Re: [PATCH] tty: Correct tty buffer flush.

Ilya Zykov
2013-03-04T19:19:43Z

On 03.12.2012 13:54, Ilya Zykov wrote:
> The root of problem is carelessly zeroing pointer(in function __tty_buffer_flush()),
> when another thread can use it. It can be cause of "NULL pointer dereference".
> Main idea of the patch, this is never release last (struct tty_buffer) in the active buffer.
> Only flush data for ldisc(tty->buf.head->read = tty->buf.head->commit).
> At that moment driver can collect(write) data in buffer without conflict.
> It is repeat behavior of flush_to_ldisc(), only without feeding data to ldisc.
> Test program and bug report you can see:
> https://lkml.org/lkml/2012/11/29/368
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Ilya Zykov <ilya@ilyx.ru>
> ---
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 6c9b7cd..4f02f9c 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -114,11 +114,14 @@ static void __tty_buffer_flush(struct tty_struct *tty)
> {
> struct tty_buffer *thead;
>
> - while ((thead = tty->buf.head) != NULL) {
> - tty->buf.head = thead->next;
> - tty_buffer_free(tty, thead);
> + if (tty->buf.head == NULL)
> + return;
> + while ((thead = tty->buf.head->next) != NULL) {
> + tty_buffer_free(tty, tty->buf.head);
> + tty->buf.head = thead;
> }
> - tty->buf.tail = NULL;
> + WARN_ON(tty->buf.head != tty->buf.tail);
> + tty->buf.head->read = tty->buf.head->commit;
> }
>
> /**
>

You can include this patch, in 3.2 series , for improve stability,
it would be merged in upstream 3.9-rc1.

Thank you.
Ilya.