bug-hunting.rst

Bug hunting
+++++++++++

Last updated: 20 December 2005

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

Always try the latest kernel from kernel.org and build from source. If you are
not confident in doing that please report the bug to your distribution vendor
instead of to a kernel developer.

Finding bugs is not always easy. Have a go though. If you can't find it don't
give up. Report as much as you have found to the relevant maintainer. See
MAINTAINERS for who that is for the subsystem you have worked on.

Before you submit a bug report read
:ref:`Documentation/REPORTING-BUGS <reportingbugs>`.

Devices not appearing
=====================

Often this is caused by udev. Check that first before blaming it on the
kernel.

Finding patch that caused a bug
===============================



Finding using ``git-bisect``
----------------------------

Using the provided tools with ``git`` makes finding bugs easy provided the bug
is reproducible.

Steps to do it:

- start using git for the kernel source
- read the man page for ``git-bisect``
- have fun

Finding it the old way
----------------------

[Sat Mar  2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]

This is how to track down a bug if you know nothing about kernel hacking.
It's a brute force approach but it works pretty well.

You need:

        - A reproducible bug - it has to happen predictably (sorry)
        - All the kernel tar files from a revision that worked to the
          revision that doesn't

You will then do:

        - Rebuild a revision that you believe works, install, and verify that.
        - Do a binary search over the kernels to figure out which one
          introduced the bug.  I.e., suppose 1.3.28 didn't have the bug, but
          you know that 1.3.69 does.  Pick a kernel in the middle and build
          that, like 1.3.50.  Build & test; if it works, pick the mid point
          between .50 and .69, else the mid point between .28 and .50.
        - You'll narrow it down to the kernel that introduced the bug.  You
          can probably do better than this but it gets tricky.

        - Narrow it down to a subdirectory

          - Copy kernel that works into "test".  Let's say that 3.62 works,
            but 3.63 doesn't.  So you diff -r those two kernels and come
            up with a list of directories that changed.  For each of those
            directories:

                Copy the non-working directory next to the working directory
                as "dir.63".
                One directory at time, try moving the working directory to
                "dir.62" and mv dir.63 dir"time, try::

                        mv dir dir.62
                        mv dir.63 dir
                        find dir -name '*.[oa]' -print | xargs rm -f

                And then rebuild and retest.  Assuming that all related
                changes were contained in the sub directory, this should
                isolate the change to a directory.

                Problems: changes in header files may have occurred; I've
                found in my case that they were self explanatory - you may
                or may not want to give up when that happens.

        - Narrow it down to a file

          - You can apply the same technique to each file in the directory,
            hoping that the changes in that file are self contained.

        - Narrow it down to a routine

          - You can take the old file and the new file and manually create
            a merged file that has::

                #ifdef VER62
                routine()
                {
                        ...
                }
                #else
                routine()
                {
                        ...
                }
                #endif

            And then walk through that file, one routine at a time and
            prefix it with::

                #define VER62
                /* both routines here */
                #undef VER62

            Then recompile, retest, move the ifdefs until you find the one
            that makes the difference.

Finally, you take all the info that you have, kernel revisions, bug
description, the extent to which you have narrowed it down, and pass
that off to whomever you believe is the maintainer of that section.
A post to linux.dev.kernel isn't such a bad idea if you've done some
work to narrow it down.

If you get it down to a routine, you'll probably get a fix in 24 hours.

My apologies to Linus and the other kernel hackers for describing this
brute force approach, it's hardly what a kernel hacker would do.  However,
it does work and it lets non-hackers help fix bugs.  And it is cool
because Linux snapshots will let you do this - something that you can't
do with vendor supplied releases.

Fixing the bug
==============

Nobody is going to tell you how to fix bugs. Seriously. You need to work it
out. But below are some hints on how to use the tools.

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/dccp/ipv4.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 code you can also debug on some crash dumps
e.g. crash dump output as shown by Dave Miller::

     EIP is at ip_queue_xmit+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

In addition, you can use GDB to figure out the exact file and line
number of the OOPS from the ``vmlinux`` file. If you have
``CONFIG_DEBUG_INFO`` enabled, 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::

  make vmlinux
  gdb vmlinux
  (gdb) p vt_ioctl
  (gdb) l *(0x<address of vt_ioctl> + 0xda8)

or, as one command::

  (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 in the :jbd: module. You can load that module in gdb
and list the relevant code::

  gdb fs/jbd/jbd.ko
  (gdb) p log_wait_commit
  (gdb) l *(0x<address> + 0xa3)

or::

  (gdb) l *(log_wait_commit + 0xa3)


Another very useful option of the Kernel Hacking section in menuconfig is
Debug memory allocations. This will help you see whether data has been
initialised and not set before use etc. To see the values that get assigned
with this look at ``mm/slab.c`` and search for ``POISON_INUSE``. When using
this an Oops will often show the poisoned data instead of zero which is the
default.

Once you have worked out a fix please submit it upstream. After all open
source is about sharing what you do and don't you want to be recognised for
your genius?

Please do read :ref:`Documentation/SubmittingPatches <submittingpatches>`
though to help your code get accepted.