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-<?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="kgdbOnLinux">
- <bookinfo>
- <title>Using kgdb, kdb and the kernel debugger internals</title>
-
- <authorgroup>
- <author>
- <firstname>Jason</firstname>
- <surname>Wessel</surname>
- <affiliation>
- <address>
- <email>jason.wessel@windriver.com</email>
- </address>
- </affiliation>
- </author>
- </authorgroup>
- <copyright>
- <year>2008,2010</year>
- <holder>Wind River Systems, Inc.</holder>
- </copyright>
- <copyright>
- <year>2004-2005</year>
- <holder>MontaVista Software, Inc.</holder>
- </copyright>
- <copyright>
- <year>2004</year>
- <holder>Amit S. Kale</holder>
- </copyright>
-
- <legalnotice>
- <para>
- This file is licensed under the terms of the GNU General Public License
- version 2. This program is licensed "as is" without any warranty of any
- kind, whether express or implied.
- </para>
-
- </legalnotice>
- </bookinfo>
-
-<toc></toc>
- <chapter id="Introduction">
- <title>Introduction</title>
- <para>
- The kernel has two different debugger front ends (kdb and kgdb)
- which interface to the debug core. It is possible to use either
- of the debugger front ends and dynamically transition between them
- if you configure the kernel properly at compile and runtime.
- </para>
- <para>
- Kdb is simplistic shell-style interface which you can use on a
- system console with a keyboard or serial console. You can use it
- to inspect memory, registers, process lists, dmesg, and even set
- breakpoints to stop in a certain location. Kdb is not a source
- level debugger, although you can set breakpoints and execute some
- basic kernel run control. Kdb is mainly aimed at doing some
- analysis to aid in development or diagnosing kernel problems. You
- can access some symbols by name in kernel built-ins or in kernel
- modules if the code was built
- with <symbol>CONFIG_KALLSYMS</symbol>.
- </para>
- <para>
- Kgdb is intended to be used as a source level debugger for the
- Linux kernel. It is used along with gdb to debug a Linux kernel.
- The expectation is that gdb can be used to "break in" to the
- kernel to inspect memory, variables and look through call stack
- information similar to the way an application developer would use
- gdb to debug an application. It is possible to place breakpoints
- in kernel code and perform some limited execution stepping.
- </para>
- <para>
- Two machines are required for using kgdb. One of these machines is
- a development machine and the other is the target machine. The
- kernel to be debugged runs on the target machine. The development
- machine runs an instance of gdb against the vmlinux file which
- contains the symbols (not boot image such as bzImage, zImage,
- uImage...). In gdb the developer specifies the connection
- parameters and connects to kgdb. The type of connection a
- developer makes with gdb depends on the availability of kgdb I/O
- modules compiled as built-ins or loadable kernel modules in the test
- machine's kernel.
- </para>
- </chapter>
- <chapter id="CompilingAKernel">
- <title>Compiling a kernel</title>
- <para>
- <itemizedlist>
- <listitem><para>In order to enable compilation of kdb, you must first enable kgdb.</para></listitem>
- <listitem><para>The kgdb test compile options are described in the kgdb test suite chapter.</para></listitem>
- </itemizedlist>
- </para>
- <sect1 id="CompileKGDB">
- <title>Kernel config options for kgdb</title>
- <para>
- To enable <symbol>CONFIG_KGDB</symbol> you should first turn on
- "Prompt for development and/or incomplete code/drivers"
- (CONFIG_EXPERIMENTAL) in "General setup", then under the
- "Kernel debugging" select "KGDB: kernel debugger".
- </para>
- <para>
- While it is not a hard requirement that you have symbols in your
- vmlinux file, gdb tends not to be very useful without the symbolic
- data, so you will want to turn
- on <symbol>CONFIG_DEBUG_INFO</symbol> which is called "Compile the
- kernel with debug info" in the config menu.
- </para>
- <para>
- It is advised, but not required that you turn on the
- <symbol>CONFIG_FRAME_POINTER</symbol> kernel option which is called "Compile the
- kernel with frame pointers" in the config menu. This option
- inserts code to into the compiled executable which saves the frame
- information in registers or on the stack at different points which
- allows a debugger such as gdb to more accurately construct
- stack back traces while debugging the kernel.
- </para>
- <para>
- If the architecture that you are using supports the kernel option
- CONFIG_DEBUG_RODATA, you should consider turning it off. This
- option will prevent the use of software breakpoints because it
- marks certain regions of the kernel's memory space as read-only.
- If kgdb supports it for the architecture you are using, you can
- use hardware breakpoints if you desire to run with the
- CONFIG_DEBUG_RODATA option turned on, else you need to turn off
- this option.
