path: root/Documentation/DocBook/kgdb.tmpl

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<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>
   <sect2 id="kgdbocArgs">
   <title>kgdboc arguments</title>
   <para>Usage: <constant>kgdboc=[kbd][[,]serial_device][,baud]</constant></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 formate 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.
   <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>
   </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>
  </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