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

