From: Linus Torvalds <torvalds@cs.helsinki.fi>

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 (and new version of 
ksymoops), 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).

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
