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Commit 953ab835 authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab
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Documentation/BUG-HUNTING: convert to ReST markup 



- Add a document title and remove its own index;
- use monotonic fonts for paths;
- use quote blocks where needed;
- adjust/use spaces to properly format paragraphs;
- add it to the user book.

Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@s-opensource.com>
parent 8e7fbec6

Documentation/BUG-HUNTING

+83 −81
Original line number Diff line number Diff line 
Table of contents

=================

Bug hunting

+++++++++++



Last updated: 20 December 2005



Contents

========



- Introduction

- Devices not appearing

- Finding patch that caused a bug

-- Finding using git-bisect

-- Finding it the old way

- Fixing the bug



Introduction

============
@@ -24,7 +14,8 @@ 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 REPORTING-BUGS.

Before you submit a bug report read

:ref:`Documentation/REPORTING-BUGS <reportingbugs>`.



Devices not appearing

=====================
@@ -37,15 +28,16 @@ Finding patch that caused a bug 






Finding using git-bisect

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

Finding using ``git-bisect``

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



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

reproducible.

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

- read the man page for ``git-bisect``

- have fun



Finding it the old way
@@ -58,22 +50,22 @@ 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

        - 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

        - 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

        - 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

        - 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
@@ -83,7 +75,7 @@ You will then do: 
                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

                "dir.62" and mv dir.63 dir"time, try::



                        mv dir dir.62

                        mv dir.63 dir
@@ -97,15 +89,15 @@ You will then do: 
                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

        - 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

        - Narrow it down to a routine



          - You can take the old file and the new file and manually create

            a merged file that has

            a merged file that has::



                #ifdef VER62

                routine()
@@ -120,7 +112,7 @@ You will then do: 
                #endif



            And then walk through that file, one routine at a time and

            prefix it with

            prefix it with::



                #define VER62

                /* both routines here */
@@ -153,94 +145,104 @@ 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:

in the kernel hacking menu of the menu configuration.) For example::



    objdump -r -S -l --disassemble net/dccp/ipv4.o



NB.: you need to be at the top level of the kernel tree for this to pick up

.. 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

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:

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:

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:

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:

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



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

> ...

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.

and list the relevant code::



  gdb fs/jbd/jbd.ko

  (gdb) p log_wait_commit

  (gdb) l *(0x<address> + 0xa3)

or



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.

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 Documentation/SubmittingPatches though to help your code get

accepted.
 
Please do read :ref:`Documentation/SubmittingPatches <submittingpatches>`

though to help your code get accepted.