- </para>
- <para>
- Next you should choose one of more I/O drivers to interconnect
- debugging host and debugged target. Early boot debugging requires
- a KGDB I/O driver that supports early debugging and the driver
- must be built into the kernel directly. Kgdb I/O driver
- configuration takes place via kernel or module parameters which
- you can learn more about in the in the section that describes the
- parameter "kgdboc".
- </para>
- <para>Here is an example set of .config symbols to enable or
- disable for kgdb:
- <itemizedlist>
- <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
- <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
- <listitem><para>CONFIG_KGDB=y</para></listitem>
- <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
- </itemizedlist>
- </para>
- </sect1>
- <sect1 id="CompileKDB">
- <title>Kernel config options for kdb</title>
- <para>Kdb is quite a bit more complex than the simple gdbstub
- sitting on top of the kernel's debug core. Kdb must implement a
- shell, and also adds some helper functions in other parts of the
- kernel, responsible for printing out interesting data such as what
- you would see if you ran "lsmod", or "ps". In order to build kdb
- into the kernel you follow the same steps as you would for kgdb.
- </para>
- <para>The main config option for kdb
- is <symbol>CONFIG_KGDB_KDB</symbol> which is called "KGDB_KDB:
- include kdb frontend for kgdb" in the config menu. In theory you
- would have already also selected an I/O driver such as the
- CONFIG_KGDB_SERIAL_CONSOLE interface if you plan on using kdb on a
- serial port, when you were configuring kgdb.
- </para>
- <para>If you want to use a PS/2-style keyboard with kdb, you would
- select CONFIG_KDB_KEYBOARD which is called "KGDB_KDB: keyboard as
- input device" in the config menu. The CONFIG_KDB_KEYBOARD option
- is not used for anything in the gdb interface to kgdb. The
- CONFIG_KDB_KEYBOARD option only works with kdb.
- </para>
- <para>Here is an example set of .config symbols to enable/disable kdb:
- <itemizedlist>
- <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
- <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
- <listitem><para>CONFIG_KGDB=y</para></listitem>
- <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
- <listitem><para>CONFIG_KGDB_KDB=y</para></listitem>
- <listitem><para>CONFIG_KDB_KEYBOARD=y</para></listitem>
- </itemizedlist>
- </para>
- </sect1>
- </chapter>
- <chapter id="kgdbKernelArgs">
- <title>Kernel Debugger Boot Arguments</title>
- <para>This section describes the various runtime kernel
- parameters that affect the configuration of the kernel debugger.
- The following chapter covers using kdb and kgdb as well as
- provides some examples of the configuration parameters.</para>
- <sect1 id="kgdboc">
- <title>Kernel parameter: kgdboc</title>
- <para>The kgdboc driver was originally an abbreviation meant to
- stand for "kgdb over console". Today it is the primary mechanism
- to configure how to communicate from gdb to kgdb as well as the
- devices you want to use to interact with the kdb shell.
- </para>
- <para>For kgdb/gdb, kgdboc is designed to work with a single serial
- port. It is intended to cover the circumstance where you want to
- use a serial console as your primary console as well as using it to
- perform kernel debugging. It is also possible to use kgdb on a
- serial port which is not designated as a system console. Kgdboc
- may be configured as a kernel built-in or a kernel loadable module.
- You can only make use of <constant>kgdbwait</constant> and early
- debugging if you build kgdboc into the kernel as a built-in.
- <para>Optionally you can elect to activate kms (Kernel Mode
- Setting) integration. When you use kms with kgdboc and you have a
- video driver that has atomic mode setting hooks, it is possible to
- enter the debugger on the graphics console. When the kernel
- execution is resumed, the previous graphics mode will be restored.
- This integration can serve as a useful tool to aid in diagnosing
- crashes or doing analysis of memory with kdb while allowing the
- full graphics console applications to run.
- </para>
- </para>
- <sect2 id="kgdbocArgs">
- <title>kgdboc arguments</title>
- <para>Usage: <constant>kgdboc=[kms][[,]kbd][[,]serial_device][,baud]</constant></para>
- <para>The order listed above must be observed if you use any of the
- optional configurations together.
- </para>
- <para>Abbreviations:
- <itemizedlist>
- <listitem><para>kms = Kernel Mode Setting</para></listitem>
- <listitem><para>kbd = Keyboard</para></listitem>
- </itemizedlist>
- </para>
- <para>You can configure kgdboc to use the keyboard, and or a serial
- device depending on if you are using kdb and or kgdb, in one of the
- following scenarios. The order listed above must be observed if
- you use any of the optional configurations together. Using kms +
- only gdb is generally not a useful combination.</para>
- <sect3 id="kgdbocArgs1">
- <title>Using loadable module or built-in</title>
- <para>
- <orderedlist>
- <listitem><para>As a kernel built-in:</para>
- <para>Use the kernel boot argument: <constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para></listitem>
- <listitem>
- <para>As a kernel loadable module:</para>
- <para>Use the command: <constant>modprobe kgdboc kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
- <para>Here are two examples of how you might format the kgdboc
- string. The first is for an x86 target using the first serial port.
