Unwinding CFI gcc practice of assumed `same value' regs

[Mark Kettenis]

>  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.

Unfortunately whoever wrote that down didn't think it through.  In
Figure 3.4 on page 20, %rbp is listed as "callee-saved register;
optionally used as frame pointer".  So %rbp can be used for anything, as
long as you save its contents and restore it before you return.  Since it
may be used for anything, it may contain 0 at any point in the middle of
the call stack.  So it is unusable as a stack trace termination condition.
The only viable option is explicitly marking it as such in the CFI.

Initializing %rbp to 0 in the outermost frame is sort of pointless on amd64.