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diff --git a/Documentation/BUG-HUNTING b/Documentation/BUG-HUNTING index ca29242dbc38..65b97e1dbf70 100644 --- a/Documentation/BUG-HUNTING +++ b/Documentation/BUG-HUNTING @@ -1,3 +1,56 @@ +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. @@ -90,3 +143,63 @@ 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 + +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. |