[PATCH -next 2/2] tty: Correct tty buffer flush.

Ilya Zykov
2012-12-05T12:53:24Z

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 free last (struct tty_buffer) in the active buffer.
Only flush the 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.

Signed-off-by: Ilya Zykov <ilya@ilyx.ru>
---
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 7602df8..8a3333d 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -119,11 +119,14 @@ static void __tty_buffer_flush(struct tty_port *port)
struct tty_bufhead *buf = &port->buf;
struct tty_buffer *thead;

- while ((thead = buf->head) != NULL) {
- buf->head = thead->next;
- tty_buffer_free(port, thead);
+ if (unlikely(buf->head == NULL))
+ return;
+ while ((thead = buf->head->next) != NULL) {
+ tty_buffer_free(port, buf->head);
+ buf->head = thead;
}
- buf->tail = NULL;
+ WARN_ON(buf->head != buf->tail);
+ buf->head->read = buf->head->commit;
}

/**