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-Title : Kernel Probes (Kprobes)
-Authors : Jim Keniston <jkenisto@us.ibm.com>
- : Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
- : Masami Hiramatsu <mhiramat@redhat.com>
-
-CONTENTS
-
-1. Concepts: Kprobes, Jprobes, Return Probes
-2. Architectures Supported
-3. Configuring Kprobes
-4. API Reference
-5. Kprobes Features and Limitations
-6. Probe Overhead
-7. TODO
-8. Kprobes Example
-9. Jprobes Example
-10. Kretprobes Example
-Appendix A: The kprobes debugfs interface
-Appendix B: The kprobes sysctl interface
-
-1. Concepts: Kprobes, Jprobes, Return Probes
-
-Kprobes enables you to dynamically break into any kernel routine and
-collect debugging and performance information non-disruptively. You
-can trap at almost any kernel code address, specifying a handler
-routine to be invoked when the breakpoint is hit.
-
-There are currently three types of probes: kprobes, jprobes, and
-kretprobes (also called return probes). A kprobe can be inserted
-on virtually any instruction in the kernel. A jprobe is inserted at
-the entry to a kernel function, and provides convenient access to the
-function's arguments. A return probe fires when a specified function
-returns.
-
-In the typical case, Kprobes-based instrumentation is packaged as
-a kernel module. The module's init function installs ("registers")
-one or more probes, and the exit function unregisters them. A
-registration function such as register_kprobe() specifies where
-the probe is to be inserted and what handler is to be called when
-the probe is hit.
-
-There are also register_/unregister_*probes() functions for batch
-registration/unregistration of a group of *probes. These functions
-can speed up unregistration process when you have to unregister
-a lot of probes at once.
-
-The next four subsections explain how the different types of
-probes work and how jump optimization works. They explain certain
-things that you'll need to know in order to make the best use of
-Kprobes -- e.g., the difference between a pre_handler and
-a post_handler, and how to use the maxactive and nmissed fields of
-a kretprobe. But if you're in a hurry to start using Kprobes, you
-can skip ahead to section 2.
-
-1.1 How Does a Kprobe Work?
-
-When a kprobe is registered, Kprobes makes a copy of the probed
-instruction and replaces the first byte(s) of the probed instruction
-with a breakpoint instruction (e.g., int3 on i386 and x86_64).
-
-When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
-registers are saved, and control passes to Kprobes via the
-notifier_call_chain mechanism. Kprobes executes the "pre_handler"
-associated with the kprobe, passing the handler the addresses of the
-kprobe struct and the saved registers.
-
-Next, Kprobes single-steps its copy of the probed instruction.
-(It would be simpler to single-step the actual instruction in place,
-but then Kprobes would have to temporarily remove the breakpoint
-instruction. This would open a small time window when another CPU
-could sail right past the probepoint.)
-
-After the instruction is single-stepped, Kprobes executes the
-"post_handler," if any, that is associated with the kprobe.
-Execution then continues with the instruction following the probepoint.
-
-1.2 How Does a Jprobe Work?
-
-A jprobe is implemented using a kprobe that is placed on a function's
-entry point. It employs a simple mirroring principle to allow
-seamless access to the probed function's arguments. The jprobe
-handler routine should have the same signature (arg list and return
-type) as the function being probed, and must always end by calling
-the Kprobes function jprobe_return().
-
-Here's how it works. When the probe is hit, Kprobes makes a copy of
-the saved registers and a generous portion of the stack (see below).
-Kprobes then points the saved instruction pointer at the jprobe's
-handler routine, and returns from the trap. As a result, control
-passes to the handler, which is presented with the same register and
-stack contents as the probed function. When it is done, the handler
-calls jprobe_return(), which traps again to restore the original stack
-contents and processor state and switch to the probed function.
-
-By convention, the callee owns its arguments, so gcc may produce code
-that unexpectedly modifies that portion of the stack. This is why
-Kprobes saves a copy of the stack and restores it after the jprobe
-handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
-64 bytes on i386.
-
-Note that the probed function's args may be passed on the stack
-or in registers. The jprobe will work in either case, so long as the
-handler's prototype matches that of the probed function.
-
-1.3 Return Probes
-
-1.3.1 How Does a Return Probe Work?
-
-When you call register_kretprobe(), Kprobes establishes a kprobe at
-the entry to the function. When the probed function is called and this
-probe is hit, Kprobes saves a copy of the return address, and replaces
-the return address with the address of a "trampoline." The trampoline
-is an arbitrary piece of code -- typically just a nop instruction.
