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SubjectRe: [PATCH] tty reference count fix

On Mon, 13 Feb 2006, Paul Fulghum wrote:

> Fix hole where tty structure can be released when reference
> count is non zero. Existing code can sleep without tty_sem
> protection between deciding to release the tty structure
> (setting local variables tty_closing and otty_closing)
> and setting TTY_CLOSING to prevent further opens.
> An open can occur during this interval causing release_dev()
> to free the tty structure while it is still referenced.
>
> This should fix bugzilla.kernel.org
> [Bug 6041] New: Unable to handle kernel paging request
>
> In Bug 6041, tty_open() oopes on accessing the tty structure
> it has successfully claimed. Bug was on SMP machine
> with the same tty being opened and closed by
> multiple processes, and DEBUG_PAGEALLOC enabled.
>

patch looks good to me. Or you could even drop the tty_sem completely from
the release_dev path (patch below) since lock_kernel() is held in both
tty_open() and the release_dev paths() (and there is no sleeping b/w
setting the tty_closing flag and setting the TTY_CLOSING bit).

-Jason


Signed-off-by: Jason Baron <jbaron@redhat.com>

--- linux-2.6-working/drivers/char/tty_io.c.bak
+++ linux-2.6-working/drivers/char/tty_io.c
@@ -1829,14 +1829,9 @@ static void release_dev(struct file * fi
* each iteration we avoid any problems.
*/
while (1) {
- /* Guard against races with tty->count changes elsewhere and
- opens on /dev/tty */
-
- down(&tty_sem);
tty_closing = tty->count <= 1;
o_tty_closing = o_tty &&
(o_tty->count <= (pty_master ? 1 : 0));
- up(&tty_sem);
do_sleep = 0;

if (tty_closing) {
@@ -1873,7 +1868,6 @@ static void release_dev(struct file * fi
* block, so it's safe to proceed with closing.
*/

- down(&tty_sem);
if (pty_master) {
if (--o_tty->count < 0) {
printk(KERN_WARNING "release_dev: bad pty slave count "
@@ -1887,7 +1881,6 @@ static void release_dev(struct file * fi
tty->count, tty_name(tty, buf));
tty->count = 0;
}
- up(&tty_sem);

/*
* We've decremented tty->count, so we need to remove this file
-
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