path: root/Documentation/dev-tools/kgdb.rst

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Using kgdb, kdb and the kernel debugger internals
=================================================

:Author: Jason Wessel

Introduction
============

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.

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
``CONFIG_KALLSYMS``.

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.

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
a 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.

Compiling a kernel
==================

-  In order to enable compilation of kdb, you must first enable kgdb.

-  The kgdb test compile options are described in the kgdb test suite
   chapter.

Kernel config options for kgdb
------------------------------

To enable ``CONFIG_KGDB`` you should look under
:menuselection:`Kernel hacking --> Kernel debugging` and select
:menuselection:`KGDB: kernel debugger`.

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 ``CONFIG_DEBUG_INFO`` which is called
:menuselection:`Compile the kernel with debug info` in the config menu.

It is advised, but not required, that you turn on the
``CONFIG_FRAME_POINTER`` kernel option which is called :menuselection:`Compile
the kernel with frame pointers` in the config menu. This option inserts code
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.

If the architecture that you are using supports the kernel option
``CONFIG_STRICT_KERNEL_RWX``, 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_STRICT_KERNEL_RWX``
option turned on, else you need to turn off this option.

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.

Here is an example set of ``.config`` symbols to enable or disable for kgdb::

  # CONFIG_STRICT_KERNEL_RWX is not set
  CONFIG_FRAME_POINTER=y
  CONFIG_KGDB=y
  CONFIG_KGDB_SERIAL_CONSOLE=y

Kernel config options for kdb
-----------------------------

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.

The main config option for kdb is ``CONFIG_KGDB_KDB`` which is called
:menuselection:`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.

If you want to use a PS/2-style keyboard with kdb, you would select
``CONFIG_KDB_KEYBOARD`` which is called :menuselection:`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.

Here is an example set of ``.config`` symbols to enable/disable kdb::

  # CONFIG_STRICT_KERNEL_RWX is not set
  CONFIG_FRAME_POINTER=y
  CONFIG_KGDB=y
  CONFIG_KGDB_SERIAL_CONSOLE=y
  CONFIG_KGDB_KDB=y
  CONFIG_KDB_KEYBOARD=y

Kernel Debugger Boot Arguments
==============================

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 providing some examples of the
configuration parameters.

Kernel parameter: kgdboc
------------------------

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.

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
``kgdbwait`` and early debugging if you build kgdboc into the kernel as
a built-in.

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.

