
+		resume_report = do_report_syscall_entry(regs, task, utrace,
+							&report, resume_report);
+	} while (resume_report);
+
+	return utrace_syscall_action(report.result) == UTRACE_SYSCALL_ABORT;
+}
+
+/*
+ * Called iff UTRACE_EVENT(SYSCALL_EXIT) flag is set.
+ */
+void utrace_report_syscall_exit(struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(SYSCALL_EXIT),
+	       report_syscall_exit, regs);
+}
+
+/*
+ * Called iff UTRACE_EVENT(CLONE) flag is set.
+ * This notification call blocks the wake_up_new_task call on the child.
+ * So we must not quiesce here.  tracehook_report_clone_complete will do
+ * a quiescence check momentarily.
+ */
+void utrace_report_clone(unsigned long clone_flags, struct task_struct *child)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	/*
+	 * We don't use the REPORT() macro here, because we need
+	 * to clear utrace->cloning before finish_report().
+	 * After finish_report(), utrace can be a stale pointer
+	 * in cases when report.action is still UTRACE_RESUME.
+	 */
+	start_report(utrace);
+	utrace->cloning = child;
+
+	REPORT_CALLBACKS(, task, utrace, &report,
+			 UTRACE_EVENT(CLONE), report_clone,
+			 report.action, engine, clone_flags, child);
+
+	utrace->cloning = NULL;
+	finish_report(task, utrace, &report, !(clone_flags & CLONE_VFORK));
+
+	/*
+	 * For a vfork, we will go into an uninterruptible block waiting
+	 * for the child.  We need UTRACE_STOP to happen before this, not
+	 * after.  For CLONE_VFORK, utrace_finish_vfork() will be called.
+	 */
+	if (report.action == UTRACE_STOP && (clone_flags & CLONE_VFORK)) {
+		spin_lock(&utrace->lock);
+		utrace->vfork_stop = 1;
+		spin_unlock(&utrace->lock);
+	}
+}
+
+/*
+ * We're called after utrace_report_clone() for a CLONE_VFORK.
+ * If UTRACE_STOP was left from the clone report, we stop here.
+ * After this, we'll enter the uninterruptible wait_for_completion()
+ * waiting for the child.
+ */
+void utrace_finish_vfork(struct task_struct *task)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	if (utrace->vfork_stop) {
+		spin_lock(&utrace->lock);
+		utrace->vfork_stop = 0;
+		spin_unlock(&utrace->lock);
+		utrace_stop(task, utrace, UTRACE_RESUME); /* XXX */
+	}
+}
+
+/*
+ * Called iff UTRACE_EVENT(JCTL) flag is set.
+ *
+ * Called with siglock held.
+ */
+void utrace_report_jctl(int notify, int what)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	spin_unlock_irq(&task->sighand->siglock);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(JCTL),
+	       report_jctl, what, notify);
+
+	spin_lock_irq(&task->sighand->siglock);
+}
+
+/*
+ * Called iff UTRACE_EVENT(EXIT) flag is set.
+ */
+void utrace_report_exit(long *exit_code)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	long orig_code = *exit_code;
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(EXIT),
+	       report_exit, orig_code, exit_code);
+
+	if (report.action == UTRACE_STOP)
+		utrace_stop(task, utrace, report.resume_action);
+}
+
+/*
+ * Called iff UTRACE_EVENT(DEATH) or UTRACE_EVENT(QUIESCE) flag is set.
+ *
+ * It is always possible that we are racing with utrace_release_task here.
+ * For this reason, utrace_release_task checks for the event bits that get
+ * us here, and delays its cleanup for us to do.
+ */
+void utrace_report_death(struct task_struct *task, struct utrace *utrace,
+			 bool group_dead, int signal)
+{
+	INIT_REPORT(report);
+
+	BUG_ON(!task->exit_state);
+
+	/*
+	 * We are presently considered "quiescent"--which is accurate
+	 * inasmuch as we won't run any more user instructions ever again.