-At boot time, Kprobes registers a kprobe at the trampoline.
-
-When the probed function executes its return instruction, control
-passes to the trampoline and that probe is hit. Kprobes' trampoline
-handler calls the user-specified return handler associated with the
-kretprobe, then sets the saved instruction pointer to the saved return
-address, and that's where execution resumes upon return from the trap.
-
-While the probed function is executing, its return address is
-stored in an object of type kretprobe_instance. Before calling
-register_kretprobe(), the user sets the maxactive field of the
-kretprobe struct to specify how many instances of the specified
-function can be probed simultaneously. register_kretprobe()
-pre-allocates the indicated number of kretprobe_instance objects.
-
-For example, if the function is non-recursive and is called with a
-spinlock held, maxactive = 1 should be enough. If the function is
-non-recursive and can never relinquish the CPU (e.g., via a semaphore
-or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
-set to a default value. If CONFIG_PREEMPT is enabled, the default
-is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
-
-It's not a disaster if you set maxactive too low; you'll just miss
-some probes. In the kretprobe struct, the nmissed field is set to
-zero when the return probe is registered, and is incremented every
-time the probed function is entered but there is no kretprobe_instance
-object available for establishing the return probe.
-
-1.3.2 Kretprobe entry-handler
-
-Kretprobes also provides an optional user-specified handler which runs
-on function entry. This handler is specified by setting the entry_handler
-field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
-function entry is hit, the user-defined entry_handler, if any, is invoked.
-If the entry_handler returns 0 (success) then a corresponding return handler
-is guaranteed to be called upon function return. If the entry_handler
-returns a non-zero error then Kprobes leaves the return address as is, and
-the kretprobe has no further effect for that particular function instance.
-
-Multiple entry and return handler invocations are matched using the unique
-kretprobe_instance object associated with them. Additionally, a user
-may also specify per return-instance private data to be part of each
-kretprobe_instance object. This is especially useful when sharing private
-data between corresponding user entry and return handlers. The size of each
-private data object can be specified at kretprobe registration time by
-setting the data_size field of the kretprobe struct. This data can be
-accessed through the data field of each kretprobe_instance object.
-
-In case probed function is entered but there is no kretprobe_instance
-object available, then in addition to incrementing the nmissed count,
-the user entry_handler invocation is also skipped.
-
-1.4 How Does Jump Optimization Work?
-
-If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
-is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
-the "debug.kprobes_optimization" kernel parameter is set to 1 (see
-sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
-instruction instead of a breakpoint instruction at each probepoint.
-
-1.4.1 Init a Kprobe
-
-When a probe is registered, before attempting this optimization,
-Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
-address. So, even if it's not possible to optimize this particular
-probepoint, there'll be a probe there.
-
-1.4.2 Safety Check
-
-Before optimizing a probe, Kprobes performs the following safety checks:
-
-- Kprobes verifies that the region that will be replaced by the jump
-instruction (the "optimized region") lies entirely within one function.
-(A jump instruction is multiple bytes, and so may overlay multiple
-instructions.)
-
-- Kprobes analyzes the entire function and verifies that there is no
-jump into the optimized region. Specifically:
- - the function contains no indirect jump;
- - the function contains no instruction that causes an exception (since
- the fixup code triggered by the exception could jump back into the
- optimized region -- Kprobes checks the exception tables to verify this);
- and
- - there is no near jump to the optimized region (other than to the first
- byte).
-
-- For each instruction in the optimized region, Kprobes verifies that
-the instruction can be executed out of line.
-
-1.4.3 Preparing Detour Buffer
-
-Next, Kprobes prepares a "detour" buffer, which contains the following
-instruction sequence:
-- code to push the CPU's registers (emulating a breakpoint trap)
-- a call to the trampoline code which calls user's probe handlers.
-- code to restore registers
-- the instructions from the optimized region
-- a jump back to the original execution path.
-
-1.4.4 Pre-optimization
-
-After preparing the detour buffer, Kprobes verifies that none of the
-following situations exist:
-- The probe has either a break_handler (i.e., it's a jprobe) or a
-post_handler.
-- Other instructions in the optimized region are probed.
-- The probe is disabled.
-In any of the above cases, Kprobes won't start optimizing the probe.
-Since these are temporary situations, Kprobes tries to start
-optimizing it again if the situation is changed.
