[ob(ish)/committed] Fix SEGV in hppa_frame_cache

[Joel Brobecker <brobecker@gnat.com>]

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I was happily testing what I hoped would be the latest version of
the next/step patch replacing a complex condition by a frame ID
comparison, when I discovered that it caused a few problems on
HP/UX... But of course, HP/UX just got frame-ified!

Here is a description of the first problem I looked into:

        (in gdb.base)
        % gdb
        (gdb) file coremaker
        (gdb) core-file corefile
        (gdb) up
        *** SEGV ***

Ooops!

What happens is that we hit the following code in hppa_frame_cache():

        /* Yow! */
        u = find_unwind_entry (frame_func_unwind (next_frame));
        if (!u)
          return;

Unfortunately, that return causes the return value to be undefined.
And we later crash while trying to dereference this undefined value
in hppa_frame_this_id().

So I fixed it with the attached patch. This fixed 8 tests.
I didn't commit it to the 6.1 branch yet, as I wanted to wait for
Andrew's comments first. Don't want to disturb the branch too much.

There is also something that bothers me. If I understand this code well,
it looks like we are going to abort the unwinding as soon as we hit a
frame for which we can't find an associated function. Is that correct?
That would be very unfortunate, especially after we manage to install
the next/step patch I was testing; Once this patch is installed, the
chances us GDB trying to unwind from an unknown location will be more
important, no? If we don't know how to find our way out of there, then
the next/step machinery will be weakened. Andrew, if you confirm my
understanding is correct, I'll try to see if we can do better.

2004-03-04  J. Brobecker  <brobecker@gnat.com>

        * hppa-tdep.c (hppa_frame_cache): Avoid undefined return value.

Comments? Ok for the branch? (already committed in HEAD as, err,
obvious - well, at least we don't crash anymore :-)

Thanks,
-- 
Joel

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Index: hppa-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/hppa-tdep.c,v
retrieving revision 1.129
diff -u -p -r1.129 hppa-tdep.c
--- hppa-tdep.c	27 Feb 2004 21:47:53 -0000	1.129
+++ hppa-tdep.c	5 Mar 2004 04:46:32 -0000
@@ -4617,7 +4617,7 @@ hppa_frame_cache (struct frame_info *nex
   /* Yow! */
   u = find_unwind_entry (frame_func_unwind (next_frame));
   if (!u)
-    return;
+    return (*this_cache);
 
   /* Turn the Entry_GR field into a bitmask.  */
   saved_gr_mask = 0;