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Re: Unwinding CFI gcc practice of assumed `same value' regs
Mark Kettenis writes:
> > 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.
Null-terminating the call stack is too well-established practice to be
changed now.
In practice, %ebp either points to a call frame -- not necessarily the
most recent one -- or is null. I don't think that having an optional
frame pointer mees you can use %ebp for anything random at all, but we
need to make a clarification request of the ABI.
> 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.
The right way to fix the ABI is to specify that %ebp mustn't be
[mis]used in this way, not to add a bunch more unwinder data.
Andrew.