1 files changed, 62 insertions, 30 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 7aeabb719526..5793aa0d3bfb 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -225,8 +225,6 @@ static void n_tty_check_throttle(struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 
-	if (tty->driver->type == TTY_DRIVER_TYPE_PTY)
-		return;
 	/*
 	 * Check the remaining room for the input canonicalization
 	 * mode.  We don't want to throttle the driver if we're in
@@ -1483,23 +1481,6 @@ n_tty_receive_char_lnext(struct tty_struct *tty, unsigned char c, char flag)
 		n_tty_receive_char_flagged(tty, c, flag);
 }
 
-/**
- *	n_tty_receive_buf	-	data receive
- *	@tty: terminal device
- *	@cp: buffer
- *	@fp: flag buffer
- *	@count: characters
- *
- *	Called by the terminal driver when a block of characters has
- *	been received. This function must be called from soft contexts
- *	not from interrupt context. The driver is responsible for making
- *	calls one at a time and in order (or using flush_to_ldisc)
- *
- *	n_tty_receive_buf()/producer path:
- *		claims non-exclusive termios_rwsem
- *		publishes commit_head or canon_head
- */
-
 static void
 n_tty_receive_buf_real_raw(struct tty_struct *tty, const unsigned char *cp,
 			   char *fp, int count)
@@ -1658,12 +1639,45 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
 	}
 }
 
+/**
+ *	n_tty_receive_buf_common	-	process input
+ *	@tty: device to receive input
+ *	@cp: input chars
+ *	@fp: flags for each char (if NULL, all chars are TTY_NORMAL)
+ *	@count: number of input chars in @cp
+ *
+ *	Called by the terminal driver when a block of characters has
+ *	been received. This function must be called from soft contexts
+ *	not from interrupt context. The driver is responsible for making
+ *	calls one at a time and in order (or using flush_to_ldisc)
+ *
+ *	Returns the # of input chars from @cp which were processed.
+ *
+ *	In canonical mode, the maximum line length is 4096 chars (including
+ *	the line termination char); lines longer than 4096 chars are
+ *	truncated. After 4095 chars, input data is still processed but
+ *	not stored. Overflow processing ensures the tty can always
+ *	receive more input until at least one line can be read.
+ *
+ *	In non-canonical mode, the read buffer will only accept 4095 chars;
+ *	this provides the necessary space for a newline char if the input
+ *	mode is switched to canonical.
+ *
+ *	Note it is possible for the read buffer to _contain_ 4096 chars
+ *	in non-canonical mode: the read buffer could already contain the
+ *	maximum canon line of 4096 chars when the mode is switched to
+ *	non-canonical.
+ *
+ *	n_tty_receive_buf()/producer path:
+ *		claims non-exclusive termios_rwsem
+ *		publishes commit_head or canon_head
+ */
 static int
 n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
 			 char *fp, int count, int flow)
 {
 	struct n_tty_data *ldata = tty->disc_data;
-	int room, n, rcvd = 0;
+	int room, n, rcvd = 0, overflow;
 
 	down_read(&tty->termios_rwsem);
 
@@ -1683,19 +1697,27 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
 		 */
 		size_t tail = smp_load_acquire(&ldata->read_tail);
 
-		room = N_TTY_BUF_SIZE - (ldata->read_head - tail) - 1;
+		room = N_TTY_BUF_SIZE - (ldata->read_head - tail);
 		if (I_PARMRK(tty))
-			room /= 3;
-		if (room <= 0)
-			room = ldata->icanon && ldata->canon_head == tail;
+			room = (room + 2) / 3;
+		room--;
+		if (room <= 0) {
+			overflow = ldata->icanon && ldata->canon_head == tail;
+			if (overflow && room < 0)
+				ldata->read_head--;
+			room = overflow;
+			ldata->no_room = flow && !room;
+		} else
+			overflow = 0;
 
 		n = min(count, room);
-		if (!n) {
-			if (flow && !room)
-				ldata->no_room = 1;
+		if (!n)
 			break;
-		}
-		__receive_buf(tty, cp, fp, n);
+
+		/* ignore parity errors if handling overflow */
+		if (!overflow || !fp || *fp != TTY_PARITY)
+			__receive_buf(tty, cp, fp, n);
+
 		cp += n;
 		if (fp)
 			fp += n;
@@ -1704,7 +1726,17 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
 	}
 
 	tty->receive_room = room;
-	n_tty_check_throttle(tty);
+
+	/* Unthrottle if handling overflow on pty */
+	if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
+		if (overflow) {
+			tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
+			tty_unthrottle_safe(tty);
+			__tty_set_flow_change(tty, 0);
+		}
+	} else
+		n_tty_check_throttle(tty);
+
 	up_read(&tty->termios_rwsem);
 
 	return rcvd;