[patch] More suggestive displaced-stepping memory error message

[Jan Kratochvil <jan.kratochvil@redhat.com>]

Hi,

with
	gdbserver ld-linux-x86-64.so.2 /bin/prog
and
	gdb /bin/prog
in non-stop mode during ld.so initialization happens an error as
displaced-stepping buffer at /bin/prog's _start is not yet available.

Also I think in single-threaded mode displaced-stepping is not useful.
But I understand turning off displaced-stepping if inferior has only one
thread would be too complicating all the troubleshooting.

With all the races and errors I would definitely find this message helpful.
It should at least state the error occured for the displaced-stepping buffer,
not just generic
	Cannot access memory at address %s

No regressions on {x86_64,x86_64-m32,i686}-fedora17-linux-gnu.
I may check it in on my own without the "you may want .*" part.


Thanks,
Jan


gdb/
2012-04-10  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Provide more specific displaced-stepping memory error message.
	* infrun.c (displaced_step_prepare): New variable status.  Call
	target_read_memory instead of read_memory, provide more specific
	error message.

--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1296,6 +1296,7 @@ displaced_step_prepare (ptid_t ptid)
   ULONGEST len;
   struct displaced_step_closure *closure;
   struct displaced_step_inferior_state *displaced;
+  int status;
 
   /* We should never reach this function if the architecture does not
      support displaced stepping.  */
@@ -1356,7 +1357,13 @@ displaced_step_prepare (ptid_t ptid)
   displaced->step_saved_copy = xmalloc (len);
   ignore_cleanups = make_cleanup (free_current_contents,
 				  &displaced->step_saved_copy);
-  read_memory (copy, displaced->step_saved_copy, len);
+  status = target_read_memory (copy, displaced->step_saved_copy, len);
+  if (status != 0)
+    throw_error (MEMORY_ERROR,
+		 _("Error accessing memory address %s (%s) for "
+		   "displaced-stepping buffer, you may want to temporarily "
+		   "turn it off if there is only one inferior thread."),
+		 paddress (target_gdbarch, copy), safe_strerror (status));
   if (debug_displaced)
     {
       fprintf_unfiltered (gdb_stdlog, "displaced: saved %s: ",