* amd64-linux-nat.c (compat_int_t, compat_uptr_t, compat_time_t)

(compat_timer_t, compat_clock_t, struct compat_timeval)
	(compat_sigval_t, compat_siginfo_t): New types.
	(cpt_si_pid, cpt_si_uid, cpt_si_timerid, cpt_si_overrun)
	(cpt_si_status, cpt_si_utime, cpt_si_stime, cpt_si_ptr)
	(cpt_si_addr, cpt_si_band, cpt_si_fd): New defines.
	(compat_siginfo_from_siginfo, siginfo_from_compat_siginfo)
	(amd64_linux_siginfo_fixup): New.
	* linux-nat.c (linux_nat_siginfo_fixup): New.
	(siginfo_fixup): New.
	(linux_xfer_siginfo): Use siginfo_fixup to convert between the
	siginfo layout expected by ptrace and the siginfo layout of the
	inferior.
	(linux_nat_set_siginfo_fixup): New.
	* linux-nat.h (linux_nat_set_siginfo_fixup): Declare.

gdb/linux-nat.c

@@ -205,6 +205,13 @@ static struct target_ops linux_ops_saved;
 /* The method to call, if any, when a new thread is attached.  */
 static void (*linux_nat_new_thread) (ptid_t);
 
+/* The method to call, if any, when the siginfo object needs to be
+   converted between the layout returned by ptrace, and the layout in
+   the architecture of the inferior.  */
+static int (*linux_nat_siginfo_fixup) (struct siginfo *,
+				       gdb_byte *,
+				       int);
+
 /* The saved to_xfer_partial method, inherited from inf-ptrace.c.
    Called by our to_xfer_partial.  */
 static LONGEST (*super_xfer_partial) (struct target_ops *, 
@@ -3223,6 +3230,28 @@ linux_nat_mourn_inferior (struct target_ops *ops)
     linux_fork_mourn_inferior ();
 }
 
+/* Convert a native/host siginfo object, into/from the siginfo in the
+   layout of the inferiors' architecture.  */
+
+static void
+siginfo_fixup (struct siginfo *siginfo, gdb_byte *inf_siginfo, int direction)
+{
+  int done = 0;
+
+  if (linux_nat_siginfo_fixup != NULL)
+    done = linux_nat_siginfo_fixup (siginfo, inf_siginfo, direction);
+
+  /* If there was no callback, or the callback didn't do anything,
+     then just do a straight memcpy.  */
+  if (!done)
+    {
+      if (direction == 1)
+	memcpy (siginfo, inf_siginfo, sizeof (struct siginfo));
+      else
+	memcpy (inf_siginfo, siginfo, sizeof (struct siginfo));
+    }
+}
+
 static LONGEST
 linux_xfer_siginfo (struct target_ops *ops, enum target_object object,
                     const char *annex, gdb_byte *readbuf,
@@ -3232,6 +3261,7 @@ linux_xfer_siginfo (struct target_ops *ops, enum target_object object,
   LONGEST n;
   int pid;
   struct siginfo siginfo;
+  gdb_byte inf_siginfo[sizeof (struct siginfo)];
 
   gdb_assert (object == TARGET_OBJECT_SIGNAL_INFO);
   gdb_assert (readbuf || writebuf);
@@ -3248,14 +3278,26 @@ linux_xfer_siginfo (struct target_ops *ops, enum target_object object,
   if (errno != 0)
     return -1;
 
+  /* When GDB is built as a 64-bit application, ptrace writes into
+     SIGINFO an object with 64-bit layout.  Since debugging a 32-bit
+     inferior with a 64-bit GDB should look the same as debugging it
+     with a 32-bit GDB, we need to convert it.  GDB core always sees
+     the converted layout, so any read/write will have to be done
+     post-conversion.  */
+  siginfo_fixup (&siginfo, inf_siginfo, 0);
+
   if (offset + len > sizeof (siginfo))
     len = sizeof (siginfo) - offset;
 
   if (readbuf != NULL)
-    memcpy (readbuf, (char *)&siginfo + offset, len);
+    memcpy (readbuf, inf_siginfo + offset, len);
   else
     {
-      memcpy ((char *)&siginfo + offset, writebuf, len);
+      memcpy (inf_siginfo + offset, writebuf, len);
+
+      /* Convert back to ptrace layout before flushing it out.  */
+      siginfo_fixup (&siginfo, inf_siginfo, 1);
+
       errno = 0;
       ptrace (PTRACE_SETSIGINFO, pid, (PTRACE_TYPE_ARG3) 0, &siginfo);
       if (errno != 0)
@@ -4720,6 +4762,19 @@ linux_nat_set_new_thread (struct target_ops *t, void (*new_thread) (ptid_t))
   linux_nat_new_thread = new_thread;
 }
 
+/* Register a method that converts a siginfo object between the layout
+   that ptrace returns, and the layout in the architecture of the
+   inferior.  */
+void
+linux_nat_set_siginfo_fixup (struct target_ops *t,
+			     int (*siginfo_fixup) (struct siginfo *,
+						   gdb_byte *,
+						   int))
+{
+  /* Save the pointer.  */
+  linux_nat_siginfo_fixup = siginfo_fixup;
+}
+
 /* Return the saved siginfo associated with PTID.  */
 struct siginfo *
 linux_nat_get_siginfo (ptid_t ptid)