Re: Unwinding CFI gcc practice of assumed `same value' regs

[Andrew Haley <aph at redhat dot com>]

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.