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admin-guide: merge oops-tracing with bug-hunting 

Now that oops-tracing.rst has only information about
stack dumps found on OOPS, and bug-hunting.rst has only
information about how to identify the source code line
associated with a stack dump, let's merge them and
improve the information inside it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mauro Carvalho Chehab 2016-11-07 17:03:19 -02:00 committed by Jonathan Corbet
parent 337c188dff
commit f226e46087
3 changed files with 302 additions and 316 deletions
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376
Documentation/admin-guide/bug-hunting.rst
View File

@ -1,16 +1,190 @@
Bug hunting
+++++++++++
===========
Last updated: 28 October 2016
Kernel bug reports often come with a stack dump like the one below::
Fixing the bug
==============
------------[ cut here ]------------
WARNING: CPU: 1 PID: 28102 at kernel/module.c:1108 module_put+0x57/0x70
Modules linked in: dvb_usb_gp8psk(-) dvb_usb dvb_core nvidia_drm(PO) nvidia_modeset(PO) snd_hda_codec_hdmi snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_timer snd soundcore nvidia(PO) [last unloaded: rc_core]
CPU: 1 PID: 28102 Comm: rmmod Tainted: P WC O 4.8.4-build.1 #1
Hardware name: MSI MS-7309/MS-7309, BIOS V1.12 02/23/2009
00000000 c12ba080 00000000 00000000 c103ed6a c1616014 00000001 00006dc6
c1615862 00000454 c109e8a7 c109e8a7 00000009 ffffffff 00000000 f13f6a10
f5f5a600 c103ee33 00000009 00000000 00000000 c109e8a7 f80ca4d0 c109f617
Call Trace:
[<c12ba080>] ? dump_stack+0x44/0x64
[<c103ed6a>] ? __warn+0xfa/0x120
[<c109e8a7>] ? module_put+0x57/0x70
[<c109e8a7>] ? module_put+0x57/0x70
[<c103ee33>] ? warn_slowpath_null+0x23/0x30
[<c109e8a7>] ? module_put+0x57/0x70
[<f80ca4d0>] ? gp8psk_fe_set_frontend+0x460/0x460 [dvb_usb_gp8psk]
[<c109f617>] ? symbol_put_addr+0x27/0x50
[<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
[<f80bb3bf>] ? dvb_usb_exit+0x2f/0xd0 [dvb_usb]
[<c13d03bc>] ? usb_disable_endpoint+0x7c/0xb0
[<f80bb48a>] ? dvb_usb_device_exit+0x2a/0x50 [dvb_usb]
[<c13d2882>] ? usb_unbind_interface+0x62/0x250
[<c136b514>] ? __pm_runtime_idle+0x44/0x70
[<c13620d8>] ? __device_release_driver+0x78/0x120
[<c1362907>] ? driver_detach+0x87/0x90
[<c1361c48>] ? bus_remove_driver+0x38/0x90
[<c13d1c18>] ? usb_deregister+0x58/0xb0
[<c109fbb0>] ? SyS_delete_module+0x130/0x1f0
[<c1055654>] ? task_work_run+0x64/0x80
[<c1000fa5>] ? exit_to_usermode_loop+0x85/0x90
[<c10013f0>] ? do_fast_syscall_32+0x80/0x130
[<c1549f43>] ? sysenter_past_esp+0x40/0x6a
---[ end trace 6ebc60ef3981792f ]---
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.
Such stack traces provide enough information to identify the line inside the
Kernel's source code where the bug happened. Depending on the severity of
the issue, it may also contain the word **Oops**, as on this one::
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<c06969d4>] iret_exc+0x7d0/0xa59
*pdpt = 000000002258a001 *pde = 0000000000000000
Oops: 0002 [#1] PREEMPT SMP
...
Despite being an **Oops** or some other sort of stack trace, the offended
line is usually required to identify and handle the bug. Along this chapter,
we'll refer to "Oops" for all kinds of stack traces that need to be analized.
.. note::
``ksymoops`` is useless on 2.6 or upper. Please use the Oops in its original
format (from ``dmesg``, etc). Ignore any references in this or other docs to
"decoding the Oops" or "running it through ksymoops".
If you post an Oops from 2.6+ that has been run through ``ksymoops``,
people will just tell you to repost it.
Where is the Oops message is located?
-------------------------------------
Normally the Oops text is read from the kernel buffers by klogd and
handed to ``syslogd`` which writes it to a syslog file, typically
``/var/log/messages`` (depends on ``/etc/syslog.conf``). On systems with
systemd, it may also be stored by the ``journald`` daemon, and accessed
by running ``journalctl`` command.
