[PATCH v3 6/6] n_tty: Fix read buffer overwrite when no newline

[Peter Hurley <peter@hurleysoftware.com>]

In canon mode, the read buffer head will advance over the buffer tail
if the input > 4095 bytes without receiving a line termination char.

Discard additional input until a line termination is received.
Before evaluating for overflow, the 'room' value is normalized for
I_PARMRK and 1 byte is reserved for line termination (even in !icanon
mode, in case the mode is switched). The following table shows the
transform:

 actual buffer |  'room' value before overflow calc
  space avail  |    !I_PARMRK    |    I_PARMRK
 --------------------------------------------------
      0        |       -1        |       -1
      1        |        0        |        0
      2        |        1        |        0
      3        |        2        |        0
      4+       |        3        |        1

When !icanon or when icanon and the read buffer contains newlines,
normalized 'room' values of -1 and 0 are clamped to 0, and
'overflow' is 0, so read_head is not adjusted and the input i/o loop
exits (setting no_room if called from flush_to_ldisc()). No input
is discarded since the reader does have input available to read
which ensures forward progress.

When icanon and the read buffer does not contain newlines and the
normalized 'room' value is 0, then overflow and room are reset to 1,
so that the i/o loop will process the next input char normally
(except for parity errors which are ignored). Thus, erasures, signalling
chars, 7-bit mode, etc. will continue to be handled properly.

If the input char processed was not a line termination char, then
the canon_head index will not have advanced, so the normalized 'room'
value will now be -1 and 'overflow' will be set, which indicates the
read_head can safely be reset, effectively erasing the last char
processed.

If the input char processed was a line termination, then the
canon_head index will have advanced, so 'overflow' is cleared to 0,
the read_head is not reset, and 'room' is cleared to 0, which exits
the i/o loop (because the reader now have input available to read
which ensures forward progress).

Note that it is possible for a line termination to be received, and
for the reader to copy the line to the user buffer before the
input i/o loop is ready to process the next input char. This is
why the i/o loop recomputes the room/overflow state with every
input char while handling overflow.

Finally, if the input data was processed without receiving
a line termination (so that overflow is still set), the pty
driver must receive a write wakeup. A pty writer may be waiting
to write more data in n_tty_write() but without unthrottling
here that wakeup will not arrive, and forward progress will halt.
(Normally, the pty writer is woken when the reader reads data out
of the buffer and more space become available).

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
 drivers/tty/n_tty.c | 92 ++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 62 insertions(+), 30 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 7aeabb7..5793aa0 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;
-- 
2.2.2