tty: clean up the tty time logic a bit

We only care if anything other than the lower 3 bits of the tty has
changed, so just check that way, which makes it a bit faster, and more
obvious what is going on.  Also, document this for future developers to
understand why we did this.
Reported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

drivers/tty/tty_io.c

@@ -1025,11 +1025,17 @@ void start_tty(struct tty_struct *tty)
 }
 EXPORT_SYMBOL(start_tty);
 
-/* We limit tty time update visibility to every 8 seconds or so. */
 static void tty_update_time(struct timespec *time)
 {
 	unsigned long sec = get_seconds();
-	if (abs(sec - time->tv_sec) & ~7)
+
+	/*
+	 * We only care if the two values differ in anything other than the
+	 * lower three bits (i.e every 8 seconds).  If so, then we can update
+	 * the time of the tty device, otherwise it could be construded as a
+	 * security leak to let userspace know the exact timing of the tty.
+	 */
+	if ((sec ^ time->tv_sec) & ~7)
 		time->tv_sec = sec;
 }