FreeBSD/Linux Kernel Cross Reference
sys/Documentation/BUG-HUNTING

     Table of contents
     =================
     
     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
    ============
    
    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 REPORTING-BUGS.
    
    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
   
   NB.: 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 Documentation/SubmittingPatches though to help your code get
   accepted.

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