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Re: Unwinding CFI gcc practice of assumed `same value' regs
- From: Andrew Haley <aph at redhat dot com>
- To: Jan Kratochvil <jan dot kratochvil at redhat dot com>
- Cc: gcc at gcc dot gnu dot org, libc-alpha at sources dot redhat dot com, gdb at sourceware dot org, Jakub Jelinek <jakub at redhat dot com>, Richard Henderson <rth at redhat dot com>
- Date: Tue, 12 Dec 2006 13:54:46 +0000
- Subject: Re: Unwinding CFI gcc practice of assumed `same value' regs
- References: <20061211190300.GA4372@host0.dyn.jankratochvil.net>
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.