Sometimes ``klogd`` dies, in which case you can run ``dmesg > file`` to
read the data from the kernel buffers and save it. Or you can
``cat /proc/kmsg > file``, however you have to break in to stop the transfer,
``kmsg`` is a "never ending file".
If the machine has crashed so badly that you cannot enter commands or
the disk is not available then you have three options:
(1) Hand copy the text from the screen and type it in after the machine
has restarted. Messy but it is the only option if you have not
planned for a crash. Alternatively, you can take a picture of
the screen with a digital camera - not nice, but better than
nothing. If the messages scroll off the top of the console, you
may find that booting with a higher resolution (eg, ``vga=791``)
will allow you to read more of the text. (Caveat: This needs ``vesafb``,
so won't help for 'early' oopses)
(2) Boot with a serial console (see
:ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
run a null modem to a second machine and capture the output there
using your favourite communication program. Minicom works well.
(3) Use Kdump (see Documentation/kdump/kdump.txt),
extract the kernel ring buffer from old memory with using dmesg
gdbmacro in Documentation/kdump/gdbmacros.txt.
Finding the bug's location
--------------------------
Reporting a bug works best if you point the location of the bug at the
Kernel source file. There are two methods for doing that. Usually, using
``gdb`` is easier, but the Kernel should be pre-compiled with debug info.
gdb
^^^
The GNU debug (``gdb``) is the best way to figure out the exact file and line
number of the OOPS from the ``vmlinux`` file.
The usage of gdb works best on a kernel compiled with ``CONFIG_DEBUG_INFO``.
This can be set by running::
$ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
On a kernel compiled with ``CONFIG_DEBUG_INFO``, 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::
$ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
$ make vmlinux
$ gdb vmlinux
(gdb) l *vt_ioctl+0xda8
0x1888 is in vt_ioctl (drivers/tty/vt/vt_ioctl.c:293).
288 {
289 struct vc_data *vc = NULL;
290 int ret = 0;
291
292 console_lock();
293 if (VT_BUSY(vc_num))
294 ret = -EBUSY;
295 else if (vc_num)
296 vc = vc_deallocate(vc_num);
297 console_unlock();
or, if you want to be more verbose::
(gdb) p vt_ioctl
$1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl>
(gdb) l *0xae0+0xda8
You could, instead, use the object file::
$ make drivers/tty/
$ gdb drivers/tty/vt/vt_ioctl.o
(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 likely in the :jbd: module. You can load that module
in gdb and list the relevant code::
$ gdb fs/jbd/jbd.ko
(gdb) l *log_wait_commit+0xa3
.. note::
You can also do the same for any function call at the stack trace,
like this one::
[<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
The position where the above call happened can be seen with::
$ gdb drivers/media/usb/dvb-usb/dvb-usb.o
(gdb) l *dvb_usb_adapter_frontend_exit+0x3a
objdump
-------
^^^^^^^
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
@ -56,86 +230,140 @@ e.g. crash dump output as shown by Dave Miller::
mov 0x8(%ebp), %ebx ! %ebx = skb->sk
mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
gdb
---
Reporting the bug
-----------------
In addition, you can use GDB to figure out the exact file and line
number of the OOPS from the ``vmlinux`` file.
Once you find where the bug happened, by inspecting its location,
you could either try to fix it yourself or report it upstream.
The usage of gdb requires a kernel compiled with ``CONFIG_DEBUG_INFO``.
This can be set by running::
In order to report it upstream, you should identify the mailing list
used for the development of the affected code. This can be done by using
the ``get_maintainer.pl`` script.
$ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
For example, if you find a bug at the gspca's conex.c file, you can get
their maintainers with::
On a kernel compiled with ``CONFIG_DEBUG_INFO``, you can simply copy the
EIP value from the OOPS::
$ ./scripts/get_maintainer.pl -f drivers/media/usb/gspca/sonixj.c
Hans Verkuil <hverkuil@xs4all.nl> (odd fixer:GSPCA USB WEBCAM DRIVER,commit_signer:1/1=100%)
Mauro Carvalho Chehab <mchehab@kernel.org> (maintainer:MEDIA INPUT INFRASTRUCTURE (V4L/DVB),commit_signer:1/1=100%)
Tejun Heo <tj@kernel.org> (commit_signer:1/1=100%)
Bhaktipriya Shridhar <bhaktipriya96@gmail.com> (commit_signer:1/1=100%,authored:1/1=100%,added_lines:4/4=100%,removed_lines:9/9=100%)
linux-media@vger.kernel.org (open list:GSPCA USB WEBCAM DRIVER)
linux-kernel@vger.kernel.org (open list)
EIP: 0060:[<c021e50e>] Not tainted VLI
Please notice that it will point to:
And use GDB to translate that to human-readable form::
- The last developers that touched on the source code. On the above example,
Tejun and Bhaktipriya (in this specific case, none really envolved on the
development of this file);
- The driver maintainer (Hans Verkuil);
- The subsystem maintainer (Mauro Carvalho Chehab)
- The driver and/or subsystem mailing list (linux-media@vger.kernel.org);
- the Linux Kernel mailing list (linux-kernel@vger.kernel.org).
