Re: Question about core files

On Sat, 10 Oct 2009, Glynn Clements wrote:

>
> Holger Kiehl wrote:
>
>> How can I find which register is fsa_pos?
>
> fsa_pos is a parameter, and doesn't appear to be changed within the
> function, so I would expect "print fsa_pos" to give the correct value.
>
Unfortunately not:

    (gdb)
    #4  0x0000000000404b5f in start_process ()
    (gdb) print fsa_pos
    No symbol "fsa_pos" in current context.

> AFAICT, the following portion of the disassembly:
>
>    0x0000000000404b49 <start_process+121>: movslq 0x24(%rsp),%rax
>    0x0000000000404b4e <start_process+126>: imul   $0x8f8,%rax,%r14
>    0x0000000000404b55 <start_process+133>: mov    %r14,%rax
>    0x0000000000404b58 <start_process+136>: add    0x225441(%rip),%rax        # 0x629fa0 <fsa>
>    0x0000000000404b5f <start_process+143>: mov    0xec(%rax),%edx
>    0x0000000000404b65 <start_process+149>: test   $0x1,%dl
>
> corresponds to the expression
>
> 	fsa[fsa_pos].host_status & DO_NOT_DELETE_DATA
>
> 0x24(%rsp) is fsa_pos, $0x8f8 (2296) is the size of each element of
> fsa[], 0x225441(%rip) is fsa, 0xec is the offset of the host_status
> field.
>
> So:
>
>    movslq 0x24(%rsp),%rax	# %rax = fsa_pos
>    imul   $0x8f8,%rax,%r14	# %r14 = fsa_pos * sizeof(fsa[i]) = &fsa[fsa_pos] - &fsa[0]
>    mov    %r14,%rax		# %rax = &fsa[fsa_pos] - &fsa[0]
>    add    0x225441(%rip),%rax	# %rax = &fsa[fsa_pos]
>    mov    0xec(%rax),%edx      # %edx = fsa[fsa_pos].host_status
>
> Based upon this, %r14 should contain fsa_pos * 2296, so:
>
>>     (gdb) info registers
>>     r14            0xfffffffffffff708       -2296
>
> Which suggests that fsa_pos is -1.
>
Many thanks for this very detailed explanation and confirming that
fsa_pos was indeed -1. I would never have thought that one could find
so much information of a core from an optimized binary without debug
information.

Thanks to you and Manish Katiyar for this valuable help!

Regards,
Holger