Documentation: Update to BUG-HUNTING
Signed-off-by: Ian McDonald <imcdnzl@gmail.com> Signed-off-by: Adrian Bunk <bunk@stusta.de>
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Ian McDonald
Adrian Bunk
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Table of contents
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=================
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Last updated: 20 December 2005
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Contents
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========
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- Introduction
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- Devices not appearing
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- Finding patch that caused a bug
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-- Finding using git-bisect
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-- Finding it the old way
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- Fixing the bug
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Introduction
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============
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Always try the latest kernel from kernel.org and build from source. If you are
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not confident in doing that please report the bug to your distribution vendor
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instead of to a kernel developer.
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Finding bugs is not always easy. Have a go though. If you can't find it don't
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give up. Report as much as you have found to the relevant maintainer. See
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MAINTAINERS for who that is for the subsystem you have worked on.
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Before you submit a bug report read REPORTING-BUGS.
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Devices not appearing
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=====================
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Often this is caused by udev. Check that first before blaming it on the
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kernel.
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Finding patch that caused a bug
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===============================
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Finding using git-bisect
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------------------------
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Using the provided tools with git makes finding bugs easy provided the bug is
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reproducible.
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Steps to do it:
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- start using git for the kernel source
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- read the man page for git-bisect
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- have fun
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Finding it the old way
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----------------------
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[Sat Mar 2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]
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This is how to track down a bug if you know nothing about kernel hacking.
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@ -90,3 +143,63 @@ it does work and it lets non-hackers help fix bugs. And it is cool
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because Linux snapshots will let you do this - something that you can't
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do with vendor supplied releases.
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Fixing the bug
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==============
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Nobody is going to tell you how to fix bugs. Seriously. You need to work it
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out. But below are some hints on how to use the tools.
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To debug a kernel, use objdump and look for the hex offset from the crash
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output to find the valid line of code/assembler. Without debug symbols, you
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will see the assembler code for the routine shown, but if your kernel has
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debug symbols the C code will also be available. (Debug symbols can be enabled
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in the kernel hacking menu of the menu configuration.) For example:
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objdump -r -S -l --disassemble net/dccp/ipv4.o
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NB.: you need to be at the top level of the kernel tree for this to pick up
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your C files.
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If you don't have access to the code you can also debug on some crash dumps
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e.g. crash dump output as shown by Dave Miller.
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> EIP is at ip_queue_xmit+0x14/0x4c0
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> ...
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> Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
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> 00 00 55 57 56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
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> <8b> 83 3c 01 00 00 89 44 24 14 8b 45 28 85 c0 89 44 24 18 0f 85
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>
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> Put the bytes into a "foo.s" file like this:
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>
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> .text
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> .globl foo
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> foo:
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> .byte .... /* bytes from Code: part of OOPS dump */
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>
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> Compile it with "gcc -c -o foo.o foo.s" then look at the output of
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> "objdump --disassemble foo.o".
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>
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> Output:
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>
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> ip_queue_xmit:
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> push %ebp
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> push %edi
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> push %esi
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> push %ebx
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> sub $0xbc, %esp
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> mov 0xd0(%esp), %ebp ! %ebp = arg0 (skb)
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> mov 0x8(%ebp), %ebx ! %ebx = skb->sk
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> mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
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Another very useful option of the Kernel Hacking section in menuconfig is
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Debug memory allocations. This will help you see whether data has been
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initialised and not set before use etc. To see the values that get assigned
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with this look at mm/slab.c and search for POISON_INUSE. When using this an
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Oops will often show the poisoned data instead of zero which is the default.
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Once you have worked out a fix please submit it upstream. After all open
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source is about sharing what you do and don't you want to be recognised for
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your genius?
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Please do read Documentation/SubmittingPatches though to help your code get
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accepted.
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