Re: [RFC] Infinite backtraces...

[Andrew Cagney <cagney@gnu.org>]

>>The other thing that would help here is for glibc's CFI to identify the 
>>return-address (and CFA) column as unknown (assuming I've got my CFI 
>>term correct) on the outer most frame.  It would then be easy for 
>>dwarf2-unwind to identify this.  It's been discussed, agreed, but not 
>>implemented.
> 
> 
> This is of course a good solution, provided that you can use dwarf2.
> On 32bit HP/UX, we're stuck.

Doh!

>>Right, but it shouldn't need an additional method.  The per-architecture 
>>unwinder, when it detects a frame that the ABI specifies as final, 
>>should return a null frame ID.  For instance, the PPC ABI explicitly 
>>specifies that it's stack be terminated with a zero SP.
> 
> 
> I am not sure this is doable. Is it? Let me check that again. Perhaps
> it's ok to create the frame object, but then later compute a null frame
> ID for it? As far as I remember, the sequence of events is like this
> when trying to build the frame chain:
> 
>   . get_prev_frame (this_frame):
>       . get_frame_id (this_frame)
>           . frame_id (next_frame, this_cache)
>       . check this frame ID
>       . build previous frame
>         (frame ID unset)

Yes, gdb already creates the frame object and then later constructs the 
frame ID.  The build has been reduced to:

	prev_frame = malloc ();
	prev_frame->next = this_frame;
	return prev_frame;

Also note that, unlike the past, the frame ID is separate to the ``frame 
base'' the former can be NULL while the later is still valid.

> And then, after building each new frame, we display the information
> for that new frame.

It is what lets us "up" on to an apparently corrupt frame.

>>Finally, a more long term suggestion is that we add a mechanism for
>>creating or adding attributes to symbols (for instance for signal
>>trampolines).  An atribute of such a symbol could be that it is
>>outermost.
> 
> 
> But could we determine that a symbol is outermost. And couldn't the
> same symbol be used in both contexts?

Determining that a symbol is outer-most is somewhat osabi dependant - 
the arch dependant code would need to register or mark it up as such.

Yes, the symbol can be used in both contexts vis:

	main (argc) { if (argc > 0) main (argc - 1); }

but I suspect it is illegal.

Andrew