Re: Frame lost after 'leave' and before 'ret' instruction on i386?

[Mark Kettenis <mark dot kettenis at xs4all dot nl>]

> Date: Fri, 16 May 2008 11:28:20 -0400
> From: Aleksandar Ristovski <aristovski@qnx.com>
> 
> Hello,
> 
> I noticed that after 'leave' asm instruction (and before 'ret') one frame will be lost (skipped).
> 
> For example:
> 
> (gdb) disassemble doSth
> Dump of assembler code for function doSth:
> 0x080484f4 <doSth+0>:   push   %ebp
> 0x080484f5 <doSth+1>:   mov    %esp,%ebp
> 0x080484f7 <doSth+3>:   sub    $0x8,%esp
> 0x080484fa <doSth+6>:   sub    $0xc,%esp
> 0x080484fd <doSth+9>:   push   $0x8048570
> 0x08048502 <doSth+14>:  call   0x8048348 <printf@plt>
> 0x08048507 <doSth+19>:  add    $0x10,%esp
> 0x0804850a <doSth+22>:  leave
> 0x0804850b <doSth+23>:  ret
> End of assembler dump.
> (gdb) info registers
> ...
> eip            0x804850b        0x804850b <doSth+23>
> ...
> (gdb) bt
> #0  0x080483c3 in doSth () at main.c:6
> #1  0xb7ebffdc in __libc_start_main () from /lib/libc.so.6
> #2  0x08048331 in _start ()
> 
> (note 'main' that really made the call is missing... e.g., just before "leave" was executed:
> (gdb) bt
> #0  doSth () at main.c:6
> #1  0x080483da in main () at main.c:10
> )
> 
> What happens is, we have restored the stack pointer but gdb is unaware of that fact and tries to unwind using already unwound value...
> 
> how is this supposed to work?

The compiler is supposed to give us unwind info for the function's
epilogue, but unfortunately GCC doesn't.  There's not much we can do
about it in GDB.  Fortunately it isnt an issue that users see a lot.