Re: Question about core files

[Glynn Clements <glynn@gclements.plus.com>]

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.

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.

-- 
Glynn Clements <glynn@gclements.plus.com>