+	 * But for utrace_control and utrace_set_events to be robust, they
+	 * must be sure whether or not we will run any more callbacks.  If
+	 * a call comes in before we do, taking the lock here synchronizes
+	 * us so we don't run any callbacks just disabled.  Calls that come
+	 * in while we're running the callbacks will see the exit.death
+	 * flag and know that we are not yet fully quiescent for purposes
+	 * of detach bookkeeping.
+	 */
+	spin_lock(&utrace->lock);
+	BUG_ON(utrace->death);
+	utrace->death = 1;
+	utrace->resume = UTRACE_RESUME;
+	splice_attaching(utrace);
+	spin_unlock(&utrace->lock);
+
+	REPORT_CALLBACKS(, task, utrace, &report, UTRACE_EVENT(DEATH),
+			 report_death, engine, group_dead, signal);
+
+	utrace_maybe_reap(task, utrace, false);
+}
+
+/*
+ * Finish the last reporting pass before returning to user mode.
+ */
+static void finish_resume_report(struct task_struct *task,
+				 struct utrace *utrace,
+				 struct utrace_report *report)
+{
+	finish_report_reset(task, utrace, report);
+
+	switch (report->action) {
+	case UTRACE_STOP:
+		utrace_stop(task, utrace, report->resume_action);
+		break;
+
+	case UTRACE_INTERRUPT:
+		if (!signal_pending(task))
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		break;
+
+	case UTRACE_BLOCKSTEP:
+		if (likely(arch_has_block_step())) {
+			user_enable_block_step(task);
+			break;
+		}
+
+		/*
+		 * This means some callback is to blame for failing
+		 * to check arch_has_block_step() itself.  Warn and
+		 * then fall through to treat it as SINGLESTEP.
+		 */
+		WARN(1, "UTRACE_BLOCKSTEP when !arch_has_block_step()");
+
+	case UTRACE_SINGLESTEP:
+		if (likely(arch_has_single_step())) {
+			user_enable_single_step(task);
+		} else {
+			/*
+			 * This means some callback is to blame for failing
+			 * to check arch_has_single_step() itself.  Spew
+			 * about it so the loser will fix his module.
+			 */
+			WARN(1,
+			     "UTRACE_SINGLESTEP when !arch_has_single_step()");
+		}
+		break;
+
+	case UTRACE_REPORT:
+	case UTRACE_RESUME:
+	default:
+		user_disable_single_step(task);
+		break;
+	}
+}
+
+/*
+ * This is called when TIF_NOTIFY_RESUME had been set (and is now clear).
+ * We are close to user mode, and this is the place to report or stop.
+ * When we return, we're going to user mode or into the signals code.
+ */
+void utrace_resume(struct task_struct *task, struct pt_regs *regs)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	struct utrace_engine *engine;
+
+	/*
+	 * Some machines get here with interrupts disabled.  The same arch
+	 * code path leads to calling into get_signal_to_deliver(), which
+	 * implicitly reenables them by virtue of spin_unlock_irq.
+	 */
+	local_irq_enable();
+
+	/*
+	 * If this flag is still set it's because there was a signal
+	 * handler setup done but no report_signal following it.  Clear
+	 * the flag before we get to user so it doesn't confuse us later.
+	 */
+	if (unlikely(utrace->signal_handler)) {
+		spin_lock(&utrace->lock);
+		utrace->signal_handler = 0;
+		spin_unlock(&utrace->lock);
+	}
+
+	/*
+	 * Update our bookkeeping even if there are no callbacks made here.
+	 */
+	report.action = start_report(utrace);
+
+	switch (report.action) {
+	case UTRACE_RESUME:
+		/*
+		 * Anything we might have done was already handled by
+		 * utrace_get_signal(), or this is an entirely spurious
+		 * call.  (The arch might use TIF_NOTIFY_RESUME for other
+		 * purposes as well as calling us.)
