path: root/Documentation/oops-tracing.txt

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/serial-console.txt <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

---------------------------------------------------------------------------

::

  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


---------------------------------------------------------------------------

Tainted kernels
---------------

Some oops reports contain the string **'Tainted: '** after the program
counter. This indicates that the kernel has been tainted by some
mechanism.  The string is followed by a series of position-sensitive
characters, each representing a particular tainted value.

  1) 'G' if all modules loaded have a GPL or compatible license, 'P' if
     any proprietary module has been loaded.  Modules without a
     MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
     insmod as GPL compatible are assumed to be proprietary.

  2) ``F`` if any module was force loaded by ``insmod -f``, ``' '`` if all
     modules were loaded normally.

  3) ``S`` if the oops occurred on an SMP kernel running on hardware that
     hasn't been certified as safe to run multiprocessor.
     Currently this occurs only on various Athlons that are not
     SMP capable.

  4) ``R`` if a module was force unloaded by ``rmmod -f``, ``' '`` if all
     modules were unloaded normally.

  5) ``M`` if any processor has reported a Machine Check Exception,
     ``' '`` if no Machine Check Exceptions have occurred.

  6) ``B`` if a page-release function has found a bad page reference or
     some unexpected page flags.

  7) ``U`` if a user or user application specifically requested that the
     Tainted flag be set, ``' '`` otherwise.

  8) ``D`` if the kernel has died recently, i.e. there was an OOPS or BUG.

  9) ``A`` if the ACPI table has been overridden.

 10) ``W`` if a warning has previously been issued by the kernel.
     (Though some warnings may set more specific taint flags.)

 11) ``C`` if a staging driver has been loaded.

 12) ``I`` if the kernel is working around a severe bug in the platform
     firmware (BIOS or similar).

 13) ``O`` if an externally-built ("out-of-tree") module has been loaded.

 14) ``E`` if an unsigned module has been loaded in a kernel supporting
     module signature.

 15) ``L`` if a soft lockup has previously occurred on the system.

 16) ``K`` if the kernel has been live patched.

The primary reason for the **'Tainted: '** string is to tell kernel
debuggers if this is a clean kernel or if anything unusual has
occurred.  Tainting is permanent: even if an offending module is
unloaded, the tainted value remains to indicate that the kernel is not
trustworthy.