[RFC][PATCH] n_tty : Loss of sync following a buffer overflow

[RFC][PATCH] n_tty : Loss of sync following a buffer overflow

[Rupesh Sugathan]

From: Rupesh Sugathan <Rupesh.Sugathan@gmail.com>

There seems to be a synchronization issue with the n_tty.c driver when
working in canonical mode.
The n_tty rightly discards data received following a buffer overflow and
hence the tty->read_cnt is not updated. However, the newline characters
received following a buffer overflow seems to increment the tty-
>canon_data index. This may result in a loss of sync between the tty-
>canon_data & tty->read_cnt while processing read in the read_chan().
This loss of sync might be irrecoverable even when the data is later
received at a slower rate.

----
I am not very sure if there is any deliberate rationale to process the
newlines even when the buffer has overflown. Please comment and review
the patch

diff -pu a/drivers/char/n_tty.c b/drivers/char/n_tty.c

--- a/drivers/char/n_tty.c	2008-03-10 17:11:54.000000000 -0700
+++ b/drivers/char/n_tty.c	2008-03-10 16:49:23.000000000 -0700
@@ -838,15 +838,17 @@ send_signal:
 				put_tty_queue(c, tty);
 
 		handle_newline:
-			spin_lock_irqsave(&tty->read_lock, flags);
-			set_bit(tty->read_head, tty->read_flags);
-			put_tty_queue_nolock(c, tty);
-			tty->canon_head = tty->read_head;
-			tty->canon_data++;
-			spin_unlock_irqrestore(&tty->read_lock, flags);
-			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
-			if (waitqueue_active(&tty->read_wait))
-				wake_up_interruptible(&tty->read_wait);
+			if (tty->read_cnt < N_TTY_BUF_SIZE) {
+				spin_lock_irqsave(&tty->read_lock, flags);
+				set_bit(tty->read_head, tty->read_flags);
+				put_tty_queue_nolock(c, tty);
+				tty->canon_head = tty->read_head;
+				tty->canon_data++;
+				spin_unlock_irqrestore(&tty->read_lock, flags);
+				kill_fasync(&tty->fasync, SIGIO, POLL_IN);
+				if (waitqueue_active(&tty->read_wait))
+					wake_up_interruptible(&tty->read_wait);
+			}
 			return;
 		}
 	}

Signed-off-by: Rupesh Sugathan <Rupesh.Sugathan@gmail.com>

Re: [RFC][PATCH] n_tty : Loss of sync following a buffer overflow

[Paul Fulghum]

Rupesh Sugathan wrote:
> There seems to be a synchronization issue with the n_tty.c driver when
> working in canonical mode.
> The n_tty rightly discards data received following a buffer overflow and
> hence the tty->read_cnt is not updated. However, the newline characters
> received following a buffer overflow seems to increment the tty-
> canon_data index. This may result in a loss of sync between the tty-
> canon_data & tty->read_cnt while processing read in the read_chan().
> This loss of sync might be irrecoverable even when the data is later
> received at a slower rate.
> 
> ----
> I am not very sure if there is any deliberate rationale to process the
> newlines even when the buffer has overflown. Please comment and review
> the patch
> 
> diff -pu a/drivers/char/n_tty.c b/drivers/char/n_tty.c

Some more information:

This is not purely theoretical. Rupesh is actually
working a problem where he has seen the N_TTY buffer overflow
and the canonical state variables get out of sync with read_cnt, etc.

This patch seems obviously correct, if N_TTY drops a
char due to a full buffer the rest of the character
processing should not take place. Particularly in this
case where the extra processing leaves the tty structure
in an inconsistent state.

--
Paul Fulghum
Microgate Systems, Ltd

[RFC][PATCH] n_tty : Loss of sync following a buffer overflow

[gmail]

From: Rupesh Sugathan <Rupesh.Sugathan@gmail.com>

There seems to be a synchronization issue with the n_tty.c driver when
working in canonical mode.
The n_tty rightly discards data received following a buffer overflow and
hence the tty->read_cnt is not updated. However, the newline characters
received following a buffer overflow seems to increment the tty-
>canon_data index. This may result in a loss of sync between the tty-
>canon_data & tty->read_cnt while processing read in the read_chan().
This loss of sync might be irrecoverable even when the data is later
received at a slower rate.

----
I am not very sure if there is any deliberate rationale to process the
newlines even when the buffer has overflown. Please comment and review
the patch

diff -pu a/drivers/char/n_tty.c b/drivers/char/n_tty.c

--- a/drivers/char/n_tty.c	2008-03-10 17:11:54.000000000 -0700
+++ b/drivers/char/n_tty.c	2008-03-10 16:49:23.000000000 -0700
@@ -838,15 +838,17 @@ send_signal:
 				put_tty_queue(c, tty);
 
 		handle_newline:
-			spin_lock_irqsave(&tty->read_lock, flags);
-			set_bit(tty->read_head, tty->read_flags);
-			put_tty_queue_nolock(c, tty);
-			tty->canon_head = tty->read_head;
-			tty->canon_data++;
-			spin_unlock_irqrestore(&tty->read_lock, flags);
-			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
-			if (waitqueue_active(&tty->read_wait))
-				wake_up_interruptible(&tty->read_wait);
+			if (tty->read_cnt < N_TTY_BUF_SIZE) {
+				spin_lock_irqsave(&tty->read_lock, flags);
+				set_bit(tty->read_head, tty->read_flags);
+				put_tty_queue_nolock(c, tty);
+				tty->canon_head = tty->read_head;
+				tty->canon_data++;
+				spin_unlock_irqrestore(&tty->read_lock, flags);
+				kill_fasync(&tty->fasync, SIGIO, POLL_IN);
+				if (waitqueue_active(&tty->read_wait))
+					wake_up_interruptible(&tty->read_wait);
+			}
 			return;
 		}
 	}

Signed-off-by: Rupesh Sugathan <Rupesh.Sugathan@gmail.com>