+		 */
+		return;
+	case UTRACE_REPORT:
+		if (unlikely(!(task->utrace_flags & UTRACE_EVENT(QUIESCE))))
+			break;
+		/*
+		 * Do a simple reporting pass, with no specific
+		 * callback after report_quiesce.
+		 */
+		report.action = UTRACE_RESUME;
+		list_for_each_entry(engine, &utrace->attached, entry)
+			start_callback(utrace, &report, engine, task, 0);
+		break;
+	default:
+		/*
+		 * Even if this report was truly spurious, there is no need
+		 * for utrace_reset() now.  TIF_NOTIFY_RESUME was already
+		 * cleared--it doesn't stay spuriously set.
+		 */
+		report.spurious = false;
+		break;
+	}
+
+	/*
+	 * Finish the report and either stop or get ready to resume.
+	 * If utrace->resume was not UTRACE_REPORT, this applies its
+	 * effect now (i.e. step or interrupt).
+	 */
+	finish_resume_report(task, utrace, &report);
+}
+
+/*
+ * Return true if current has forced signal_pending().
+ *
+ * This is called only when current->utrace_flags is nonzero, so we know
+ * that current->utrace must be set.  It's not inlined in tracehook.h
+ * just so that struct utrace can stay opaque outside this file.
+ */
+bool utrace_interrupt_pending(void)
+{
+	return task_utrace_struct(current)->resume == UTRACE_INTERRUPT;
+}
+
+/*
+ * Take the siglock and push @info back on our queue.
+ * Returns with @task->sighand->siglock held.
+ */
+static void push_back_signal(struct task_struct *task, siginfo_t *info)
+	__acquires(task->sighand->siglock)
+{
+	struct sigqueue *q;
+
+	if (unlikely(!info->si_signo)) { /* Oh, a wise guy! */
+		spin_lock_irq(&task->sighand->siglock);
+		return;
+	}
+
+	q = sigqueue_alloc();
+	if (likely(q)) {
+		q->flags = 0;
+		copy_siginfo(&q->info, info);
+	}
+
+	spin_lock_irq(&task->sighand->siglock);
+
+	sigaddset(&task->pending.signal, info->si_signo);
+	if (likely(q))
+		list_add(&q->list, &task->pending.list);
+
+	set_tsk_thread_flag(task, TIF_SIGPENDING);
+}
+
+/*
+ * This is the hook from the signals code, called with the siglock held.
+ * Here is the ideal place to stop.  We also dequeue and intercept signals.
+ */
+int utrace_get_signal(struct task_struct *task, struct pt_regs *regs,
+		      siginfo_t *info, struct k_sigaction *return_ka)
+	__releases(task->sighand->siglock)
+	__acquires(task->sighand->siglock)
+{
+	struct utrace *utrace;
+	struct k_sigaction *ka;
+	INIT_REPORT(report);
+	struct utrace_engine *engine;
+	const struct utrace_engine_ops *ops;
+	unsigned long event, want;
+	u32 ret;
+	int signr;
+
+	utrace = task_utrace_struct(task);
+	if (utrace->resume < UTRACE_RESUME ||
+	    utrace->pending_attach || utrace->signal_handler) {
+		enum utrace_resume_action resume;
+
+		/*
+		 * We've been asked for an explicit report before we
+		 * even check for pending signals.
+		 */
+
+		spin_unlock_irq(&task->sighand->siglock);
+
+		spin_lock(&utrace->lock);
+
+		splice_attaching(utrace);
+
+		report.result = utrace->signal_handler ?
+			UTRACE_SIGNAL_HANDLER : UTRACE_SIGNAL_REPORT;
+		utrace->signal_handler = 0;
+
+		resume = utrace->resume;
+		utrace->resume = UTRACE_RESUME;
+
+		spin_unlock(&utrace->lock);
+
+		/*
+		 * Make sure signal_pending() only returns true
+		 * if there are real signals pending.