-
-If the kprobe can be optimized, Kprobes enqueues the kprobe to an
-optimizing list, and kicks the kprobe-optimizer workqueue to optimize
-it. If the to-be-optimized probepoint is hit before being optimized,
-Kprobes returns control to the original instruction path by setting
-the CPU's instruction pointer to the copied code in the detour buffer
--- thus at least avoiding the single-step.
-
-1.4.5 Optimization
-
-The Kprobe-optimizer doesn't insert the jump instruction immediately;
-rather, it calls synchronize_sched() for safety first, because it's
-possible for a CPU to be interrupted in the middle of executing the
-optimized region(*). As you know, synchronize_sched() can ensure
-that all interruptions that were active when synchronize_sched()
-was called are done, but only if CONFIG_PREEMPT=n. So, this version
-of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
-
-After that, the Kprobe-optimizer calls stop_machine() to replace
-the optimized region with a jump instruction to the detour buffer,
-using text_poke_smp().
-
-1.4.6 Unoptimization
-
-When an optimized kprobe is unregistered, disabled, or blocked by
-another kprobe, it will be unoptimized. If this happens before
-the optimization is complete, the kprobe is just dequeued from the
-optimized list. If the optimization has been done, the jump is
-replaced with the original code (except for an int3 breakpoint in
-the first byte) by using text_poke_smp().
-
-(*)Please imagine that the 2nd instruction is interrupted and then
-the optimizer replaces the 2nd instruction with the jump *address*
-while the interrupt handler is running. When the interrupt
-returns to original address, there is no valid instruction,
-and it causes an unexpected result.
-
-(**)This optimization-safety checking may be replaced with the
-stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
-kernel.
-
-NOTE for geeks:
-The jump optimization changes the kprobe's pre_handler behavior.
-Without optimization, the pre_handler can change the kernel's execution
-path by changing regs->ip and returning 1. However, when the probe
-is optimized, that modification is ignored. Thus, if you want to
-tweak the kernel's execution path, you need to suppress optimization,
-using one of the following techniques:
-- Specify an empty function for the kprobe's post_handler or break_handler.
- or
-- Execute 'sysctl -w debug.kprobes_optimization=n'
-
-2. Architectures Supported
-
-Kprobes, jprobes, and return probes are implemented on the following
-architectures:
-
-- i386 (Supports jump optimization)
-- x86_64 (AMD-64, EM64T) (Supports jump optimization)
-- ppc64
-- ia64 (Does not support probes on instruction slot1.)
-- sparc64 (Return probes not yet implemented.)
-- arm
-- ppc
-- mips
-
-3. Configuring Kprobes
-
-When configuring the kernel using make menuconfig/xconfig/oldconfig,
-ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation
-Support", look for "Kprobes".
-
-So that you can load and unload Kprobes-based instrumentation modules,
-make sure "Loadable module support" (CONFIG_MODULES) and "Module
-unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
-
-Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
-are set to "y", since kallsyms_lookup_name() is used by the in-kernel
-kprobe address resolution code.
-
-If you need to insert a probe in the middle of a function, you may find
-it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
-so you can use "objdump -d -l vmlinux" to see the source-to-object
-code mapping.
-
-4. API Reference
-
-The Kprobes API includes a "register" function and an "unregister"
-function for each type of probe. The API also includes "register_*probes"
-and "unregister_*probes" functions for (un)registering arrays of probes.
-Here are terse, mini-man-page specifications for these functions and
-the associated probe handlers that you'll write. See the files in the
-samples/kprobes/ sub-directory for examples.
-
-4.1 register_kprobe
-
-#include <linux/kprobes.h>
-int register_kprobe(struct kprobe *kp);
-
-Sets a breakpoint at the address kp->addr. When the breakpoint is
-hit, Kprobes calls kp->pre_handler. After the probed instruction
-is single-stepped, Kprobe calls kp->post_handler. If a fault
-occurs during execution of kp->pre_handler or kp->post_handler,
-or during single-stepping of the probed instruction, Kprobes calls
-kp->fault_handler. Any or all handlers can be NULL. If kp->flags
-is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
-so, its handlers aren't hit until calling enable_kprobe(kp).
-
-NOTE:
-1. With the introduction of the "symbol_name" field to struct kprobe,
-the probepoint address resolution will now be taken care of by the kernel.
