[rfa/mips] Stop backtraces when we've lost the PC

Daniel Jacobowitz
2004-03-06T23:17:44Z

Here's an updated version of a little hack I've been using since GDB 6.0.
If we are in a nested normal frame, i.e. something whose next frame is a
function that it called in the normal way, and we didn't find a saved PC,
we're going to be stuck in a loop. We might have been able to figure out
the frame size, but not where the return address was stored; as the comment
says, this happens in glibc's clone function. Of course the problem there
is that it _doesn't_ save $ra in the normal fashion; it won't return.

Without this patch schedlock.exp falls apart, because backtraces continue
forever printing "clone()" on every line.

OK? Or a better way to do this?

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer

2004-03-06 Daniel Jacobowitz <drow@mvista.com>

* mips-tdep.c (mips_mdebug_frame_this_id): Terminate unwinding if
we haven't found a saved PC.

Index: mips-tdep.c
===================================================================
RCS file: /big/fsf/rsync/src-cvs/src/gdb/mips-tdep.c,v
retrieving revision 1.283
diff -u -p -r1.283 mips-tdep.c
--- mips-tdep.c 17 Feb 2004 15:21:21 -0000 1.283
+++ mips-tdep.c 6 Mar 2004 22:52:29 -0000
@@ -1672,6 +1672,24 @@ mips_mdebug_frame_this_id (struct frame_
{
struct mips_frame_cache *info = mips_mdebug_frame_cache (next_frame,
this_cache);
+
+ /* If the return address is not saved for two frames in a row,
+ then we are probably hosed. Not necessarily - it's possible to
+ write working assembly that violates this rule - but we can't
+ backtrace through that either. Eventually MIPS will support
+ DWARF2 unwind information, allowing assembly programmers to
+ avoid this problem.
+
+ One place this check triggers is in the GNU/Linux clone syscall
+ wrapper. */
+ if (frame_relative_level (next_frame) >= 0
+ && get_frame_type (next_frame) == NORMAL_FRAME
+ && !trad_frame_addr_p (info->saved_regs, NUM_REGS + PC_REGNUM))
+ {
+ (*this_id) = null_frame_id;
+ return;
+ }
+
(*this_id) = frame_id_build (info->base, frame_func_unwind (next_frame));
}