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: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 3949d95..e569546 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/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;
 }