Re: [PATCH 06/11] docs: Get rid of the "bug-hunting" guide
From: Mauro Carvalho Chehab
Date: Wed Oct 26 2016 - 20:23:27 EST
Em Wed, 26 Oct 2016 17:19:34 -0600
Jonathan Corbet <corbet@xxxxxxx> escreveu:
> Larry McVoy's advice on how to manually bisect 1.3.x kernel bugs is of
> historical interest, but that's what the repository is for. It is not
> useful to users now.
In the specific case of this file, I think that the information there
about how to disassemble a file and how to use gdb and objdump to get the
error line associated with an OOPS very useful.
So, I prefer to keep this one. See the patch I sent you today.
> Signed-off-by: Jonathan Corbet <corbet@xxxxxxx>
> ---
> Documentation/admin-guide/bug-hunting.rst | 249 ------------------------------
> Documentation/admin-guide/index.rst | 1 -
> 2 files changed, 250 deletions(-)
> delete mode 100644 Documentation/admin-guide/bug-hunting.rst
>
> diff --git a/Documentation/admin-guide/bug-hunting.rst b/Documentation/admin-guide/bug-hunting.rst
> deleted file mode 100644
> index d35dd9fd1af0..000000000000
> --- a/Documentation/admin-guide/bug-hunting.rst
> +++ /dev/null
> @@ -1,249 +0,0 @@
> -Bug hunting
> -+++++++++++
> -
> -Last updated: 20 December 2005
> -
> -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
> -:ref:`Documentation/admin-guide/reporting-bugs.rst <reportingbugs>`.
> -
> -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@xxxxxxx (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
> -
> -.. 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
> -
> -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
> -ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though
> -to help your code get accepted.
> diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
> index 2872c0c70ea4..2d0a302e8773 100644
> --- a/Documentation/admin-guide/index.rst
> +++ b/Documentation/admin-guide/index.rst
> @@ -25,7 +25,6 @@ problems and bugs in particular.
>
> reporting-bugs
> security-bugs
> - bug-hunting
> oops-tracing
> ramoops
> dynamic-debug-howto
Thanks,
Mauro