kgdboc arguments
~~~~~~~~~~~~~~~~

Usage::

	kgdboc=[kms][[,]kbd][[,]serial_device][,baud]

The order listed above must be observed if you use any of the optional
configurations together.

Abbreviations:

-  kms = Kernel Mode Setting

-  kbd = Keyboard

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.

Using loadable module or built-in
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1. As a kernel built-in:

   Use the kernel boot argument::

	kgdboc=<tty-device>,[baud]

2. As a kernel loadable module:

   Use the command::

	modprobe kgdboc kgdboc=<tty-device>,[baud]

   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.

   1. ``kgdboc=ttyS0,115200``

   2. ``kgdboc=ttyAMA1,115200``

Configure kgdboc at runtime with sysfs
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

At run time you can enable or disable kgdboc by echoing a parameters
into the sysfs. Here are two examples:

1. Enable kgdboc on ttyS0::

	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc

2. Disable kgdboc::

	echo "" > /sys/module/kgdboc/parameters/kgdboc

.. note::

   You do not need to specify the baud if you are configuring the
   console on tty which is already configured or open.

More examples
^^^^^^^^^^^^^

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.

1. kdb and kgdb over only a serial port::

	kgdboc=<serial_device>[,baud]

   Example::

	kgdboc=ttyS0,115200

2. kdb and kgdb with keyboard and a serial port::

	kgdboc=kbd,<serial_device>[,baud]

   Example::

	kgdboc=kbd,ttyS0,115200

3. kdb with a keyboard::

	kgdboc=kbd

4. kdb with kernel mode setting::

	kgdboc=kms,kbd

5. kdb with kernel mode setting and kgdb over a serial port::

	kgdboc=kms,kbd,ttyS0,115200

.. note::

   Kgdboc does not support interrupting the target via the gdb remote
   protocol. You must manually send a :kbd:`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 :kbd:`SysRq-G`
   for you.

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 :kbd:`Sysrq` sequence and then type the letter :kbd:`g`. Then you
disconnect the terminal session and connect gdb. Your options if you
don't like this are to hack gdb to send the :kbd:`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.

Kernel parameter: ``kgdboc_earlycon``
-------------------------------------

If you specify the kernel parameter ``kgdboc_earlycon`` and your serial
driver registers a boot console that supports polling (doesn't need
interrupts and implements a nonblocking read() function) kgdb will attempt
to work using the boot console until it can transition to the regular
tty driver specified by the ``kgdboc`` parameter.

Normally there is only one boot console (especially that implements the
read() function) so just adding ``kgdboc_earlycon`` on its own is
sufficient to make this work. If you have more than one boot console you
can add the boot console's name to differentiate. Note that names that
are registered through the boot console layer and the tty layer are not
the same for the same port.

For instance, on one board to be explicit you might do::

   kgdboc_earlycon=qcom_geni kgdboc=ttyMSM0

If the only boot console on the device was "qcom_geni", you could simplify::

   kgdboc_earlycon kgdboc=ttyMSM0

Kernel parameter: ``kgdbwait``
------------------------------

The Kernel command line option ``kgdbwait`` makes kgdb wait for a
debugger connection during booting of a kernel. You can only use this
option if 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.

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.

Kernel parameter: ``kgdbcon``
-----------------------------

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.

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.

1. Activate with the kernel command line option::

	kgdbcon

2. Use sysfs before configuring an I/O driver::

	echo 1 > /sys/module/kgdb/parameters/kgdb_use_con

.. 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.

.. important::

   You cannot use kgdboc + kgdbcon on a tty that is an
   active system console. An example of incorrect usage is::

	console=ttyS0,115200 kgdboc=ttyS0 kgdbcon

It is possible to use this option with kgdboc on a tty that is not a
system console.

Run time parameter: ``kgdbreboot``
----------------------------------

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.

.. tabularcolumns:: |p{0.4cm}|p{11.5cm}|p{5.6cm}|

.. flat-table::
  :widths: 1 10 8

  * - 1
    - ``echo -1 > /sys/module/debug_core/parameters/kgdbreboot``
    - Ignore the reboot notification entirely.

  * - 2
    - ``echo 0 > /sys/module/debug_core/parameters/kgdbreboot``
    - Send the detach message to any attached debugger client.

  * - 3
    - ``echo 1 > /sys/module/debug_core/parameters/kgdbreboot``
    - Enter the debugger on reboot notify.

Kernel parameter: ``nokaslr``
-----------------------------

If the architecture that you are using enable KASLR by default,
you should consider turning it off.  KASLR randomizes the
virtual address where the kernel image is mapped and confuse
gdb which resolve kernel symbol address from symbol table
of vmlinux.

Using kdb
=========

Quick start for kdb on a serial port
------------------------------------

This is a quick example of how to use kdb.

1. Configure kgdboc at boot using kernel parameters::

	console=ttyS0,115200 kgdboc=ttyS0,115200 nokaslr

   OR

   Configure kgdboc after the kernel has booted; assuming you are using
   a serial port console::

	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc

2. 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 :kbd:`SysRq-G`, which means you must have
   enabled ``CONFIG_MAGIC_SysRq=y`` in your kernel config.

   -  When logged in as root or with a super user session you can run::

	echo g > /proc/sysrq-trigger

   -  Example using minicom 2.2

      Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`

   -  When you have telneted to a terminal server that supports sending
      a remote break

      Press: :kbd:`CTRL-]`

      Type in: ``send break``

      Press: :kbd:`Enter` :kbd:`g`

3. From the kdb prompt you can run the ``help`` command to see a complete
   list of the commands that are available.

   Some useful commands in kdb include:

   =========== =================================================================
   ``lsmod``   Shows where kernel modules are loaded
   ``ps``      Displays only the active processes
   ``ps A``    Shows all the processes
   ``summary`` Shows kernel version info and memory usage
   ``bt``      Get a backtrace of the current process using dump_stack()
   ``dmesg``   View the kernel syslog buffer
   ``go``      Continue the system
   =========== =================================================================

4. 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.

Quick start for kdb using a keyboard connected console
------------------------------------------------------

This is a quick example of how to use kdb with a keyboard.

1. Configure kgdboc at boot using kernel parameters::

	kgdboc=kbd

   OR

   Configure kgdboc after the kernel has booted::

	echo kbd > /sys/module/kgdboc/parameters/kgdboc

2. 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 :kbd:`SysRq-G`, which means you must have
   enabled ``CONFIG_MAGIC_SysRq=y`` in your kernel config.

   -  When logged in as root or with a super user session you can run::

	echo g > /proc/sysrq-trigger

   -  Example using a laptop keyboard:

      Press and hold down: :kbd:`Alt`

      Press and hold down: :kbd:`Fn`

      Press and release the key with the label: :kbd:`SysRq`

      Release: :kbd:`Fn`

      Press and release: :kbd:`g`

      Release: :kbd:`Alt`

   -  Example using a PS/2 101-key keyboard

      Press and hold down: :kbd:`Alt`

      Press and release the key with the label: :kbd:`SysRq`

      Press and release: :kbd:`g`

      Release: :kbd:`Alt`

3. Now type in a kdb command such as ``help``, ``dmesg``, ``bt`` or ``go`` to
   continue kernel execution.

Using kgdb / gdb
================

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.

All kgdb I/O drivers can be reconfigured at run time, if
``CONFIG_SYSFS`` and ``CONFIG_MODULES`` are enabled, by echo'ing a new
config string to ``/sys/module/<driver>/parameter/<option>``. 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 ``detach`` command prior to trying to unconfigure a
kgdb I/O driver.

Connecting with gdb to a serial port
------------------------------------

1. Configure kgdboc

   Configure kgdboc at boot using kernel parameters::

	kgdboc=ttyS0,115200

   OR

   Configure kgdboc after the kernel has booted::

	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc

2. Stop kernel execution (break into the debugger)

   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 :kbd:`SysRq-G`, or running the
   kernel until it takes an exception where it waits for the debugger to
   attach.

   -  When logged in as root or with a super user session you can run::

	echo g > /proc/sysrq-trigger

   -  Example using minicom 2