-The following will now work:
-
- kp.symbol_name = "symbol_name";
-
-(64-bit powerpc intricacies such as function descriptors are handled
-transparently)
-
-2. Use the "offset" field of struct kprobe if the offset into the symbol
-to install a probepoint is known. This field is used to calculate the
-probepoint.
-
-3. Specify either the kprobe "symbol_name" OR the "addr". If both are
-specified, kprobe registration will fail with -EINVAL.
-
-4. With CISC architectures (such as i386 and x86_64), the kprobes code
-does not validate if the kprobe.addr is at an instruction boundary.
-Use "offset" with caution.
-
-register_kprobe() returns 0 on success, or a negative errno otherwise.
-
-User's pre-handler (kp->pre_handler):
-#include <linux/kprobes.h>
-#include <linux/ptrace.h>
-int pre_handler(struct kprobe *p, struct pt_regs *regs);
-
-Called with p pointing to the kprobe associated with the breakpoint,
-and regs pointing to the struct containing the registers saved when
-the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
-
-User's post-handler (kp->post_handler):
-#include <linux/kprobes.h>
-#include <linux/ptrace.h>
-void post_handler(struct kprobe *p, struct pt_regs *regs,
- unsigned long flags);
-
-p and regs are as described for the pre_handler. flags always seems
-to be zero.
-
-User's fault-handler (kp->fault_handler):
-#include <linux/kprobes.h>
-#include <linux/ptrace.h>
-int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
-
-p and regs are as described for the pre_handler. trapnr is the
-architecture-specific trap number associated with the fault (e.g.,
-on i386, 13 for a general protection fault or 14 for a page fault).
-Returns 1 if it successfully handled the exception.
-
-4.2 register_jprobe
-
-#include <linux/kprobes.h>
-int register_jprobe(struct jprobe *jp)
-
-Sets a breakpoint at the address jp->kp.addr, which must be the address
-of the first instruction of a function. When the breakpoint is hit,
-Kprobes runs the handler whose address is jp->entry.
-
-The handler should have the same arg list and return type as the probed
-function; and just before it returns, it must call jprobe_return().
-(The handler never actually returns, since jprobe_return() returns
-control to Kprobes.) If the probed function is declared asmlinkage
-or anything else that affects how args are passed, the handler's
-declaration must match.
-
-register_jprobe() returns 0 on success, or a negative errno otherwise.
-
-4.3 register_kretprobe
-
-#include <linux/kprobes.h>
-int register_kretprobe(struct kretprobe *rp);
-
-Establishes a return probe for the function whose address is
-rp->kp.addr. When that function returns, Kprobes calls rp->handler.
-You must set rp->maxactive appropriately before you call
-register_kretprobe(); see "How Does a Return Probe Work?" for details.
-
-register_kretprobe() returns 0 on success, or a negative errno
-otherwise.
-
-User's return-probe handler (rp->handler):
-#include <linux/kprobes.h>
-#include <linux/ptrace.h>
-int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
-
-regs is as described for kprobe.pre_handler. ri points to the
-kretprobe_instance object, of which the following fields may be
-of interest:
-- ret_addr: the return address
-- rp: points to the corresponding kretprobe object
-- task: points to the corresponding task struct
-- data: points to per return-instance private data; see "Kretprobe
- entry-handler" for details.
-
-The regs_return_value(regs) macro provides a simple abstraction to
-extract the return value from the appropriate register as defined by
-the architecture's ABI.
-
-The handler's return value is currently ignored.
-
-4.4 unregister_*probe
-
-#include <linux/kprobes.h>
-void unregister_kprobe(struct kprobe *kp);
-void unregister_jprobe(struct jprobe *jp);
-void unregister_kretprobe(struct kretprobe *rp);
-
-Removes the specified probe. The unregister function can be called
-at any time after the probe has been registered.
-
-NOTE:
-If the functions find an incorrect probe (ex. an unregistered probe),
-they clear the addr field of the probe.
-
-4.5 register_*probes
-
-#include <linux/kprobes.h>
-int register_kprobes(struct kprobe **kps, int num);
-int register_kretprobes(struct kretprobe **rps, int num);
-int register_jprobes(struct jprobe **jps, int num);
-
-Registers each of the num probes in the specified array. If any
-error occurs during registration, all probes in the array, up to
-the bad probe, are safely unregistered before the register_*probes
-function returns.
-- kps/rps/jps: an array of pointers to *probe data structures
-- num: the number of the array entries.
