tty: fix up atime/mtime mess, take three

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

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index a62798fcc014f743df4b05065b4514cc54ed2830..59bfaecc4e1438ecc771573c44c161b26fc3548d 100644 (file)
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -681,6 +681,9 @@ 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 97ebc8c5864e7f092e443810a53d5e42b4238501..6464029e4860968945840c252ad632b556f9f947 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -988,10 +988,10 @@ 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;
 }