+		 */
+		if (signal_pending(task)) {
+			spin_lock_irq(&task->sighand->siglock);
+			recalc_sigpending();
+			spin_unlock_irq(&task->sighand->siglock);
+		}
+
+		if (resume > UTRACE_REPORT) {
+			/*
+			 * We only got here to process utrace->resume.
+			 * Despite no callbacks, this report is not spurious.
+			 */
+			report.action = resume;
+			report.spurious = false;
+			finish_resume_report(task, utrace, &report);
+			return -1;
+		} else if (!(task->utrace_flags & UTRACE_EVENT(QUIESCE))) {
+			/*
+			 * We only got here to clear utrace->signal_handler.
+			 */
+			return -1;
+		}
+
+		/*
+		 * Do a reporting pass for no signal, just for EVENT(QUIESCE).
+		 * The engine callbacks can fill in *info and *return_ka.
+		 * We'll pass NULL for the @orig_ka argument to indicate
+		 * that there was no original signal.
+		 */
+		event = 0;
+		ka = NULL;
+		memset(return_ka, 0, sizeof *return_ka);
+	} else if (!(task->utrace_flags & UTRACE_EVENT_SIGNAL_ALL) ||
+		   unlikely(task->signal->group_stop_count)) {
+		/*
+		 * If no engine is interested in intercepting signals or
+		 * we must stop, let the caller just dequeue them normally
+		 * or participate in group-stop.
+		 */
+		return 0;
+	} else {
+		/*
+		 * Steal the next signal so we can let tracing engines
+		 * examine it.  From the signal number and sigaction,
+		 * determine what normal delivery would do.  If no
+		 * engine perturbs it, we'll do that by returning the
+		 * signal number after setting *return_ka.
+		 */
+		signr = dequeue_signal(task, &task->blocked, info);
+		if (signr == 0)
+			return signr;
+		BUG_ON(signr != info->si_signo);
+
+		ka = &task->sighand->action[signr - 1];
+		*return_ka = *ka;
+
+		/*
+		 * We are never allowed to interfere with SIGKILL.
+		 * Just punt after filling in *return_ka for our caller.
+		 */
+		if (signr == SIGKILL)
+			return signr;
+
+		if (ka->sa.sa_handler == SIG_IGN) {
+			event = UTRACE_EVENT(SIGNAL_IGN);
+			report.result = UTRACE_SIGNAL_IGN;
+		} else if (ka->sa.sa_handler != SIG_DFL) {
+			event = UTRACE_EVENT(SIGNAL);
+			report.result = UTRACE_SIGNAL_DELIVER;
+		} else if (sig_kernel_coredump(signr)) {
+			event = UTRACE_EVENT(SIGNAL_CORE);
+			report.result = UTRACE_SIGNAL_CORE;
+		} else if (sig_kernel_ignore(signr)) {
+			event = UTRACE_EVENT(SIGNAL_IGN);
+			report.result = UTRACE_SIGNAL_IGN;
+		} else if (signr == SIGSTOP) {
+			event = UTRACE_EVENT(SIGNAL_STOP);
+			report.result = UTRACE_SIGNAL_STOP;
+		} else if (sig_kernel_stop(signr)) {
+			event = UTRACE_EVENT(SIGNAL_STOP);
+			report.result = UTRACE_SIGNAL_TSTP;
+		} else {
+			event = UTRACE_EVENT(SIGNAL_TERM);
+			report.result = UTRACE_SIGNAL_TERM;
+		}
+
+		/*
+		 * Now that we know what event type this signal is, we
+		 * can short-circuit if no engines care about those.
+		 */
+		if ((task->utrace_flags & (event | UTRACE_EVENT(QUIESCE))) == 0)
+			return signr;
+
+		/*
+		 * We have some interested engines, so tell them about
+		 * the signal and let them change its disposition.
+		 */
+		spin_unlock_irq(&task->sighand->siglock);
+	}
+
+	/*
+	 * This reporting pass chooses what signal disposition we'll act on.
