tty: n_gsm: fix invalid gsmtty_write_room() result

gsmtty_write() does not prevent the user to use the full fifo size of 4096
bytes as allocated in gsm_dlci_alloc(). However, gsmtty_write_room() tries
to limit the return value by 'TX_SIZE' and returns a negative value if the
fifo has more than 'TX_SIZE' bytes stored. This is obviously wrong as
'TX_SIZE' is defined as 512.
Define 'TX_SIZE' to the fifo size and use it accordingly for allocation to
keep the current behavior. Return the correct remaining size of the fifo in
gsmtty_write_room() via kfifo_avail().

Fixes: e1eaea4 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220504081733.3494-3-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dstarke-siemens

drivers/tty/n_gsm.c

@@ -137,6 +137,7 @@ struct gsm_dlci {
 	int retries;
 	/* Uplink tty if active */
 	struct tty_port port;	/* The tty bound to this DLCI if there is one */
+#define TX_SIZE		4096    /* Must be power of 2. */
 	struct kfifo fifo;	/* Queue fifo for the DLCI */
 	int adaption;		/* Adaption layer in use */
 	int prev_adaption;
@@ -1731,7 +1732,7 @@ static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr)
 		return NULL;
 	spin_lock_init(&dlci->lock);
 	mutex_init(&dlci->mutex);
-	if (kfifo_alloc(&dlci->fifo, 4096, GFP_KERNEL) < 0) {
+	if (kfifo_alloc(&dlci->fifo, TX_SIZE, GFP_KERNEL) < 0) {
 		kfree(dlci);
 		return NULL;
 	}
@@ -2976,8 +2977,6 @@ static struct tty_ldisc_ops tty_ldisc_packet = {
  *	Virtual tty side
  */
 
-#define TX_SIZE		512
-
 /**
  *	gsm_modem_upd_via_data	-	send modem bits via convergence layer
  *	@dlci: channel
@@ -3217,7 +3216,7 @@ static unsigned int gsmtty_write_room(struct tty_struct *tty)
 	struct gsm_dlci *dlci = tty->driver_data;
 	if (dlci->state == DLCI_CLOSED)
 		return 0;
-	return TX_SIZE - kfifo_len(&dlci->fifo);
+	return kfifo_avail(&dlci->fifo);
 }
 
 static unsigned int gsmtty_chars_in_buffer(struct tty_struct *tty)