tty: n_gsm: fix wrong modem processing in convergence layer type 2

The function gsm_process_modem() exists to handle modem status bits of
incoming frames. This includes incoming MSC (modem status command) frames
and convergence layer type 2 data frames. The function, however, was only
designed to handle MSC frames as it expects the command length. Within
gsm_dlci_data() it is wrongly assumed that this is the same as the data
frame length. This is only true if the data frame contains only 1 byte of
payload.

This patch names the length parameter of gsm_process_modem() in a generic
manner to reflect its association. It also corrects all calls to the
function to handle the variable number of modem status octets correctly in
both cases.

Fixes: 7263287 ("tty: n_gsm: Fixed logic to decode break signal from modem status")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-6-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dstarke-siemens

drivers/tty/n_gsm.c

@@ -1021,25 +1021,25 @@ static void gsm_control_reply(struct gsm_mux *gsm, int cmd, const u8 *data,
  *	@tty: virtual tty bound to the DLCI
  *	@dlci: DLCI to affect
  *	@modem: modem bits (full EA)
- *	@clen: command length
+ *	@slen: number of signal octets
  *
  *	Used when a modem control message or line state inline in adaption
  *	layer 2 is processed. Sort out the local modem state and throttles
  */
 
 static void gsm_process_modem(struct tty_struct *tty, struct gsm_dlci *dlci,
-							u32 modem, int clen)
+							u32 modem, int slen)
 {
 	int  mlines = 0;
 	u8 brk = 0;
 	int fc;
 
-	/* The modem status command can either contain one octet (v.24 signals)
-	   or two octets (v.24 signals + break signals). The length field will
-	   either be 2 or 3 respectively. This is specified in section
-	   5.4.6.3.7 of the  27.010 mux spec. */
+	/* The modem status command can either contain one octet (V.24 signals)
+	 * or two octets (V.24 signals + break signals). This is specified in
+	 * section 5.4.6.3.7 of the 07.10 mux spec.
+	 */
 
-	if (clen == 2)
+	if (slen == 1)
 		modem = modem & 0x7f;
 	else {
 		brk = modem & 0x7f;
@@ -1096,6 +1096,7 @@ static void gsm_control_modem(struct gsm_mux *gsm, const u8 *data, int clen)
 	unsigned int brk = 0;
 	struct gsm_dlci *dlci;
 	int len = clen;
+	int slen;
 	const u8 *dp = data;
 	struct tty_struct *tty;
 
@@ -1115,6 +1116,7 @@ static void gsm_control_modem(struct gsm_mux *gsm, const u8 *data, int clen)
 		return;
 	dlci = gsm->dlci[addr];
 
+	slen = len;
 	while (gsm_read_ea(&modem, *dp++) == 0) {
 		len--;
 		if (len == 0)
@@ -1131,7 +1133,7 @@ static void gsm_control_modem(struct gsm_mux *gsm, const u8 *data, int clen)
 		modem |= (brk & 0x7f);
 	}
 	tty = tty_port_tty_get(&dlci->port);
-	gsm_process_modem(tty, dlci, modem, clen);
+	gsm_process_modem(tty, dlci, modem, slen);
 	if (tty) {
 		tty_wakeup(tty);
 		tty_kref_put(tty);
@@ -1597,6 +1599,7 @@ static void gsm_dlci_data(struct gsm_dlci *dlci, const u8 *data, int clen)
 	struct tty_struct *tty;
 	unsigned int modem = 0;
 	int len = clen;
+	int slen = 0;
 
 	if (debug & 16)
 		pr_debug("%d bytes for tty\n", len);
@@ -1609,12 +1612,14 @@ static void gsm_dlci_data(struct gsm_dlci *dlci, const u8 *data, int clen)
 	case 2:		/* Asynchronous serial with line state in each frame */
 		while (gsm_read_ea(&modem, *data++) == 0) {
 			len--;
+			slen++;
 			if (len == 0)
 				return;
 		}
+		slen++;
 		tty = tty_port_tty_get(port);
 		if (tty) {
-			gsm_process_modem(tty, dlci, modem, clen);
+			gsm_process_modem(tty, dlci, modem, slen);
 			tty_kref_put(tty);
 		}
 		fallthrough;