Re: Strange segfaults of gdb

Eli Zaretskii
2002-04-12T12:05:48Z

> Date: Fri, 12 Apr 2002 13:27:14 +0200
> From: Michal Ludvig <mludvig@suse.cz>
>
> > What if you run GDB under another GDB--can you see where does the
> > subordinate GDB crash then?
>
> (gdb) p 1
> $1 = 1
> (gdb) r
> Starting program: /root/mludvig/tst/xmmtest
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x2a95ae759c in wait4 () at soinit.c:76
> 76 }
> (top-gdb) disassemble 0x2a95ae759c

It's more useful to type "bt" at this point. Then you will know what
kind of code in GDB called wait4.

> Dump of assembler code for function wait4:
> 0x2a95ae7590 <wait4>: mov %rcx,%r10
> 0x2a95ae7593 <wait4+3>: mov $0x3d,%rax
> 0x2a95ae759a <wait4+10>: syscall
> 0x2a95ae759c <wait4+12>: cmp $0xfffffffffffff001,%rax
> 0x2a95ae75a2 <wait4+18>: jae 0x2a95ae75a5 <wait4+21>
> 0x2a95ae75a4 <wait4+20>: retq
> 0x2a95ae75a5 <wait4+21>: xor %rdx,%rdx
> 0x2a95ae75a8 <wait4+24>: sub %rax,%rdx
> 0x2a95ae75ab <wait4+27>: push %rdx
> 0x2a95ae75ac <wait4+28>: callq 0x2a95a6fa30 <key+145504>
> 0x2a95ae75b1 <wait4+33>: pop %rdx
> 0x2a95ae75b2 <wait4+34>: mov %rdx,(%rax)
> 0x2a95ae75b5 <wait4+37>: or $0xffffffffffffffff,%rax
> 0x2a95ae75b9 <wait4+41>: jmp 0x2a95ae75a4 <wait4+20>
> 0x2a95ae75bb <wait4+43>: nop
> 0x2a95ae75bc <wait4+44>: nop
> 0x2a95ae75bd <wait4+45>: nop
> 0x2a95ae75be <wait4+46>: nop
> 0x2a95ae75bf <wait4+47>: nop
> End of assembler dump.
>

> So it appears like the segfault happend on 'cmp <imm>,<reg>'
> instruction, which shouldn't be able to generate any exception at all.

I suspect that what crashed is the syscall instruction before that:

> Dump of assembler code for function wait4:
> 0x2a95ae7590 <wait4>: mov %rcx,%r10
> 0x2a95ae7593 <wait4+3>: mov $0x3d,%rax
> 0x2a95ae759a <wait4+10>: syscall
> 0x2a95ae759c <wait4+12>: cmp $0xfffffffffffff001,%rax

It is also possible that the stack is somehow blown up, which would
explain why the first instruction after a syscall return crashes.