ARM: 6654/1: perf/oprofile: fix off-by-one in stack check

Since tail is the previous fp - 1, we need to compare the new fp with tail + 1
to ensure that we don't end up passing in the same tail again, in order to
avoid a potential infinite loop in the perf interrupt handler (which has been
observed to occur).  A similar fix seems to be needed in the OProfile code.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/perf_event.c

@@ -700,7 +700,7 @@ user_backtrace(struct frame_tail __user *tail,
 	 * Frame pointers should strictly progress back up the stack
 	 * (towards higher addresses).
 	 */
-	if (tail >= buftail.fp)
+	if (tail + 1 >= buftail.fp)
 		return NULL;
 
 	return buftail.fp - 1;