Re: [rfa] Attach vsyscall support for GNU/Linux

[Mark Kettenis <kettenis@gnu.org>]

   Date: Mon, 1 Nov 2004 11:15:52 -0500
   From: Daniel Jacobowitz <drow@false.org>

   On Tue, Oct 26, 2004 at 12:12:26AM +0200, Mark Kettenis wrote:
   >    Date: Sun, 24 Oct 2004 19:16:36 -0400
   >    From: Daniel Jacobowitz <drow@false.org>
   > 
   > Thanks for the explanation Daniel!
   > 
   >    Unwinding bugs aside, I think it's valuable for GDB to know that it is
   >    at a signal trampoline.  I think the custom display in backtrace is
   >    valuable.  That means we should look for signal handlers before looking
   >    for CFI.  For NPTL that value judgement would fall the other way - not
   >    having to special-case signal handlers is a clear win.
   > 
   > Given the fact that it is desirable for GDB to know that it's dealing
   > with a signal handler, I think the correct approach is to extend the
   > DWARF2 unwinder with a method to get the frame type, similar to what
   > we already do for pre-initializing the register state.
   > 
   > I'll see if I can come up with a patch for you to test.

   Had any time for this, Mark?  If not, I can try it.

I forgot about it :-(.  I had a look just now, but unfortunately it's
not as easy as I thought.  The frame type is currently hard-coded in
the unwinder.  This is wrong, but Andrew thinks it's wrong in a
different way than I.  At least, that's what I think.  I'll have to
learn reading UML diagrams first.  I'll throw some Feynman diagrams
into my next mail to get even with him ;-).  This is not going to be a
simple fix, therefore...

   I think this patch is very important for 6.3.

...you might want to convince Andrew to include your origional patch
in 6.3.  Or better yet, a patch that prepends a signal frame unwinder
only for Linux.  If you succeed, you have my blessing to check this in
on mainline too, provided you add a big fat warning why this is done
and that it's so wrong.

Sorry,

Mark