commit 6eeee81c8e59511962bdd83df5e7785bfdf871d2
Author: Tom Tromey <tromey@redhat.com>
Date:   Fri Nov 22 17:38:44 2013 +0000

    Detect infinite loop in value_fetch_lazy's lval_register handling.
    
    If value_fetch_lazy loops infinitely while unwrapping lval_register
    values, it means we either somehow ended up with two frames with the
    same ID in the frame chain, or some code is trying to unwind behind
    get_prev_frame's back (e.g., a frame unwind sniffer trying to unwind).
    In any case, it should always be an internal error to end up in this
    situation.
    
    This patch adds a check and throws an internal error if the same frame
    is returned.
    
    2013-11-22  Tom Tromey  <tromey@redhat.com>
    	    Pedro Alves  <palves@redhat.com>
    
    	PR backtrace/16155
    	* value.c (value_fetch_lazy): Internal error if
    	get_frame_register_value returns the same register.

Index: gdb-7.6.1/gdb/valops.c
===================================================================
--- gdb-7.6.1.orig/gdb/valops.c
+++ gdb-7.6.1/gdb/valops.c
@@ -1093,7 +1093,9 @@ value_fetch_lazy (struct value *val)
 
       while (VALUE_LVAL (new_val) == lval_register && value_lazy (new_val))
 	{
-	  frame = frame_find_by_id (VALUE_FRAME_ID (new_val));
+	  struct frame_id frame_id = VALUE_FRAME_ID (new_val);
+
+	  frame = frame_find_by_id (frame_id);
 	  regnum = VALUE_REGNUM (new_val);
 
 	  gdb_assert (frame != NULL);
@@ -1107,6 +1109,22 @@ value_fetch_lazy (struct value *val)
 						   regnum, type));
 
 	  new_val = get_frame_register_value (frame, regnum);
+
+	  /* If we get another lazy lval_register value, it means the
+	     register is found by reading it from the next frame.
+	     get_frame_register_value should never return a value with
+	     the frame id pointing to FRAME.  If it does, it means we
+	     either have two consecutive frames with the same frame id
+	     in the frame chain, or some code is trying to unwind
+	     behind get_prev_frame's back (e.g., a frame unwind
+	     sniffer trying to unwind), bypassing its validations.  In
+	     any case, it should always be an internal error to end up
+	     in this situation.  */
+	  if (VALUE_LVAL (new_val) == lval_register
+	      && value_lazy (new_val)
+	      && frame_id_eq (VALUE_FRAME_ID (new_val), frame_id))
+	    internal_error (__FILE__, __LINE__,
+			    _("infinite loop while fetching a register"));
 	}
 
       /* If it's still lazy (for instance, a saved register on the