Re: Strange segfaults of gdb

Michal Ludvig
2002-04-12T11:27:22Z

Eli Zaretskii wrote:
>>It doesn't matter which program I run, what I want to print and if I
>>then want invoke 'run', 'continue' or even 'si'. It segfaults. Core file
>>doesn't give any reasonable informations.
>
> You mean, you cannot even tell from the core file where (inside what
> function) GDB crashes? That'd be very strange indeed--what could
> prevent you from getting att his information? Is the core file
> corrupt or something?

I can see the same information as if I run gdb from gdb. Anyway I treat
them incorrect [see below].

> 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
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.
So I don't trust this information.
Or do you have an idea how to interpret it? I don't say it's a bug in
the gdb - it may be in the kernel, glibc or gcc as well, but everything
else seems to work. Only gdb doesn't...
May this be a memory corruption problem on the gdb side (perhaps it
passes a wrong address to the syscall)? I'll try to use ElectricFence to
see what happens.

Is there somewhere a tutorial on how to examine/compare core files
generated by gcore command? What should I look for?

It's somehow difficult to debug a broken debugger using a broken
debugger :-((

Michal Ludvig
--
* SuSE CR, s.r.o * mludvig@suse.cz
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