+	 */
+	list_for_each_entry(engine, &utrace->attached, entry) {
+		/*
+		 * See start_callback() comment about this barrier.
+		 */
+		utrace->reporting = engine;
+		smp_mb();
+
+		/*
+		 * This pairs with the barrier in mark_engine_detached(),
+		 * see start_callback() comments.
+		 */
+		want = engine->flags;
+		smp_rmb();
+		ops = engine->ops;
+
+		if ((want & (event | UTRACE_EVENT(QUIESCE))) == 0) {
+			utrace->reporting = NULL;
+			continue;
+		}
+
+		if (ops->report_signal)
+			ret = (*ops->report_signal)(
+				report.result | report.action, engine,
+				regs, info, ka, return_ka);
+		else
+			ret = (report.result | (*ops->report_quiesce)(
+				       report.action, engine, event));
+
+		/*
+		 * Avoid a tight loop reporting again and again if some
+		 * engine is too stupid.
+		 */
+		switch (utrace_resume_action(ret)) {
+		default:
+			break;
+		case UTRACE_INTERRUPT:
+		case UTRACE_REPORT:
+			ret = (ret & ~UTRACE_RESUME_MASK) | UTRACE_RESUME;
+			break;
+		}
+
+		finish_callback(task, utrace, &report, engine, ret);
+	}
+
+	/*
+	 * We express the chosen action to the signals code in terms
+	 * of a representative signal whose default action does it.
+	 * Our caller uses our return value (signr) to decide what to
+	 * do, but uses info->si_signo as the signal number to report.
+	 */
+	switch (utrace_signal_action(report.result)) {
+	case UTRACE_SIGNAL_TERM:
+		signr = SIGTERM;
+		break;
+
+	case UTRACE_SIGNAL_CORE:
+		signr = SIGQUIT;
+		break;
+
+	case UTRACE_SIGNAL_STOP:
+		signr = SIGSTOP;
+		break;
+
+	case UTRACE_SIGNAL_TSTP:
+		signr = SIGTSTP;
+		break;
+
+	case UTRACE_SIGNAL_DELIVER:
+		signr = info->si_signo;
+
+		if (return_ka->sa.sa_handler == SIG_DFL) {
+			/*
+			 * We'll do signr's normal default action.
+			 * For ignore, we'll fall through below.
+			 * For stop/death, break locks and returns it.
+			 */
+			if (likely(signr) && !sig_kernel_ignore(signr))
+				break;
+		} else if (return_ka->sa.sa_handler != SIG_IGN &&
+			   likely(signr)) {
+			/*
+			 * Complete the bookkeeping after the report.
+			 * The handler will run.  If an engine wanted to
+			 * stop or step, then make sure we do another
+			 * report after signal handler setup.
+			 */
+			if (report.action != UTRACE_RESUME)
+				report.action = UTRACE_INTERRUPT;
+			finish_report(task, utrace, &report, true);
+
+			if (unlikely(report.result & UTRACE_SIGNAL_HOLD))
+				push_back_signal(task, info);
+			else
+				spin_lock_irq(&task->sighand->siglock);
+
+			/*
+			 * We do the SA_ONESHOT work here since the
+			 * normal path will only touch *return_ka now.
+			 */
+			if (unlikely(return_ka->sa.sa_flags & SA_ONESHOT)) {
+				return_ka->sa.sa_flags &= ~SA_ONESHOT;
+				if (likely(valid_signal(signr))) {
+					ka = &task->sighand->action[signr - 1];
+					ka->sa.sa_handler = SIG_DFL;
+				}
+			}
+
+			return signr;
+		}
+
+		/* Fall through for an ignored signal.  */
+
+	case UTRACE_SIGNAL_IGN:
+	case UTRACE_SIGNAL_REPORT:
+	default:
+		/*
+		 * If the signal is being ignored, then we are on the way
+		 * directly back to user mode.  We can stop here, or step,
+		 * as in utrace_resume(), above.  After we've dealt with that,
+		 * our caller will relock and come back through here.
