tty: fix up atime/mtime mess, take three

commit b0b885657b6c8ef63a46bc9299b2a7715d19acde upstream.

We first tried to avoid updating atime/mtime entirely (commit
b0de59b5733d: "TTY: do not update atime/mtime on read/write"), and then
limited it to only update it occasionally (commit 37b7f3c76595: "TTY:
fix atime/mtime regression"), but it turns out that this was both
insufficient and overkill.

It was insufficient because we let people attach to the shared ptmx node
to see activity without even reading atime/mtime, and it was overkill
because the "only once a minute" means that you can't really tell an
idle person from an active one with 'w'.

So this tries to fix the problem properly.  It marks the shared ptmx
node as un-notifiable, and it lowers the "only once a minute" to a few
seconds instead - still long enough that you can't time individual
keystrokes, but short enough that you can tell whether somebody is
active or not.

Reported-by: Simon Kirby <sim@hostway.ca>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index ac35c901c8590a58a0ebc9d82310d163c8b081f8..c830b604c96f6e41a15e1ca919bf1ac0fc7be17b 100644 (file)
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -675,6 +675,9 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 
        nonseekable_open(inode, filp);
 
+       /* We refuse fsnotify events on ptmx, since it's a shared resource */
+       filp->f_mode |= FMODE_NONOTIFY;
+
        retval = tty_alloc_file(filp);
        if (retval)
                return retval;

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 892ecda5cee410e4f84f35aa0428d423874a8ef4..f34f98df509d03dfeb47174f2bc38a55d5880b5b 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -941,10 +941,10 @@ 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();
-       sec -= sec % 60;
+       unsigned long sec = get_seconds() & ~7;
        if ((long)(sec - time->tv_sec) > 0)
                time->tv_sec = sec;
 }