-
-NOTE:
-You have to allocate(or define) an array of pointers and set all
-of the array entries before using these functions.
-
-4.6 unregister_*probes
-
-#include <linux/kprobes.h>
-void unregister_kprobes(struct kprobe **kps, int num);
-void unregister_kretprobes(struct kretprobe **rps, int num);
-void unregister_jprobes(struct jprobe **jps, int num);
-
-Removes each of the num probes in the specified array at once.
-
-NOTE:
-If the functions find some incorrect probes (ex. unregistered
-probes) in the specified array, they clear the addr field of those
-incorrect probes. However, other probes in the array are
-unregistered correctly.
-
-4.7 disable_*probe
-
-#include <linux/kprobes.h>
-int disable_kprobe(struct kprobe *kp);
-int disable_kretprobe(struct kretprobe *rp);
-int disable_jprobe(struct jprobe *jp);
-
-Temporarily disables the specified *probe. You can enable it again by using
-enable_*probe(). You must specify the probe which has been registered.
-
-4.8 enable_*probe
-
-#include <linux/kprobes.h>
-int enable_kprobe(struct kprobe *kp);
-int enable_kretprobe(struct kretprobe *rp);
-int enable_jprobe(struct jprobe *jp);
-
-Enables *probe which has been disabled by disable_*probe(). You must specify
-the probe which has been registered.
-
-5. Kprobes Features and Limitations
-
-Kprobes allows multiple probes at the same address. Currently,
-however, there cannot be multiple jprobes on the same function at
-the same time. Also, a probepoint for which there is a jprobe or
-a post_handler cannot be optimized. So if you install a jprobe,
-or a kprobe with a post_handler, at an optimized probepoint, the
-probepoint will be unoptimized automatically.
-
-In general, you can install a probe anywhere in the kernel.
-In particular, you can probe interrupt handlers. Known exceptions
-are discussed in this section.
-
-The register_*probe functions will return -EINVAL if you attempt
-to install a probe in the code that implements Kprobes (mostly
-kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
-as do_page_fault and notifier_call_chain).
-
-If you install a probe in an inline-able function, Kprobes makes
-no attempt to chase down all inline instances of the function and
-install probes there. gcc may inline a function without being asked,
-so keep this in mind if you're not seeing the probe hits you expect.
-
-A probe handler can modify the environment of the probed function
--- e.g., by modifying kernel data structures, or by modifying the
-contents of the pt_regs struct (which are restored to the registers
-upon return from the breakpoint). So Kprobes can be used, for example,
-to install a bug fix or to inject faults for testing. Kprobes, of
-course, has no way to distinguish the deliberately injected faults
-from the accidental ones. Don't drink and probe.
-
-Kprobes makes no attempt to prevent probe handlers from stepping on
-each other -- e.g., probing printk() and then calling printk() from a
-probe handler. If a probe handler hits a probe, that second probe's
-handlers won't be run in that instance, and the kprobe.nmissed member
-of the second probe will be incremented.
-
-As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
-the same handler) may run concurrently on different CPUs.
-
-Kprobes does not use mutexes or allocate memory except during
-registration and unregistration.
-
-Probe handlers are run with preemption disabled. Depending on the
-architecture and optimization state, handlers may also run with
-interrupts disabled (e.g., kretprobe handlers and optimized kprobe
-handlers run without interrupt disabled on x86/x86-64). In any case,
-your handler should not yield the CPU (e.g., by attempting to acquire
-a semaphore).
-
-Since a return probe is implemented by replacing the return
-address with the trampoline's address, stack backtraces and calls
-to __builtin_return_address() will typically yield the trampoline's
-address instead of the real return address for kretprobed functions.
-(As far as we can tell, __builtin_return_address() is used only
-for instrumentation and error reporting.)
-
-If the number of times a function is called does not match the number
-of times it returns, registering a return probe on that function may
-produce undesirable results. In such a case, a line:
-kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
-gets printed. With this information, one will be able to correlate the
-exact instance of the kretprobe that caused the problem. We have the
-do_exit() case covered. do_execve() and do_fork() are not an issue.
-We're unaware of other specific cases where this could be a problem.
-
-If, upon entry to or exit from a function, the CPU is running on
-a stack other than that of the current task, registering a return
-probe on that function may produce undesirable results. For this
-reason, Kprobes doesn't support return probes (or kprobes or jprobes)
-on the x86_64 version of __switch_to(); the registration functions
-return -EINVAL.