- The second example is for the ARM Versatile AB using the second
- serial port.
- <orderedlist>
- <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
- <listitem><para><constant>kgdboc=ttyAMA1,115200</constant></para></listitem>
- </orderedlist>
- </para>
- </listitem>
- </orderedlist></para>
- </sect3>
- <sect3 id="kgdbocArgs2">
- <title>Configure kgdboc at runtime with sysfs</title>
- <para>At run time you can enable or disable kgdboc by echoing a
- parameters into the sysfs. Here are two examples:</para>
- <orderedlist>
- <listitem><para>Enable kgdboc on ttyS0</para>
- <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
- <listitem><para>Disable kgdboc</para>
- <para><constant>echo "" &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
- </orderedlist>
- <para>NOTE: You do not need to specify the baud if you are
- configuring the console on tty which is already configured or
- open.</para>
- </sect3>
- <sect3 id="kgdbocArgs3">
- <title>More examples</title>
- <para>You can configure kgdboc to use the keyboard, and or a serial
- device depending on if you are using kdb and or kgdb, in one of the
- following scenarios.</para>
- <para>You can configure kgdboc to use the keyboard, and or a serial device
- depending on if you are using kdb and or kgdb, in one of the
- following scenarios.
- <orderedlist>
- <listitem><para>kdb and kgdb over only a serial port</para>
- <para><constant>kgdboc=&lt;serial_device&gt;[,baud]</constant></para>
- <para>Example: <constant>kgdboc=ttyS0,115200</constant></para>
- </listitem>
- <listitem><para>kdb and kgdb with keyboard and a serial port</para>
- <para><constant>kgdboc=kbd,&lt;serial_device&gt;[,baud]</constant></para>
- <para>Example: <constant>kgdboc=kbd,ttyS0,115200</constant></para>
- </listitem>
- <listitem><para>kdb with a keyboard</para>
- <para><constant>kgdboc=kbd</constant></para>
- </listitem>
- <listitem><para>kdb with kernel mode setting</para>
- <para><constant>kgdboc=kms,kbd</constant></para>
- </listitem>
- <listitem><para>kdb with kernel mode setting and kgdb over a serial port</para>
- <para><constant>kgdboc=kms,kbd,ttyS0,115200</constant></para>
- </listitem>
- </orderedlist>
- </para>
- </sect3>
- <para>NOTE: Kgdboc does not support interrupting the target via the
- gdb remote protocol. You must manually send a sysrq-g unless you
- have a proxy that splits console output to a terminal program.
- A console proxy has a separate TCP port for the debugger and a separate
- TCP port for the "human" console. The proxy can take care of sending
- the sysrq-g for you.
- </para>
- <para>When using kgdboc with no debugger proxy, you can end up
- connecting the debugger at one of two entry points. If an
- exception occurs after you have loaded kgdboc, a message should
- print on the console stating it is waiting for the debugger. In
- this case you disconnect your terminal program and then connect the
- debugger in its place. If you want to interrupt the target system
- and forcibly enter a debug session you have to issue a Sysrq
- sequence and then type the letter <constant>g</constant>. Then
- you disconnect the terminal session and connect gdb. Your options
- if you don't like this are to hack gdb to send the sysrq-g for you
- as well as on the initial connect, or to use a debugger proxy that
- allows an unmodified gdb to do the debugging.
- </para>
- </sect2>
- </sect1>
- <sect1 id="kgdbwait">
- <title>Kernel parameter: kgdbwait</title>
- <para>
- The Kernel command line option <constant>kgdbwait</constant> makes
- kgdb wait for a debugger connection during booting of a kernel. You
- can only use this option you compiled a kgdb I/O driver into the
- kernel and you specified the I/O driver configuration as a kernel
- command line option. The kgdbwait parameter should always follow the
- configuration parameter for the kgdb I/O driver in the kernel
- command line else the I/O driver will not be configured prior to
- asking the kernel to use it to wait.
- </para>
- <para>
- The kernel will stop and wait as early as the I/O driver and
- architecture allows when you use this option. If you build the
- kgdb I/O driver as a loadable kernel module kgdbwait will not do
- anything.
- </para>
- </sect1>
- <sect1 id="kgdbcon">
- <title>Kernel parameter: kgdbcon</title>
- <para> The kgdbcon feature allows you to see printk() messages
- inside gdb while gdb is connected to the kernel. Kdb does not make
- use of the kgdbcon feature.
- </para>
- <para>Kgdb supports using the gdb serial protocol to send console
- messages to the debugger when the debugger is connected and running.
