Bug hunting
===========

Kernel bug reports often come with a stack dump like the one below::

	------------[ cut here ]------------
	WARNING: CPU: 1 PID: 28102 at kernel/module.c:1108 module_put+0x57/0x70
	Modules linked in: dvb_usb_gp8psk(-) dvb_usb dvb_core nvidia_drm(PO) nvidia_modeset(PO) snd_hda_codec_hdmi snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_timer snd soundcore nvidia(PO) [last unloaded: rc_core]
	CPU: 1 PID: 28102 Comm: rmmod Tainted: P        WC O 4.8.4-build.1 #1
	Hardware name: MSI MS-7309/MS-7309, BIOS V1.12 02/23/2009
	 00000000 c12ba080 00000000 00000000 c103ed6a c1616014 00000001 00006dc6
	 c1615862 00000454 c109e8a7 c109e8a7 00000009 ffffffff 00000000 f13f6a10
	 f5f5a600 c103ee33 00000009 00000000 00000000 c109e8a7 f80ca4d0 c109f617
	Call Trace:
	 [<c12ba080>] ? dump_stack+0x44/0x64
	 [<c103ed6a>] ? __warn+0xfa/0x120
	 [<c109e8a7>] ? module_put+0x57/0x70
	 [<c109e8a7>] ? module_put+0x57/0x70
	 [<c103ee33>] ? warn_slowpath_null+0x23/0x30
	 [<c109e8a7>] ? module_put+0x57/0x70
	 [<f80ca4d0>] ? gp8psk_fe_set_frontend+0x460/0x460 [dvb_usb_gp8psk]
	 [<c109f617>] ? symbol_put_addr+0x27/0x50
	 [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
	 [<f80bb3bf>] ? dvb_usb_exit+0x2f/0xd0 [dvb_usb]
	 [<c13d03bc>] ? usb_disable_endpoint+0x7c/0xb0
	 [<f80bb48a>] ? dvb_usb_device_exit+0x2a/0x50 [dvb_usb]
	 [<c13d2882>] ? usb_unbind_interface+0x62/0x250
	 [<c136b514>] ? __pm_runtime_idle+0x44/0x70
	 [<c13620d8>] ? __device_release_driver+0x78/0x120
	 [<c1362907>] ? driver_detach+0x87/0x90
	 [<c1361c48>] ? bus_remove_driver+0x38/0x90
	 [<c13d1c18>] ? usb_deregister+0x58/0xb0
	 [<c109fbb0>] ? SyS_delete_module+0x130/0x1f0
	 [<c1055654>] ? task_work_run+0x64/0x80
	 [<c1000fa5>] ? exit_to_usermode_loop+0x85/0x90
	 [<c10013f0>] ? do_fast_syscall_32+0x80/0x130
	 [<c1549f43>] ? sysenter_past_esp+0x40/0x6a
	---[ end trace 6ebc60ef3981792f ]---

Such stack traces provide enough information to identify the line inside the
Kernel's source code where the bug happened. Depending on the severity of
the issue, it may also contain the word **Oops**, as on this one::

	BUG: unable to handle kernel NULL pointer dereference at   (null)
	IP: [<c06969d4>] iret_exc+0x7d0/0xa59
	*pdpt = 000000002258a001 *pde = 0000000000000000
	Oops: 0002 [#1] PREEMPT SMP
	...

Despite being an **Oops** or some other sort of stack trace, the offended
line is usually required to identify and handle the bug. Along this chapter,
we'll refer to "Oops" for all kinds of stack traces that need to be analyzed.

If the kernel is compiled with ``CONFIG_DEBUG_INFO``, you can enhance the
quality of the stack trace by using ``scripts/decode_stacktrace.sh``.

Modules linked in
-----------------

Modules that are tainted or are being loaded or unloaded are marked with
"(...)", where the taint flags are described in
Documentation/admin-guide/tainted-kernels.rst, "being loaded" is
annotated with "+", and "being unloaded" is annotated with "-".


Where is the Oops message is located?
-------------------------------------

Normally the Oops text is read from the kernel buffers by klogd and
handed to ``syslogd`` which writes it to a syslog file, typically
``/var/log/messages`` (depends on ``/etc/syslog.conf``). On systems with
systemd, it may also be stored by the ``journald`` daemon, and accessed
by running ``journalctl`` command.

Sometimes ``klogd`` dies, in which case you can run ``dmesg > file`` to
read the data from the kernel buffers and save it.  Or you can
``cat /proc/kmsg > file``, however you have to break in to stop the transfer,
since ``kmsg`` is a "never ending file".

If the machine has crashed so badly that you cannot enter commands or
the disk is not available then you have three options:

(1) Hand copy the text from the screen and type it in after the machine
    has restarted.  Messy but it is the only option if you have not
    planned for a crash. Alternatively, you can take a picture of
    the screen with a digital camera - not nice, but better than
    nothing.  If the messages scroll off the top of the console, you
    may find that booting with a higher resolution (e.g., ``vga=791``)
    will allow you to read more of the text. (Caveat: This needs ``vesafb``,
    so won't help for 'early' oopses.)