-
-On x86/x86-64, since the Jump Optimization of Kprobes modifies
-instructions widely, there are some limitations to optimization. To
-explain it, we introduce some terminology. Imagine a 3-instruction
-sequence consisting of a two 2-byte instructions and one 3-byte
-instruction.
-
- IA
- |
-[-2][-1][0][1][2][3][4][5][6][7]
- [ins1][ins2][ ins3 ]
- [<- DCR ->]
- [<- JTPR ->]
-
-ins1: 1st Instruction
-ins2: 2nd Instruction
-ins3: 3rd Instruction
-IA: Insertion Address
-JTPR: Jump Target Prohibition Region
-DCR: Detoured Code Region
-
-The instructions in DCR are copied to the out-of-line buffer
-of the kprobe, because the bytes in DCR are replaced by
-a 5-byte jump instruction. So there are several limitations.
-
-a) The instructions in DCR must be relocatable.
-b) The instructions in DCR must not include a call instruction.
-c) JTPR must not be targeted by any jump or call instruction.
-d) DCR must not straddle the border between functions.
-
-Anyway, these limitations are checked by the in-kernel instruction
-decoder, so you don't need to worry about that.
-
-6. Probe Overhead
-
-On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
-microseconds to process. Specifically, a benchmark that hits the same
-probepoint repeatedly, firing a simple handler each time, reports 1-2
-million hits per second, depending on the architecture. A jprobe or
-return-probe hit typically takes 50-75% longer than a kprobe hit.
-When you have a return probe set on a function, adding a kprobe at
-the entry to that function adds essentially no overhead.
-
-Here are sample overhead figures (in usec) for different architectures.
-k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
-on same function; jr = jprobe + return probe on same function
-
-i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
-k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
-
-x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
-k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
-
-ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
-k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
-
-6.1 Optimized Probe Overhead
-
-Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
-process. Here are sample overhead figures (in usec) for x86 architectures.
-k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
-r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
-
-i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
-k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
-
-x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
-k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
-
-7. TODO
-
-a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
-programming interface for probe-based instrumentation. Try it out.
-b. Kernel return probes for sparc64.
-c. Support for other architectures.
-d. User-space probes.
-e. Watchpoint probes (which fire on data references).
-
-8. Kprobes Example
-
-See samples/kprobes/kprobe_example.c
-
-9. Jprobes Example
-
-See samples/kprobes/jprobe_example.c
-
-10. Kretprobes Example
-
-See samples/kprobes/kretprobe_example.c
-
-For additional information on Kprobes, refer to the following URLs:
-http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
-http://www.redhat.com/magazine/005mar05/features/kprobes/
-http://www-users.cs.umn.edu/~boutcher/kprobes/
-http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
-
-
-Appendix A: The kprobes debugfs interface
-
-With recent kernels (> 2.6.20) the list of registered kprobes is visible
-under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
-
-/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
-
-c015d71a k vfs_read+0x0
-c011a316 j do_fork+0x0
-c03dedc5 r tcp_v4_rcv+0x0
-
-The first column provides the kernel address where the probe is inserted.
-The second column identifies the type of probe (k - kprobe, r - kretprobe
-and j - jprobe), while the third column specifies the symbol+offset of
-the probe. If the probed function belongs to a module, the module name
-is also specified. Following columns show probe status. If the probe is on
-a virtual address that is no longer valid (module init sections, module
-virtual addresses that correspond to modules that've been unloaded),
-such probes are marked with [GONE]. If the probe is temporarily disabled,
-such probes are marked with [DISABLED]. If the probe is optimized, it is
-marked with [OPTIMIZED].
-
-/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
-
-Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
-By default, all kprobes are enabled. By echoing "0" to this file, all
-registered probes will be disarmed, till such time a "1" is echoed to this
-file. Note that this knob just disarms and arms all kprobes and doesn't
-change each probe's disabling state. This means that disabled kprobes (marked
-[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
-
-
-Appendix B: The kprobes sysctl interface
-
-/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
-
-When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
-a knob to globally and forcibly turn jump optimization (see section
-1.4) ON or OFF. By default, jump optimization is allowed (ON).
-If you echo "0" to this file or set "debug.kprobes_optimization" to
-0 via sysctl, all optimized probes will be unoptimized, and any new
-probes registered after that will not be optimized. Note that this
-knob *changes* the optimized state. This means that optimized probes
-(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
-removed). If the knob is turned on, they will be optimized again.
-