- There are two ways to activate this feature.
- <orderedlist>
- <listitem><para>Activate with the kernel command line option:</para>
- <para><constant>kgdbcon</constant></para>
- </listitem>
- <listitem><para>Use sysfs before configuring an I/O driver</para>
- <para>
- <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
- </para>
- <para>
- NOTE: If you do this after you configure the kgdb I/O driver, the
- setting will not take effect until the next point the I/O is
- reconfigured.
- </para>
- </listitem>
- </orderedlist>
- <para>IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an
- active system console. An example incorrect usage is <constant>console=ttyS0,115200 kgdboc=ttyS0 kgdbcon</constant>
- </para>
- <para>It is possible to use this option with kgdboc on a tty that is not a system console.
- </para>
- </para>
- </sect1>
- <sect1 id="kgdbreboot">
- <title>Run time parameter: kgdbreboot</title>
- <para> The kgdbreboot feature allows you to change how the debugger
- deals with the reboot notification. You have 3 choices for the
- behavior. The default behavior is always set to 0.</para>
- <orderedlist>
- <listitem><para>echo -1 > /sys/module/debug_core/parameters/kgdbreboot</para>
- <para>Ignore the reboot notification entirely.</para>
- </listitem>
- <listitem><para>echo 0 > /sys/module/debug_core/parameters/kgdbreboot</para>
- <para>Send the detach message to any attached debugger client.</para>
- </listitem>
- <listitem><para>echo 1 > /sys/module/debug_core/parameters/kgdbreboot</para>
- <para>Enter the debugger on reboot notify.</para>
- </listitem>
- </orderedlist>
- </sect1>
- </chapter>
- <chapter id="usingKDB">
- <title>Using kdb</title>
- <para>
- </para>
- <sect1 id="quickKDBserial">
- <title>Quick start for kdb on a serial port</title>
- <para>This is a quick example of how to use kdb.</para>
- <para><orderedlist>
- <listitem><para>Boot kernel with arguments:
- <itemizedlist>
- <listitem><para><constant>console=ttyS0,115200 kgdboc=ttyS0,115200</constant></para></listitem>
- </itemizedlist></para>
- <para>OR</para>
- <para>Configure kgdboc after the kernel booted; assuming you are using a serial port console:
- <itemizedlist>
- <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
- </itemizedlist>
- </para>
- </listitem>
- <listitem><para>Enter the kernel debugger manually or by waiting for an oops or fault. There are several ways you can enter the kernel debugger manually; all involve using the sysrq-g, which means you must have enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.</para>
- <itemizedlist>
- <listitem><para>When logged in as root or with a super user session you can run:</para>
- <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
- <listitem><para>Example using minicom 2.2</para>
- <para>Press: <constant>Control-a</constant></para>
- <para>Press: <constant>f</constant></para>
- <para>Press: <constant>g</constant></para>
- </listitem>
- <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
- <para>Press: <constant>Control-]</constant></para>
- <para>Type in:<constant>send break</constant></para>
- <para>Press: <constant>Enter</constant></para>
- <para>Press: <constant>g</constant></para>
- </listitem>
- </itemizedlist>
- </listitem>
- <listitem><para>From the kdb prompt you can run the "help" command to see a complete list of the commands that are available.</para>
- <para>Some useful commands in kdb include:
- <itemizedlist>
- <listitem><para>lsmod -- Shows where kernel modules are loaded</para></listitem>
- <listitem><para>ps -- Displays only the active processes</para></listitem>
- <listitem><para>ps A -- Shows all the processes</para></listitem>
- <listitem><para>summary -- Shows kernel version info and memory usage</para></listitem>
- <listitem><para>bt -- Get a backtrace of the current process using dump_stack()</para></listitem>
- <listitem><para>dmesg -- View the kernel syslog buffer</para></listitem>
- <listitem><para>go -- Continue the system</para></listitem>
- </itemizedlist>
- </para>
- </listitem>
- <listitem>
- <para>When you are done using kdb you need to consider rebooting the
- system or using the "go" command to resuming normal kernel
- execution. If you have paused the kernel for a lengthy period of
- time, applications that rely on timely networking or anything to do
- with real wall clock time could be adversely affected, so you
- should take this into consideration when using the kernel
- debugger.</para>
- </listitem>
- </orderedlist></para>
- </sect1>
- <sect1 id="quickKDBkeyboard">
- <title>Quick start for kdb using a keyboard connected console</title>
- <para>This is a quick example of how to use kdb with a keyboard.</para>
- <para><orderedlist>
- <listitem><para>Boot kernel with arguments:
- <itemizedlist>
- <listitem><para><constant>kgdboc=kbd</constant></para></listitem>
- </itemizedlist></para>
- <para>OR</para>
- <para>Configure kgdboc after the kernel booted:
- <itemizedlist>
- <listitem><para><constant>echo kbd &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
- </itemizedlist>
- </para>
- </listitem>
