Re: 5.1, Data Corruption, Intel, Oh my! [patch] - Fatal trap 12

On Tue, 2003-08-12 at 12:52, Terry Lambert wrote:
> Bosko Milekic wrote:
> > > db> trace
> > > _mtx_lock_flags(0,0,c07aa287,11e,c0c21aaa) at _mtx_lock_flags+0x43
> > > vm_fault(c102f000,c0000000,2,0,c08205c0) at vm_fault+0x2b4
> > > trap_pfault(c0c21b9e,0,c00004d8,100000,c00004d8) at trap_pfault+0x152
> > > trap(6c200018,10,1bc40060,1c,0) at trap+0x30d
> > > calltrap() at calltrap+0x5
> > > --- trap 0xc, eip = 0x5949, esp = 0xc0c21bde, dbp = 0xc0c21be4 ---
> > > (null)(1bf80058,0,530e0102,80202,505a61) at 0x5949
> > > db>
> 
> ... He might also want to look for any function pointer
> that takes 5 arguments; 

Nice tactic, but misleading in this case, methinks.

I assume your basing this on the 5 arguments shown in the backtrace.
The 5 arguments passed to the "function" at 0x5949 is probably just
defaulted; I doubt it has any significance.

Long version:

ddb tries to work out the number of arguments passed to a function at a
particular stack frame first based on symbolic information for the
function itself (obviously not an option here), then based on the
instruction at the return address in that frame. This works at best
sporadically in the face of -O compiled C code. The fact that there's no
function under the "(null)" would strongly suggest that ddb got confused
with the frame pointer here and didn't get any useful information with
which to work out the argument count.

In the face of failure, ddb just wildly prints out the 5 words under the
stack pointer.

Given that there's no real function at 0x5949, the stack frame won't
have been set up at all, the frame pointer is still pointing to the
caller's frame, which could be foobar anyway.

What can be useful is to print out the values on the stack symbolically.
(in gdb,  p/a ((void **)$sp)[0]_at_100. I'm sure ddb can do something
similar, but no idea how...). And hope to find the caller's return
address lying in the output.

HTH,
Peter.