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
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