- <listitem><para>Enter the kernel debugger manually or by waiting for an oops or fault. There are several ways you can enter the kernel debugger manually; all involve using the sysrq-g, which means you must have enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.</para>
- <itemizedlist>
- <listitem><para>When logged in as root or with a super user session you can run:</para>
- <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
- <listitem><para>Example using a laptop keyboard</para>
- <para>Press and hold down: <constant>Alt</constant></para>
- <para>Press and hold down: <constant>Fn</constant></para>
- <para>Press and release the key with the label: <constant>SysRq</constant></para>
- <para>Release: <constant>Fn</constant></para>
- <para>Press and release: <constant>g</constant></para>
- <para>Release: <constant>Alt</constant></para>
- </listitem>
- <listitem><para>Example using a PS/2 101-key keyboard</para>
- <para>Press and hold down: <constant>Alt</constant></para>
- <para>Press and release the key with the label: <constant>SysRq</constant></para>
- <para>Press and release: <constant>g</constant></para>
- <para>Release: <constant>Alt</constant></para>
- </listitem>
- </itemizedlist>
- </listitem>
- <listitem>
- <para>Now type in a kdb command such as "help", "dmesg", "bt" or "go" to continue kernel execution.</para>
- </listitem>
- </orderedlist></para>
- </sect1>
- </chapter>
- <chapter id="EnableKGDB">
- <title>Using kgdb / gdb</title>
- <para>In order to use kgdb you must activate it by passing
- configuration information to one of the kgdb I/O drivers. If you
- do not pass any configuration information kgdb will not do anything
- at all. Kgdb will only actively hook up to the kernel trap hooks
- if a kgdb I/O driver is loaded and configured. If you unconfigure
- a kgdb I/O driver, kgdb will unregister all the kernel hook points.
- </para>
- <para> All kgdb I/O drivers can be reconfigured at run time, if
- <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
- are enabled, by echo'ing a new config string to
- <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
- The driver can be unconfigured by passing an empty string. You cannot
- change the configuration while the debugger is attached. Make sure
- to detach the debugger with the <constant>detach</constant> command
- prior to trying to unconfigure a kgdb I/O driver.
- </para>
- <sect1 id="ConnectingGDB">
- <title>Connecting with gdb to a serial port</title>
- <orderedlist>
- <listitem><para>Configure kgdboc</para>
- <para>Boot kernel with arguments:
- <itemizedlist>
- <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
- </itemizedlist></para>
- <para>OR</para>
- <para>Configure kgdboc after the kernel booted:
- <itemizedlist>
- <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
- </itemizedlist></para>
- </listitem>
- <listitem>
- <para>Stop kernel execution (break into the debugger)</para>
- <para>In order to connect to gdb via kgdboc, the kernel must
- first be stopped. There are several ways to stop the kernel which
- include using kgdbwait as a boot argument, via a sysrq-g, or running
- the kernel until it takes an exception where it waits for the
- debugger to attach.
- <itemizedlist>
- <listitem><para>When logged in as root or with a super user session you can run:</para>
- <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
- <listitem><para>Example using minicom 2.2</para>
- <para>Press: <constant>Control-a</constant></para>
- <para>Press: <constant>f</constant></para>
- <para>Press: <constant>g</constant></para>
- </listitem>
- <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
- <para>Press: <constant>Control-]</constant></para>
- <para>Type in:<constant>send break</constant></para>
- <para>Press: <constant>Enter</constant></para>
- <para>Press: <constant>g</constant></para>
- </listitem>
- </itemizedlist>
- </para>
- </listitem>
- <listitem>
- <para>Connect from from gdb</para>
- <para>
- Example (using a directly connected port):
- </para>
- <programlisting>
- % gdb ./vmlinux
- (gdb) set remotebaud 115200
- (gdb) target remote /dev/ttyS0
- </programlisting>
- <para>
- Example (kgdb to a terminal server on TCP port 2012):
- </para>
- <programlisting>
- % gdb ./vmlinux
- (gdb) target remote 192.168.2.2:2012
- </programlisting>
- <para>
- Once connected, you can debug a kernel the way you would debug an
- application program.
- </para>
- <para>
- If you are having problems connecting or something is going
- seriously wrong while debugging, it will most often be the case
- that you want to enable gdb to be verbose about its target
- communications. You do this prior to issuing the <constant>target
- remote</constant> command by typing in: <constant>set debug remote 1</constant>
- </para>
- </listitem>
- </orderedlist>
- <para>Remember if you continue in gdb, and need to "break in" again,
- you need to issue an other sysrq-g. It is easy to create a simple
- entry point by putting a breakpoint at <constant>sys_sync</constant>
- and then you can run "sync" from a shell or script to break into the
- debugger.</para>
- </sect1>
- </chapter>
- <chapter id="switchKdbKgdb">
- <title>kgdb and kdb interoperability</title>
- <para>It is possible to transition between kdb and kgdb dynamically.
