Unwinding CFI gcc practice of assumed `same value' regs

[Andrew Haley]

Jan Kratochvil writes:

 > 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).

That's how it's defined to work: %rbp is zero.  

 > 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.

Not by my understanding it isn't.  It's set up by the runtime system,
and 0 (i.e. NULL on x86-64) marks the end of the stack.  Officially.

See page 28, AMD64 ABI Draft 0.98 \u2013 September 27, 2006 -- 9:24.

Andrew.