[PATCH 3.16.y-ckt 090/165] tty: fix up atime/mtime mess, take four

[Luis Henriques]

3.16.7-ckt9 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jiri Slaby <jslaby at suse.cz>

commit f0bf0bd07943bfde8f5ac39a32664810a379c7d3 upstream.

This problem was taken care of three times already in
* b0de59b5733d18b0d1974a060860a8b5c1b36a2e (TTY: do not update
  atime/mtime on read/write),
* 37b7f3c76595e23257f61bd80b223de8658617ee (TTY: fix atime/mtime
  regression), and
* b0b885657b6c8ef63a46bc9299b2a7715d19acde (tty: fix up atime/mtime
  mess, take three)

But it still misses one point. As John Paul correctly points out, we
do not care about setting date. If somebody ever changes wall
time backwards (by mistake for example), tty timestamps are never
updated until the original wall time passes.

So check the absolute difference of times and if it large than "8
seconds or so", always update the time. That means we will update
immediatelly when changing time. Ergo, CAP_SYS_TIME can foul the
check, but it was always that way.

Thanks John for serving me this so nicely debugged.

Signed-off-by: Jiri Slaby <jslaby at suse.cz>
Reported-by: John Paul Perry <john_paul.perry at alcatel-lucent.com>
Acked-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/tty/tty_io.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index fd4095cd4bd6..3ab9cca090be 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -996,8 +996,8 @@ 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() & ~7;
-	if ((long)(sec - time->tv_sec) > 0)
+	unsigned long sec = get_seconds();
+	if (abs(sec - time->tv_sec) & ~7)
 		time->tv_sec = sec;
 }