When does gdb show something 'on the stack'?

Question

With the following assembly:

mov $2, %rax
push %rcx
call func

The 'stack' section in gdb looks like this:

─── Stack ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
[0] from 0x0000000000400109 in func
[1] from 0x00000000004000be in _start

And it doesn't matter whether I add or remove the push %rcx instruction. Why does gdb only show the 'function address stack', and not show any modifications to the stack when manually pushed? That is, both push %rcx and call func change the stack register rsp but only the latter actually changes what gdb shows in the "stack" section.

Could someone please explain the reason for this?

That's some custom gdb modification and it prints the call stack, otherwise known as backtrace. It only shows the nested function calls. To look at the actual stack do something like x/8x $rsp — Jester
@Jester: Yes, but how does it figure out which qwords on the stack are from call and which are from push, for hand-written asm without debug info? I'd guess it might look for possible return addresses, i.e. values that point into the text section? Or maybe the OP isn't showing us a complete enough full picture. — Peter Cordes

Employed Russian Employed Russian · Accepted Answer · 2020-08-30T03:15:23

The 'stack' section in gdb looks like this:

The 'stack' here stands for 'call stack', not 'stack memory'.

When does gdb show something 'on the stack'?

1 Answers