Unwinding CFI gcc practice of assumed `same value' regs

[Michael Matz]

Hi,

On Mon, 11 Dec 2006, Jan Kratochvil wrote:

> currently (on x86_64) the gdb backtrace does not properly stop at the outermost
> frame:
> 
> #3  0x00000036ddb0610a in start_thread () from /lib64/tls/libpthread.so.0
> #4  0x00000036dd0c68c3 in clone () from /lib64/tls/libc.so.6
> #5  0x0000000000000000 in ?? ()
> 
> Currently it relies only on clearing %rbp (0x0000000000000000 above is
> unrelated to it, it got read from uninitialized memory).
> 
> http://sourceware.org/ml/gdb/2004-08/msg00060.html suggests frame 
> pointer 0x0 should be enough for a debugger not finding CFI to stop 
> unwinding, still it is a heuristic.  In the -fno-frame-pointer compiled 
> code there is no indication the frame pointer register became a regular 
> one and 0x0 is its valid value.

Right.  Unwinding through functions (without frame pointer) requires CFI.  
If there is CFI for a function the unwinder must not look at %rbp for stop 
condition.  If there's no CFI for a function it can't be unwound (strictly 
per ABI).  If one relaxes that and wants to unwind through CFI-less 
functions it has to have a frame pointer.  In that case zero in that frame 
pointer could indicate the outermost frame (_if_ the suggestion in the ABI 
is adhered to, which noone is required to).


Ciao,
Michael.