+		 */
+		finish_resume_report(task, utrace, &report);
+
+		if (unlikely(fatal_signal_pending(task))) {
+			/*
+			 * The only reason we woke up now was because of a
+			 * SIGKILL.  Don't do normal dequeuing in case it
+			 * might get a signal other than SIGKILL.  That would
+			 * perturb the death state so it might differ from
+			 * what the debugger would have allowed to happen.
+			 * Instead, pluck out just the SIGKILL to be sure
+			 * we'll die immediately with nothing else different
+			 * from the quiescent state the debugger wanted us in.
+			 */
+			sigset_t sigkill_only;
+			siginitsetinv(&sigkill_only, sigmask(SIGKILL));
+			spin_lock_irq(&task->sighand->siglock);
+			signr = dequeue_signal(task, &sigkill_only, info);
+			BUG_ON(signr != SIGKILL);
+			*return_ka = task->sighand->action[SIGKILL - 1];
+			return signr;
+		}
+
+		if (unlikely(report.result & UTRACE_SIGNAL_HOLD)) {
+			push_back_signal(task, info);
+			spin_unlock_irq(&task->sighand->siglock);
+		}
+
+		return -1;
+	}
+
+	/*
+	 * Complete the bookkeeping after the report.
+	 * This sets utrace->resume if UTRACE_STOP was used.
+	 */
+	finish_report(task, utrace, &report, true);
+
+	return_ka->sa.sa_handler = SIG_DFL;
+
+	/*
+	 * If this signal is fatal, si_signo gets through as exit_code.
+	 * We can't allow a completely bogus value there or else core
+	 * kernel code can freak out.  (If an engine wants to control
+	 * the exit_code value exactly, it can do so in report_exit.)
+	 * We'll produce a big complaint in dmesg, but won't crash.
+	 * That's nicer for debugging your utrace engine.
+	 */
+	if (unlikely(info->si_signo & 0x80)) {
+		WARN(1, "utrace engine left bogus si_signo value!");
+		info->si_signo = SIGTRAP;
+	}
+
+	if (unlikely(report.result & UTRACE_SIGNAL_HOLD))
+		push_back_signal(task, info);
+	else
+		spin_lock_irq(&task->sighand->siglock);
+
+	if (sig_kernel_stop(signr))
+		task->signal->flags |= SIGNAL_STOP_DEQUEUED;
+
+	return signr;
+}
+
+/*
+ * This gets called after a signal handler has been set up.
+ * We set a flag so the next report knows it happened.
+ * If we're already stepping, make sure we do a report_signal.
+ * If not, make sure we get into utrace_resume() where we can
+ * clear the signal_handler flag before resuming.
+ */
+void utrace_signal_handler(struct task_struct *task, int stepping)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	spin_lock(&utrace->lock);
+
+	utrace->signal_handler = 1;
+	if (utrace->resume > UTRACE_INTERRUPT) {
+		if (stepping) {
+			utrace->resume = UTRACE_INTERRUPT;
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		} else if (utrace->resume == UTRACE_RESUME) {
+			set_tsk_thread_flag(task, TIF_NOTIFY_RESUME);
+		}
+	}
+
+	spin_unlock(&utrace->lock);
+}
+
+/**
+ * utrace_prepare_examine - prepare to examine thread state
+ * @target:		thread of interest, a &struct task_struct pointer
+ * @engine:		engine pointer returned by utrace_attach_task()
+ * @exam:		temporary state, a &struct utrace_examiner pointer
+ *
+ * This call prepares to safely examine the thread @target using
+ * &struct user_regset calls, or direct access to thread-synchronous fields.
+ *
+ * When @target is current, this call is superfluous.  When @target is
+ * another thread, it must be held stopped via %UTRACE_STOP by @engine.