- The debug core will remember which you used the last time and
- automatically start in the same mode.</para>
- <sect1>
- <title>Switching between kdb and kgdb</title>
- <sect2>
- <title>Switching from kgdb to kdb</title>
- <para>
- There are two ways to switch from kgdb to kdb: you can use gdb to
- issue a maintenance packet, or you can blindly type the command $3#33.
- Whenever kernel debugger stops in kgdb mode it will print the
- message <constant>KGDB or $3#33 for KDB</constant>. It is important
- to note that you have to type the sequence correctly in one pass.
- You cannot type a backspace or delete because kgdb will interpret
- that as part of the debug stream.
- <orderedlist>
- <listitem><para>Change from kgdb to kdb by blindly typing:</para>
- <para><constant>$3#33</constant></para></listitem>
- <listitem><para>Change from kgdb to kdb with gdb</para>
- <para><constant>maintenance packet 3</constant></para>
- <para>NOTE: Now you must kill gdb. Typically you press control-z and
- issue the command: kill -9 %</para></listitem>
- </orderedlist>
- </para>
- </sect2>
- <sect2>
- <title>Change from kdb to kgdb</title>
- <para>There are two ways you can change from kdb to kgdb. You can
- manually enter kgdb mode by issuing the kgdb command from the kdb
- shell prompt, or you can connect gdb while the kdb shell prompt is
- active. The kdb shell looks for the typical first commands that gdb
- would issue with the gdb remote protocol and if it sees one of those
- commands it automatically changes into kgdb mode.</para>
- <orderedlist>
- <listitem><para>From kdb issue the command:</para>
- <para><constant>kgdb</constant></para>
- <para>Now disconnect your terminal program and connect gdb in its place</para></listitem>
- <listitem><para>At the kdb prompt, disconnect the terminal program and connect gdb in its place.</para></listitem>
- </orderedlist>
- </sect2>
- </sect1>
- <sect1>
- <title>Running kdb commands from gdb</title>
- <para>It is possible to run a limited set of kdb commands from gdb,
- using the gdb monitor command. You don't want to execute any of the
- run control or breakpoint operations, because it can disrupt the
- state of the kernel debugger. You should be using gdb for
- breakpoints and run control operations if you have gdb connected.
- The more useful commands to run are things like lsmod, dmesg, ps or
- possibly some of the memory information commands. To see all the kdb
- commands you can run <constant>monitor help</constant>.</para>
- <para>Example:
- <informalexample><programlisting>
-(gdb) monitor ps
-1 idle process (state I) and
-27 sleeping system daemon (state M) processes suppressed,
-use 'ps A' to see all.
-Task Addr Pid Parent [*] cpu State Thread Command
-
-0xc78291d0 1 0 0 0 S 0xc7829404 init
-0xc7954150 942 1 0 0 S 0xc7954384 dropbear
-0xc78789c0 944 1 0 0 S 0xc7878bf4 sh
-(gdb)
- </programlisting></informalexample>
- </para>
- </sect1>
- </chapter>
- <chapter id="KGDBTestSuite">
- <title>kgdb Test Suite</title>
- <para>
- When kgdb is enabled in the kernel config you can also elect to
- enable the config parameter KGDB_TESTS. Turning this on will
- enable a special kgdb I/O module which is designed to test the
- kgdb internal functions.
- </para>
- <para>
- The kgdb tests are mainly intended for developers to test the kgdb
- internals as well as a tool for developing a new kgdb architecture
- specific implementation. These tests are not really for end users
- of the Linux kernel. The primary source of documentation would be
- to look in the drivers/misc/kgdbts.c file.
- </para>
- <para>
- The kgdb test suite can also be configured at compile time to run
- the core set of tests by setting the kernel config parameter
- KGDB_TESTS_ON_BOOT. This particular option is aimed at automated
- regression testing and does not require modifying the kernel boot
- config arguments. If this is turned on, the kgdb test suite can
- be disabled by specifying "kgdbts=" as a kernel boot argument.
