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

[Michael Matz <matz@suse.de>]

Hi,

On Tue, 12 Dec 2006, Andrew Haley wrote:

>  > > 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.
>  > 
>  > I don't see that as feasible.  If %ebp/%rbp may be used as a general 
>  > callee-saved register, then it can hold any value.
> 
> Sure, we already know that, as has been clear.  The question is *if* 
> %rbp may be used as a general callee-saved register that can hold any 
> value.

Yes of course it was meant to be used such.  The ABI actually only gives a 
recommendation that %rbp should be zero in the outermost frame, it's not a 
must.  The ABI _requires_ proper .eh_frame descriptors when unwinding is 
desired; so it's useless (and wrong) for any unwinder to look at %rbp and 
determine if it should stop.

Alternatively (though not sanctioned by the ABI) all functions through 
which unwinding is desired but for which no unwind info is created _have_ 
to use %rbp as frame pointer and not as general register.  In that case 
the zeroing of %rbp would be a usable stop condition for functions without 
unwind info.  But that's already outside the ABI.


Ciao,
Michael.