[PATCH][V2] n_tty: check for negative and zero space return from tty_write_room

[Colin King]

From: Colin Ian King <colin.king@xxxxxxxxxxxxx>

The return from tty_write_room could potentially be negative if
a tty write_room driver returns an error number (not that any seem
to do). Rather than just check for a zero return, also check for
a -ve return. This avoids the unsigned nr being set to a large unsigned
value on the assignment from variable space and can lead to overflowing
the buffer buf.  Better to be safe than assume all write_room
implementations in tty drivers are going to do the right thing.

Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
---

V2: return the error code from tty_write_room rather than zero,
thanks to Dan Carpenter for suggesting this improvement.

---
 drivers/tty/n_tty.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 9cdb0fa3c4bf..f9c584244f72 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -550,9 +550,9 @@ static ssize_t process_output_block(struct tty_struct *tty,
 	mutex_lock(&ldata->output_lock);
 
 	space = tty_write_room(tty);
-	if (!space) {
+	if (space <= 0) {
 		mutex_unlock(&ldata->output_lock);
-		return 0;
+		return space;
 	}
 	if (nr > space)
 		nr = space;
-- 
2.20.1