- </para>
- </chapter>
- <chapter id="CommonBackEndReq">
- <title>Kernel Debugger Internals</title>
- <sect1 id="kgdbArchitecture">
- <title>Architecture Specifics</title>
- <para>
- The kernel debugger is organized into a number of components:
- <orderedlist>
- <listitem><para>The debug core</para>
- <para>
- The debug core is found in kernel/debugger/debug_core.c. It contains:
- <itemizedlist>
- <listitem><para>A generic OS exception handler which includes
- sync'ing the processors into a stopped state on an multi-CPU
- system.</para></listitem>
- <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
- <listitem><para>The API to make calls to the arch-specific kgdb implementation</para></listitem>
- <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
- <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
- <listitem><para>The API to invoke either the kdb or kgdb frontend to the debug core.</para></listitem>
- <listitem><para>The structures and callback API for atomic kernel mode setting.</para>
- <para>NOTE: kgdboc is where the kms callbacks are invoked.</para></listitem>
- </itemizedlist>
- </para>
- </listitem>
- <listitem><para>kgdb arch-specific implementation</para>
- <para>
- This implementation is generally found in arch/*/kernel/kgdb.c.
- As an example, arch/x86/kernel/kgdb.c contains the specifics to
- implement HW breakpoint as well as the initialization to
- dynamically register and unregister for the trap handlers on
- this architecture. The arch-specific portion implements:
- <itemizedlist>
- <listitem><para>contains an arch-specific trap catcher which
- invokes kgdb_handle_exception() to start kgdb about doing its
- work</para></listitem>
- <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
- <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
- <listitem><para>Any special exception handling and cleanup</para></listitem>
- <listitem><para>NMI exception handling and cleanup</para></listitem>
- <listitem><para>(optional)HW breakpoints</para></listitem>
- </itemizedlist>
- </para>
- </listitem>
- <listitem><para>gdbstub frontend (aka kgdb)</para>
- <para>The gdbstub is located in kernel/debug/gdbstub.c. It contains:</para>
- <itemizedlist>
- <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
- </itemizedlist>
- </listitem>
- <listitem><para>kdb frontend</para>
- <para>The kdb debugger shell is broken down into a number of
- components. The kdb core is located in kernel/debug/kdb. There
- are a number of helper functions in some of the other kernel
- components to make it possible for kdb to examine and report
- information about the kernel without taking locks that could
- cause a kernel deadlock. The kdb core contains implements the following functionality.</para>
- <itemizedlist>
- <listitem><para>A simple shell</para></listitem>
- <listitem><para>The kdb core command set</para></listitem>
- <listitem><para>A registration API to register additional kdb shell commands.</para>
- <itemizedlist>
- <listitem><para>A good example of a self-contained kdb module
- is the "ftdump" command for dumping the ftrace buffer. See:
- kernel/trace/trace_kdb.c</para></listitem>
- <listitem><para>For an example of how to dynamically register
- a new kdb command you can build the kdb_hello.ko kernel module
- from samples/kdb/kdb_hello.c. To build this example you can
- set CONFIG_SAMPLES=y and CONFIG_SAMPLE_KDB=m in your kernel
- config. Later run "modprobe kdb_hello" and the next time you
- enter the kdb shell, you can run the "hello"
- command.</para></listitem>
- </itemizedlist></listitem>
- <listitem><para>The implementation for kdb_printf() which
- emits messages directly to I/O drivers, bypassing the kernel
- log.</para></listitem>
- <listitem><para>SW / HW breakpoint management for the kdb shell</para></listitem>
- </itemizedlist>
- </listitem>
- <listitem><para>kgdb I/O driver</para>
- <para>
- Each kgdb I/O driver has to provide an implementation for the following:
- <itemizedlist>
- <listitem><para>configuration via built-in or module</para></listitem>
- <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
- <listitem><para>read and write character interface</para></listitem>
- <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
- <listitem><para>(optional) Early debug methodology</para></listitem>
- </itemizedlist>
- Any given kgdb I/O driver has to operate very closely with the
- hardware and must do it in such a way that does not enable
- interrupts or change other parts of the system context without
- completely restoring them. The kgdb core will repeatedly "poll"
- a kgdb I/O driver for characters when it needs input. The I/O
- driver is expected to return immediately if there is no data
- available. Doing so allows for the future possibility to touch
- watch dog hardware in such a way as to have a target system not
- reset when these are enabled.
- </para>
- </listitem>
- </orderedlist>
- </para>
- <para>
- If you are intent on adding kgdb architecture specific support
- for a new architecture, the architecture should define
- <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
- Kconfig file. This will enable kgdb for the architecture, and
- at that point you must create an architecture specific kgdb
- implementation.
- </para>
- <para>
- There are a few flags which must be set on every architecture in
- their &lt;asm/kgdb.h&gt; file. These are:
- <itemizedlist>
- <listitem>
- <para>
- NUMREGBYTES: The size in bytes of all of the registers, so
- that we can ensure they will all fit into a packet.
- </para>
- <para>
- BUFMAX: The size in bytes of the buffer GDB will read into.
- This must be larger than NUMREGBYTES.
- </para>
- <para>
- CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
- flush_cache_range or flush_icache_range. On some architectures,
- these functions may not be safe to call on SMP since we keep other
- CPUs in a holding pattern.
