Tracing call stack in disassembled code - c++

Question

I am trying to debug a tricky core dump (from an -O2 optimized binary).
// Caller Function
void caller(Container* c)
{
std::list < Message*> msgs;
if(!decoder.called(c->buf_, msgs))
{
....
.....
}
// Called Function
bool
Decoder::called(Buffer* buf, list < Message*>& msgs)
{
add_data(buf); // Inlined code to append buf to decoders buf chain
while(m_data_in && m_data_in->length() > 0)
{
.....
}
}
In both caller and the callee, the first argument is optimized out, that means it must be somewhere in the register.
Caller Disassembly:
push %r15
mov %rdi,%r15
push %r14
push %r13
push %r12
push %rbp
push %rbx
sub $0x68,%rsp
test %rsi,%rsi
je 0x8ccd62
cmpq $0x0,(%rsi)
je 0x8ccd62
lea 0x40(%rsp),%rax
lea 0x1b8(%rdi),%rdi
mov %rax,(%rsp)
mov %rax,0x40(%rsp)
mov %rax,%rdx
mov %rax,0x48(%rsp)
mov (%rsi),%rsi
callq 0x8cc820
Caller Register Info:
rax 0x7fbfffc7e0 548682057696
rbx 0x2a97905ba0 182931446688
rcx 0x0 0
rdx 0x2 2
rsi 0x1 1
rdi 0x7fbfffc7e2 548682057698
rbp 0x4f 0x4f
rsp 0x7fbfffc870 0x7fbfffc870
r8 0x40 64
r9 0x20 32
r10 0x7fbfffc7e0 548682057696
r11 0x2abe466600 183580911104
r12 0x7fbfffd910 548682062096 // THIS IS HOLDING buf_
r13 0x7fbfffdec0 548682063552
r14 0x5dc 1500
r15 0x2a97905ba0 182931446688
rip 0x8cca89 0x8cca89
eflags 0x206 [ PF IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
Called function Disassembly:
push %r14
push %r13
mov %rdx,%r13
push %r12
mov %rdi,%r12
push %rbp
push %rbx
sub $0x10,%rsp
mov 0x8(%rdi),%rdx
test %rdx,%rdx
jne 0x8cc843
jmpq 0x8cc9cb
mov %rax,%rdx
mov 0x8(%rdx),%rax
test %rax,%rax
mov %rsi,0x8(%rdx)
mov 0x8(%r12),%rax
test %rax,%rax
xor %edx,%edx
add 0x4(%rax),%edx
mov 0x8(%rax),%rax
lea 0x8(%rsp),%rsi
mov %r12,%rdi
movq $0x0,0x8(%rsp)
Called function Register Info :
rax 0x7fbfffc7e0 548682057696
rbx 0x2abc49f9c0 183547591104
rcx 0x0 0
rdx 0x2 2
rsi 0x1 1
rdi 0x7fbfffc7e2 548682057698
rbp 0xffffffff 0xffffffff
rsp 0x7fbfffc830 0x7fbfffc830
r8 0x40 64
r9 0x20 32
r10 0x7fbfffc7e0 548682057696
r11 0x2abe466600 183580911104
r12 0x2a97905d58 182931447128
r13 0x7fbfffc8b0 548682057904
r14 0x5dc 1500
r15 0x2a97905ba0 182931446688
rip 0x8cc88a 0x8cc88a
eflags 0x206 [ PF IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
The issue is, in the called function, it appears that "add_data" function achieved nothing.
So, wanted to know whether in disassembly of called function, do we see the "buf_" pointer being used anywhere (Register r12 in callee function).
I do understand assembly to some level, but all those code inlining has left me confused.
Would appreciate some help in demistifying called function disassembly.
UPDATE:
add_data does below:
if (m_data_in) {
m_data_in->next = data;
} else {
m_data_in = data;
}

Answer 1

This looks like if (m_data_in)
mov 0x8(%rdi),%rdx
test %rdx,%rdx
test %rdx,%rdx
jne 0x8cc843
jmpq 0x8cc9cb
Now, I don't quite know where 0x8cc843 and 0x8cc9cb are located in your code, so can't really follow the code further. There is still not enough code & information to say exactly what is going on in the original question. I'm happy to fill in more of this answer if more information is provided.