summaryrefslogtreecommitdiffstats
path: root/Documentation/BUG-HUNTING
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/BUG-HUNTING')
-rw-r--r--Documentation/BUG-HUNTING246
1 files changed, 0 insertions, 246 deletions
diff --git a/Documentation/BUG-HUNTING b/Documentation/BUG-HUNTING
deleted file mode 100644
index 65022a8..0000000
--- a/Documentation/BUG-HUNTING
+++ /dev/null
@@ -1,246 +0,0 @@
-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.