- </para>
- </listitem>
- </itemizedlist>
- </para>
- <para>
- There are also the following functions for the common backend,
- found in kernel/kgdb.c, that must be supplied by the
- architecture-specific backend unless marked as (optional), in
- which case a default function maybe used if the architecture
- does not need to provide a specific implementation.
- </para>
-!Iinclude/linux/kgdb.h
- </sect1>
- <sect1 id="kgdbocDesign">
- <title>kgdboc internals</title>
- <sect2>
- <title>kgdboc and uarts</title>
- <para>
- The kgdboc driver is actually a very thin driver that relies on the
- underlying low level to the hardware driver having "polling hooks"
- which the to which the tty driver is attached. In the initial
- implementation of kgdboc it the serial_core was changed to expose a
- low level UART hook for doing polled mode reading and writing of a
- single character while in an atomic context. When kgdb makes an I/O
- request to the debugger, kgdboc invokes a callback in the serial
- core which in turn uses the callback in the UART driver.</para>
- <para>
- When using kgdboc with a UART, the UART driver must implement two callbacks in the <constant>struct uart_ops</constant>. Example from drivers/8250.c:<programlisting>
-#ifdef CONFIG_CONSOLE_POLL
- .poll_get_char = serial8250_get_poll_char,
- .poll_put_char = serial8250_put_poll_char,
-#endif
- </programlisting>
- Any implementation specifics around creating a polling driver use the
- <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
- Keep in mind that polling hooks have to be implemented in such a way
- that they can be called from an atomic context and have to restore
- the state of the UART chip on return such that the system can return
- to normal when the debugger detaches. You need to be very careful
- with any kind of lock you consider, because failing here is most likely
- going to mean pressing the reset button.
- </para>
- </sect2>
- <sect2 id="kgdbocKbd">
- <title>kgdboc and keyboards</title>
- <para>The kgdboc driver contains logic to configure communications
- with an attached keyboard. The keyboard infrastructure is only
- compiled into the kernel when CONFIG_KDB_KEYBOARD=y is set in the
- kernel configuration.</para>
- <para>The core polled keyboard driver driver for PS/2 type keyboards
- is in drivers/char/kdb_keyboard.c. This driver is hooked into the
- debug core when kgdboc populates the callback in the array
- called <constant>kdb_poll_funcs[]</constant>. The
- kdb_get_kbd_char() is the top-level function which polls hardware
- for single character input.
- </para>
- </sect2>
- <sect2 id="kgdbocKms">
- <title>kgdboc and kms</title>
- <para>The kgdboc driver contains logic to request the graphics
- display to switch to a text context when you are using
- "kgdboc=kms,kbd", provided that you have a video driver which has a
- frame buffer console and atomic kernel mode setting support.</para>
- <para>
- Every time the kernel
- debugger is entered it calls kgdboc_pre_exp_handler() which in turn
- calls con_debug_enter() in the virtual console layer. On resuming kernel
- execution, the kernel debugger calls kgdboc_post_exp_handler() which
- in turn calls con_debug_leave().</para>
- <para>Any video driver that wants to be compatible with the kernel
- debugger and the atomic kms callbacks must implement the
- mode_set_base_atomic, fb_debug_enter and fb_debug_leave operations.
- For the fb_debug_enter and fb_debug_leave the option exists to use
- the generic drm fb helper functions or implement something custom for
- the hardware. The following example shows the initialization of the
- .mode_set_base_atomic operation in
- drivers/gpu/drm/i915/intel_display.c:
- <informalexample>
- <programlisting>
-static const struct drm_crtc_helper_funcs intel_helper_funcs = {
-[...]
- .mode_set_base_atomic = intel_pipe_set_base_atomic,
-[...]
-};
- </programlisting>
- </informalexample>
- </para>
- <para>Here is an example of how the i915 driver initializes the fb_debug_enter and fb_debug_leave functions to use the generic drm helpers in
- drivers/gpu/drm/i915/intel_fb.c:
- <informalexample>
- <programlisting>
-static struct fb_ops intelfb_ops = {
-[...]
- .fb_debug_enter = drm_fb_helper_debug_enter,
- .fb_debug_leave = drm_fb_helper_debug_leave,
-[...]
-};
- </programlisting>
- </informalexample>
- </para>
- </sect2>
- </sect1>
- </chapter>
- <chapter id="credits">
- <title>Credits</title>
- <para>
- The following people have contributed to this document:
- <orderedlist>
- <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
- <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
- </orderedlist>
- In March 2008 this document was completely rewritten by:
- <itemizedlist>
- <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
- </itemizedlist>
- In Jan 2010 this document was updated to include kdb.
- <itemizedlist>
- <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
- </itemizedlist>
- </para>
- </chapter>
-</book>
-