(2) Boot with a serial console (see
    :ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
    run a null modem to a second machine and capture the output there
    using your favourite communication program.  Minicom works well.

(3) Use Kdump (see Documentation/admin-guide/kdump/kdump.rst),
    extract the kernel ring buffer from old memory with using dmesg
    gdbmacro in Documentation/admin-guide/kdump/gdbmacros.txt.

Finding the bug's location
--------------------------

Reporting a bug works best if you point the location of the bug at the
Kernel source file. There are two methods for doing that. Usually, using
``gdb`` is easier, but the Kernel should be pre-compiled with debug info.

gdb
^^^

The GNU debugger (``gdb``) is the best way to figure out the exact file and line
number of the OOPS from the ``vmlinux`` file.

The usage of gdb works best on a kernel compiled with ``CONFIG_DEBUG_INFO``.
This can be set by running::

  $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO

On a kernel compiled with ``CONFIG_DEBUG_INFO``, you can simply copy the
EIP value from the OOPS::

 EIP:    0060:[<c021e50e>]    Not tainted VLI

And use GDB to translate that to human-readable form::

  $ gdb vmlinux
  (gdb) l *0xc021e50e

If you don't have ``CONFIG_DEBUG_INFO`` enabled, you use the function
offset from the OOPS::

 EIP is at vt_ioctl+0xda8/0x1482

And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled::

  $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
  $ make vmlinux
  $ gdb vmlinux
  (gdb) l *vt_ioctl+0xda8
  0x1888 is in vt_ioctl (drivers/tty/vt/vt_ioctl.c:293).
  288	{
  289		struct vc_data *vc = NULL;
  290		int ret = 0;
  291
  292		console_lock();
  293		if (VT_BUSY(vc_num))
  294			ret = -EBUSY;
  295		else if (vc_num)
  296			vc = vc_deallocate(vc_num);
  297		console_unlock();

or, if you want to be more verbose::

  (gdb) p vt_ioctl
  $1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl>
  (gdb) l *0xae0+0xda8

You could, instead, use the object file::

  $ make drivers/tty/
  $ gdb drivers/tty/vt/vt_ioctl.o
  (gdb) l *vt_ioctl+0xda8

If you have a call trace, such as::

     Call Trace:
      [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
      [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
      [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
      ...

this shows the problem likely is in the :jbd: module. You can load that module
in gdb and list the relevant code::

  $ gdb fs/jbd/jbd.ko
  (gdb) l *log_wait_commit+0xa3

.. note::

     You can also do the same for any function call at the stack trace,
     like this one::

	 [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]

     The position where the above call happened can be seen with::

	$ gdb drivers/media/usb/dvb-usb/dvb-usb.o
	(gdb) l *dvb_usb_adapter_frontend_exit+0x3a

objdump
^^^^^^^

To debug a kernel, use objdump and look for the hex offset from the crash
output to find the valid line of code/assembler. Without debug symbols, you
will see the assembler code for the routine shown, but if your kernel has
debug symbols the C code will also be available. (Debug symbols can be enabled
in the kernel hacking menu of the menu configuration.) For example::

    $ objdump -r -S -l --disassemble net/ipv4/tcp.o

.. note::

   You need to be at the top level of the kernel tree for this to pick up
   your C files.

If you don't have access to the source code you can still debug some crash
dumps using the following method (example crash dump output as shown by
Dave Miller)::

     EIP is at 	+0x14/0x4c0
      ...
     Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
     00 00 55 57  56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
     <8b> 83 3c 01 00 00 89 44  24 14 8b 45 28 85 c0 89 44 24 18 0f 85

     Put the bytes into a "foo.s" file like this:

            .text
            .globl foo
     foo:
            .byte  .... /* bytes from Code: part of OOPS dump */

     Compile it with "gcc -c -o foo.o foo.s" then look at the output of
     "objdump --disassemble foo.o".

     Output:

     ip_queue_xmit:
         push       %ebp
         push       %edi
         push       %esi
         push       %ebx
         sub        $0xbc, %esp
         mov        0xd0(%esp), %ebp        ! %ebp = arg0 (skb)
         mov        0x8(%ebp), %ebx         ! %ebx = skb->sk
         mov        0x13c(%ebx), %eax       ! %eax = inet_sk(sk)->opt

``scripts/decodecode`` can be used to automate most of this, depending
on what CPU architecture is being debugged.

Reporting the bug
-----------------

Once you find where the bug happened, by inspecting its location,
you could either try to fix it yourself or report it upstream.

In order to report it upstream, you should identify the bug tracker, if any, or
mailing list used for the development of the affected code. This can be done by
using the ``get_maintainer.pl`` script.