$ gdb vmlinux
(gdb) l *0xc021e50e
Usually, the fastest way to have your bug fixed is to report it to mailing
list used for the development of the code (linux-media ML) copying the driver maintainer (Hans).
If you don't have ``CONFIG_DEBUG_INFO`` enabled, you use the function
offset from the OOPS::
If you are totally stumped as to whom to send the report, and
``get_maintainer.pl`` didn't provide you anything useful, send it to
linux-kernel@vger.kernel.org.
EIP is at vt_ioctl+0xda8/0x1482
Thanks for your help in making Linux as stable as humanly possible.
And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled::
Fixing the bug
--------------
$ make vmlinux
$ gdb vmlinux
(gdb) l *vt_ioctl+0xda8
0x1888 is in vt_ioctl (drivers/tty/vt/vt_ioctl.c:293).
288 {
289 struct vc_data *vc = NULL;
290 int ret = 0;
291
292 console_lock();
293 if (VT_BUSY(vc_num))
294 ret = -EBUSY;
295 else if (vc_num)
296 vc = vc_deallocate(vc_num);
297 console_unlock();
If you know programming, you could help us by not only reporting the bug,
but also providing us with a solution. After all open source is about
sharing what you do and don't you want to be recognised for your genius?
or, if you want to be more verbose::
(gdb) p vt_ioctl
$1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl>
(gdb) l *0xae0+0xda8
You could, instead, use the object file::
$ make drivers/tty/
$ gdb drivers/tty/vt/vt_ioctl.o
(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 likely in the :jbd: module. You can load that module
in gdb and list the relevant code::
$ gdb fs/jbd/jbd.ko
(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?
If you decide to take this way, once you have worked out a fix please submit
it upstream.
Please do read
ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though
to help your code get accepted.
---------------------------------------------------------------------------
Notes on Oops tracing with ``klogd``
------------------------------------
In order to help Linus and the other kernel developers there has been
substantial support incorporated into ``klogd`` for processing protection
faults. In order to have full support for address resolution at least
version 1.3-pl3 of the ``sysklogd`` package should be used.
When a protection fault occurs the ``klogd`` daemon automatically
translates important addresses in the kernel log messages to their
symbolic equivalents. This translated kernel message is then
forwarded through whatever reporting mechanism ``klogd`` is using. The
protection fault message can be simply cut out of the message files
and forwarded to the kernel developers.
Two types of address resolution are performed by ``klogd``. The first is
static translation and the second is dynamic translation. Static
translation uses the System.map file in much the same manner that
ksymoops does. In order to do static translation the ``klogd`` daemon
must be able to find a system map file at daemon initialization time.
See the klogd man page for information on how ``klogd`` searches for map
files.
Dynamic address translation is important when kernel loadable modules
are being used. Since memory for kernel modules is allocated from the
kernel's dynamic memory pools there are no fixed locations for either
the start of the module or for functions and symbols in the module.
The kernel supports system calls which allow a program to determine
which modules are loaded and their location in memory. Using these
system calls the klogd daemon builds a symbol table which can be used
to debug a protection fault which occurs in a loadable kernel module.
At the very minimum klogd will provide the name of the module which
generated the protection fault. There may be additional symbolic
information available if the developer of the loadable module chose to
export symbol information from the module.
Since the kernel module environment can be dynamic there must be a
mechanism for notifying the ``klogd`` daemon when a change in module
environment occurs. There are command line options available which
allow klogd to signal the currently executing daemon that symbol
information should be refreshed. See the ``klogd`` manual page for more
information.
A patch is included with the sysklogd distribution which modifies the
``modules-2.0.0`` package to automatically signal klogd whenever a module
is loaded or unloaded. Applying this patch provides essentially
seamless support for debugging protection faults which occur with
kernel loadable modules.
