tty: n_tty: use uint for space returned by tty_write_room()

tty_write_room() returns an "unsigned int". So in case some insane
driver (like my tty test driver) returns (legitimate) UINT_MAX from its
tty_operations::write_room(), n_tty is confused on several places.

For example, in process_output_block(), the result of tty_write_room()
is stored into (signed) "int". So this UINT_MAX suddenly becomes -1. And
that is extended to ssize_t and returned from process_output_block().
This causes a write() to such a node to receive -EPERM (which is -1).

Fix that by using proper "unsigned int" and proper "== 0" test. And
return 0 constant directly in that "if", so that it is immediately clear
what is returned ("space" equals to 0 at that point).

Similarly for process_output() and __process_echoes().

Note this does not fix any in-tree driver as of now.

If you want "Fixes: something", it would be commit 03b3b1a ("tty:
make tty_operations::write_room return uint"). I intentionally do not
mark this patch by a real tag below.

Signed-off-by: Jiri Slaby (SUSE) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Jiri Slaby (SUSE)

drivers/tty/n_tty.c

@@ -488,7 +488,8 @@ static int do_output_char(u8 c, struct tty_struct *tty, int space)
 static int process_output(u8 c, struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
-	int	space, retval;
+	unsigned int space;
+	int retval;
 
 	mutex_lock(&ldata->output_lock);
 
@@ -524,16 +525,16 @@ static ssize_t process_output_block(struct tty_struct *tty,
 				    const u8 *buf, unsigned int nr)
 {
 	struct n_tty_data *ldata = tty->disc_data;
-	int	space;
-	int	i;
+	unsigned int space;
+	int i;
 	const u8 *cp;
 
 	mutex_lock(&ldata->output_lock);
 
 	space = tty_write_room(tty);
-	if (space <= 0) {
+	if (space == 0) {
 		mutex_unlock(&ldata->output_lock);
-		return space;
+		return 0;
 	}
 	if (nr > space)
 		nr = space;
@@ -698,7 +699,7 @@ static int n_tty_process_echo_ops(struct tty_struct *tty, size_t *tail,
 static size_t __process_echoes(struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
-	int	space, old_space;
+	unsigned int space, old_space;
 	size_t tail;
 	u8 c;