For example, if you find a bug at the gspca's sonixj.c file, you can get
its maintainers with::

	$ ./scripts/get_maintainer.pl --bug -f drivers/media/usb/gspca/sonixj.c
	Hans Verkuil <hverkuil@kernel.org> (odd fixer:GSPCA USB WEBCAM DRIVER,commit_signer:1/1=100%)
	Mauro Carvalho Chehab <mchehab@kernel.org> (maintainer:MEDIA INPUT INFRASTRUCTURE (V4L/DVB),commit_signer:1/1=100%)
	Tejun Heo <tj@kernel.org> (commit_signer:1/1=100%)
	Bhaktipriya Shridhar <bhaktipriya96@gmail.com> (commit_signer:1/1=100%,authored:1/1=100%,added_lines:4/4=100%,removed_lines:9/9=100%)
	linux-media@vger.kernel.org (open list:GSPCA USB WEBCAM DRIVER)
	linux-kernel@vger.kernel.org (open list)

Please notice that it will point to:

- The last developers that touched the source code (if this is done inside
  a git tree). On the above example, Tejun and Bhaktipriya (in this
  specific case, none really involved on the development of this file);
- The driver maintainer (Hans Verkuil);
- The subsystem maintainer (Mauro Carvalho Chehab);
- The driver and/or subsystem mailing list (linux-media@vger.kernel.org);
- The Linux Kernel mailing list (linux-kernel@vger.kernel.org);
- The bug reporting URIs for the driver/subsystem (none in the above example).

If the listing contains bug reporting URIs at the end, please prefer them over
email. Otherwise, please report bugs to the mailing list used for the
development of the code (linux-media ML) copying the driver maintainer (Hans).

If you are totally stumped as to whom to send the report, and
``get_maintainer.pl`` didn't provide you anything useful, send it to
linux-kernel@vger.kernel.org.

Thanks for your help in making Linux as stable as humanly possible.

Fixing the bug
--------------

If you know programming, you could help us by not only reporting the bug,
but also providing us with a solution. After all, open source is about
sharing what you do and don't you want to be recognised for your genius?

If you decide to take this way, once you have worked out a fix please submit
it upstream.

Please do read
:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though
to help your code get accepted.


---------------------------------------------------------------------------

Notes on Oops tracing with ``klogd``
------------------------------------

In order to help Linus and the other kernel developers there has been
substantial support incorporated into ``klogd`` for processing protection
faults.  In order to have full support for address resolution at least
version 1.3-pl3 of the ``sysklogd`` package should be used.

When a protection fault occurs the ``klogd`` daemon automatically
translates important addresses in the kernel log messages to their
symbolic equivalents.  This translated kernel message is then
forwarded through whatever reporting mechanism ``klogd`` is using.  The
protection fault message can be simply cut out of the message files
and forwarded to the kernel developers.

Two types of address resolution are performed by ``klogd``.  The first is
static translation and the second is dynamic translation.
Static translation uses the System.map file.
In order to do static translation the ``klogd`` daemon
must be able to find a system map file at daemon initialization time.
See the klogd man page for information on how ``klogd`` searches for map
files.

Dynamic address translation is important when kernel loadable modules
are being used.  Since memory for kernel modules is allocated from the
kernel's dynamic memory pools there are no fixed locations for either
the start of the module or for functions and symbols in the module.

The kernel supports system calls which allow a program to determine
which modules are loaded and their location in memory.  Using these
system calls the klogd daemon builds a symbol table which can be used
to debug a protection fault which occurs in a loadable kernel module.

At the very minimum klogd will provide the name of the module which
generated the protection fault.  There may be additional symbolic
information available if the developer of the loadable module chose to
export symbol information from the module.

Since the kernel module environment can be dynamic there must be a
mechanism for notifying the ``klogd`` daemon when a change in module
environment occurs.  There are command line options available which
allow klogd to signal the currently executing daemon that symbol
information should be refreshed.  See the ``klogd`` manual page for more
information.

A patch is included with the sysklogd distribution which modifies the
``modules-2.0.0`` package to automatically signal klogd whenever a module
is loaded or unloaded.  Applying this patch provides essentially
seamless support for debugging protection faults which occur with
kernel loadable modules.

The following is an example of a protection fault in a loadable module
processed by ``klogd``::

	Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
	Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
	Aug 29 09:51:01 blizard kernel: *pde = 00000000
	Aug 29 09:51:01 blizard kernel: Oops: 0002
	Aug 29 09:51:01 blizard kernel: CPU:    0
	Aug 29 09:51:01 blizard kernel: EIP:    0010:[oops:_oops+16/3868]
	Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
	Aug 29 09:51:01 blizard kernel: eax: 315e97cc   ebx: 003a6f80   ecx: 001be77b   edx: 00237c0c
	Aug 29 09:51:01 blizard kernel: esi: 00000000   edi: bffffdb3   ebp: 00589f90   esp: 00589f8c
	Aug 29 09:51:01 blizard kernel: ds: 0018   es: 0018   fs: 002b   gs: 002b   ss: 0018
	Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
	Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
	Aug 29 09:51:01 blizard kernel:        00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
	Aug 29 09:51:01 blizard kernel:        bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
	Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
	Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3