The following is an example of a protection fault in a loadable module
processed by ``klogd``::
Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
Aug 29 09:51:01 blizard kernel: *pde = 00000000
Aug 29 09:51:01 blizard kernel: Oops: 0002
Aug 29 09:51:01 blizard kernel: CPU: 0
Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3
---------------------------------------------------------------------------
::
Dr. G.W. Wettstein Oncology Research Div. Computing Facility
Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com
820 4th St. N.
Fargo, ND 58122
Phone: 701-234-7556

1
Documentation/admin-guide/index.rst
View File

@ -27,7 +27,6 @@ problems and bugs in particular.
security-bugs
bug-hunting
bug-bisect
oops-tracing
tainted-kernels
ramoops
dynamic-debug-howto

241
Documentation/admin-guide/oops-tracing.rst
View File

@ -1,241 +0,0 @@
OOPS tracing
============
.. note::
``ksymoops`` is useless on 2.6 or upper. Please use the Oops in its original
format (from ``dmesg``, etc). Ignore any references in this or other docs to
"decoding the Oops" or "running it through ksymoops".
If you post an Oops from 2.6+ that has been run through ``ksymoops``,
people will just tell you to repost it.
Quick Summary
-------------
Find the Oops and send it to the maintainer of the kernel area that seems to be
involved with the problem. Don't worry too much about getting the wrong person.
If you are unsure send it to the person responsible for the code relevant to
what you were doing. If it occurs repeatably try and describe how to recreate
it. That's worth even more than the oops.
If you are totally stumped as to whom to send the report, send it to
linux-kernel@vger.kernel.org. Thanks for your help in making Linux as
stable as humanly possible.
Where is the Oops?
----------------------
Normally the Oops text is read from the kernel buffers by klogd and
handed to ``syslogd`` which writes it to a syslog file, typically
``/var/log/messages`` (depends on ``/etc/syslog.conf``). Sometimes ``klogd``
dies, in which case you can run ``dmesg > file`` to read the data from the
kernel buffers and save it. Or you can ``cat /proc/kmsg > file``, however you
have to break in to stop the transfer, ``kmsg`` is a "never ending file".
If the machine has crashed so badly that you cannot enter commands or
the disk is not available then you have three options :
(1) Hand copy the text from the screen and type it in after the machine
has restarted. Messy but it is the only option if you have not
planned for a crash. Alternatively, you can take a picture of
the screen with a digital camera - not nice, but better than
nothing. If the messages scroll off the top of the console, you
may find that booting with a higher resolution (eg, ``vga=791``)
will allow you to read more of the text. (Caveat: This needs ``vesafb``,
so won't help for 'early' oopses)
(2) Boot with a serial console (see
:ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
run a null modem to a second machine and capture the output there
using your favourite communication program. Minicom works well.
(3) Use Kdump (see Documentation/kdump/kdump.txt),
extract the kernel ring buffer from old memory with using dmesg
gdbmacro in Documentation/kdump/gdbmacros.txt.
Full Information
----------------
.. note::
the message from Linus below applies to 2.4 kernel. I have preserved it
for historical reasons, and because some of the information in it still
applies. Especially, please ignore any references to ksymoops.
::
From: Linus Torvalds <torvalds@osdl.org>
How to track down an Oops.. [originally a mail to linux-kernel]
The main trick is having 5 years of experience with those pesky oops
messages ;-)
Actually, there are things you can do that make this easier. I have two
separate approaches::
gdb /usr/src/linux/vmlinux
gdb> disassemble <offending_function>
That's the easy way to find the problem, at least if the bug-report is
well made (like this one was - run through ``ksymoops`` to get the
information of which function and the offset in the function that it
happened in).
Oh, it helps if the report happens on a kernel that is compiled with the
same compiler and similar setups.
The other thing to do is disassemble the "Code:" part of the bug report:
ksymoops will do this too with the correct tools, but if you don't have
the tools you can just do a silly program::
char str[] = "\xXX\xXX\xXX...";
main(){}
and compile it with ``gcc -g`` and then do ``disassemble str`` (where the ``XX``
stuff are the values reported by the Oops - you can just cut-and-paste
and do a replace of spaces to ``\x`` - that's what I do, as I'm too lazy
to write a program to automate this all).
Alternatively, you can use the shell script in ``scripts/decodecode``.