+ *
+ * This call may block the caller until @target stays stopped, so it must
+ * be called only after the caller is sure @target is about to unschedule.
+ * This means a zero return from a utrace_control() call on @engine giving
+ * %UTRACE_STOP, or a report_quiesce() or report_signal() callback to
+ * @engine that used %UTRACE_STOP in its return value.
+ *
+ * Returns -%ESRCH if @target is dead or -%EINVAL if %UTRACE_STOP was
+ * not used.  If @target has started running again despite %UTRACE_STOP
+ * (for %SIGKILL or a spurious wakeup), this call returns -%EAGAIN.
+ *
+ * When this call returns zero, it's safe to use &struct user_regset
+ * calls and task_user_regset_view() on @target and to examine some of
+ * its fields directly.  When the examination is complete, a
+ * utrace_finish_examine() call must follow to check whether it was
+ * completed safely.
+ */
+int utrace_prepare_examine(struct task_struct *target,
+			   struct utrace_engine *engine,
+			   struct utrace_examiner *exam)
+{
+	int ret = 0;
+
+	if (unlikely(target == current))
+		return 0;
+
+	rcu_read_lock();
+	if (unlikely(!engine_wants_stop(engine)))
+		ret = -EINVAL;
+	else if (unlikely(target->exit_state))
+		ret = -ESRCH;
+	else {
+		exam->state = target->state;
+		if (unlikely(exam->state == TASK_RUNNING))
+			ret = -EAGAIN;
+		else
+			get_task_struct(target);
+	}
+	rcu_read_unlock();
+
+	if (likely(!ret)) {
+		exam->ncsw = wait_task_inactive(target, exam->state);
+		put_task_struct(target);
+		if (unlikely(!exam->ncsw))
+			ret = -EAGAIN;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_prepare_examine);
+
+/**
+ * utrace_finish_examine - complete an examination of thread state
+ * @target:		thread of interest, a &struct task_struct pointer
+ * @engine:		engine pointer returned by utrace_attach_task()
+ * @exam:		pointer passed to utrace_prepare_examine() call
+ *
+ * This call completes an examination on the thread @target begun by a
+ * paired utrace_prepare_examine() call with the same arguments that
+ * returned success (zero).
+ *
+ * When @target is current, this call is superfluous.  When @target is
+ * another thread, this returns zero if @target has remained unscheduled
+ * since the paired utrace_prepare_examine() call returned zero.
+ *
+ * When this returns an error, any examination done since the paired
+ * utrace_prepare_examine() call is unreliable and the data extracted
+ * should be discarded.  The error is -%EINVAL if @engine is not
+ * keeping @target stopped, or -%EAGAIN if @target woke up unexpectedly.
+ */
+int utrace_finish_examine(struct task_struct *target,
+			  struct utrace_engine *engine,
+			  struct utrace_examiner *exam)
+{
+	int ret = 0;
+
+	if (unlikely(target == current))
+		return 0;
+
+	rcu_read_lock();
+	if (unlikely(!engine_wants_stop(engine)))
+		ret = -EINVAL;
+	else if (unlikely(target->state != exam->state))
+		ret = -EAGAIN;
+	else
+		get_task_struct(target);
+	rcu_read_unlock();
+
+	if (likely(!ret)) {
+		unsigned long ncsw = wait_task_inactive(target, exam->state);
+		if (unlikely(ncsw != exam->ncsw))
+			ret = -EAGAIN;
+		put_task_struct(target);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_finish_examine);
+
+/*
+ * This is declared in linux/regset.h and defined in machine-dependent
+ * code.  We put the export here to ensure no machine forgets it.
+ */
+EXPORT_SYMBOL_GPL(task_user_regset_view);
+
+/*
+ * Called with rcu_read_lock() held.
+ */
+void task_utrace_proc_status(struct seq_file *m, struct task_struct *p)
+{
+	seq_printf(m, "Utrace:\t%lx\n", p->utrace_flags);
+}