Its usage is::
decodecode < oops.txt
The hex bytes that follow "Code:" may (in some architectures) have a series
of bytes that precede the current instruction pointer as well as bytes at and
following the current instruction pointer. In some cases, one instruction
byte or word is surrounded by ``<>`` or ``()``, as in ``<86>`` or ``(f00d)``.
These ``<>`` or ``()`` markings indicate the current instruction pointer.
Example from i386, split into multiple lines for readability::
Code: f9 0f 8d f9 00 00 00 8d 42 0c e8 dd 26 11 c7 a1 60 ea 2b f9 8b 50 08 a1
64 ea 2b f9 8d 34 82 8b 1e 85 db 74 6d 8b 15 60 ea 2b f9 <8b> 43 04 39 42 54
7e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0
Finally, if you want to see where the code comes from, you can do::
cd /usr/src/linux
make fs/buffer.s # or whatever file the bug happened in
and then you get a better idea of what happens than with the gdb
disassembly.
Now, the trick is just then to combine all the data you have: the C
sources (and general knowledge of what it **should** do), the assembly
listing and the code disassembly (and additionally the register dump you
also get from the "oops" message - that can be useful to see **what** the
corrupted pointers were, and when you have the assembler listing you can
also match the other registers to whatever C expressions they were used
for).
Essentially, you just look at what doesn't match (in this case it was the
"Code" disassembly that didn't match with what the compiler generated).
Then you need to find out **why** they don't match. Often it's simple - you
see that the code uses a NULL pointer and then you look at the code and
wonder how the NULL pointer got there, and if it's a valid thing to do
you just check against it..
Now, if somebody gets the idea that this is time-consuming and requires
some small amount of concentration, you're right. Which is why I will
mostly just ignore any panic reports that don't have the symbol table
info etc looked up: it simply gets too hard to look it up (I have some
programs to search for specific patterns in the kernel code segment, and
sometimes I have been able to look up those kinds of panics too, but
that really requires pretty good knowledge of the kernel just to be able
to pick out the right sequences etc..)
**Sometimes** it happens that I just see the disassembled code sequence
from the panic, and I know immediately where it's coming from. That's when
I get worried that I've been doing this for too long ;-)
Linus
---------------------------------------------------------------------------
Notes on Oops tracing with ``klogd``
------------------------------------
In order to help Linus and the other kernel developers there has been
substantial support incorporated into ``klogd`` for processing protection
faults. In order to have full support for address resolution at least
version 1.3-pl3 of the ``sysklogd`` package should be used.
When a protection fault occurs the ``klogd`` daemon automatically
translates important addresses in the kernel log messages to their
symbolic equivalents. This translated kernel message is then
forwarded through whatever reporting mechanism ``klogd`` is using. The
protection fault message can be simply cut out of the message files
and forwarded to the kernel developers.
Two types of address resolution are performed by ``klogd``. The first is
static translation and the second is dynamic translation. Static
translation uses the System.map file in much the same manner that
ksymoops does. In order to do static translation the ``klogd`` daemon
must be able to find a system map file at daemon initialization time.
See the klogd man page for information on how ``klogd`` searches for map
files.
Dynamic address translation is important when kernel loadable modules
are being used. Since memory for kernel modules is allocated from the
kernel's dynamic memory pools there are no fixed locations for either
the start of the module or for functions and symbols in the module.
The kernel supports system calls which allow a program to determine
which modules are loaded and their location in memory. Using these
system calls the klogd daemon builds a symbol table which can be used
to debug a protection fault which occurs in a loadable kernel module.
At the very minimum klogd will provide the name of the module which
generated the protection fault. There may be additional symbolic
information available if the developer of the loadable module chose to
export symbol information from the module.
Since the kernel module environment can be dynamic there must be a
mechanism for notifying the ``klogd`` daemon when a change in module
environment occurs. There are command line options available which
allow klogd to signal the currently executing daemon that symbol
information should be refreshed. See the ``klogd`` manual page for more
information.
A patch is included with the sysklogd distribution which modifies the
``modules-2.0.0`` package to automatically signal klogd whenever a module
is loaded or unloaded. Applying this patch provides essentially
seamless support for debugging protection faults which occur with
kernel loadable modules.
The following is an example of a protection fault in a loadable module
processed by ``klogd``::
Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
Aug 29 09:51:01 blizard kernel: *pde = 00000000
Aug 29 09:51:01 blizard kernel: Oops: 0002
Aug 29 09:51:01 blizard kernel: CPU: 0
Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3
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::
Dr. G.W. Wettstein Oncology Research Div. Computing Facility
Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com
820 4th St. N.
Fargo, ND 58122
Phone: 701-234-7556
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