tty: n_gsm: fix wrong command frame length field encoding

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.4.6.1 states that each command frame shall
be made up from type, length and value. Looking for example in chapter
5.4.6.3.5 at the description for the encoding of a flow control on command
it becomes obvious, that the type and length field is always present
whereas the value may be zero bytes long. The current implementation omits
the length field if the value is not present. This is wrong.
Correct this by always sending the length in gsm_control_transmit().
So far only the modem status command (MSC) has included a value and encoded
its length directly. Therefore, also change gsmtty_modem_update().

Fixes: e1eaea46 ("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/20220414094225.4527-12-daniel.starke@siemens.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -1327,11 +1327,12 @@ static void gsm_control_response(struct gsm_mux *gsm, unsigned int command,
 
 static void gsm_control_transmit(struct gsm_mux *gsm, struct gsm_control *ctrl)
 {
-	struct gsm_msg *msg = gsm_data_alloc(gsm, 0, ctrl->len + 1, gsm->ftype);
+	struct gsm_msg *msg = gsm_data_alloc(gsm, 0, ctrl->len + 2, gsm->ftype);
 	if (msg == NULL)
 		return;
-	msg->data[0] = (ctrl->cmd << 1) | 2 | EA;	/* command */
-	memcpy(msg->data + 1, ctrl->data, ctrl->len);
+	msg->data[0] = (ctrl->cmd << 1) | CR | EA;	/* command */
+	msg->data[1] = (ctrl->len << 1) | EA;
+	memcpy(msg->data + 2, ctrl->data, ctrl->len);
 	gsm_data_queue(gsm->dlci[0], msg);
 }
 
@@ -2957,19 +2958,17 @@ static struct tty_ldisc_ops tty_ldisc_packet = {
 
 static int gsmtty_modem_update(struct gsm_dlci *dlci, u8 brk)
 {
-	u8 modembits[5];
+	u8 modembits[3];
 	struct gsm_control *ctrl;
 	int len = 2;
 
-	if (brk)
+	modembits[0] = (dlci->addr << 2) | 2 | EA;  /* DLCI, Valid, EA */
+	modembits[1] = (gsm_encode_modem(dlci) << 1) | EA;
+	if (brk) {
+		modembits[2] = (brk << 4) | 2 | EA; /* Length, Break, EA */
 		len++;
-
-	modembits[0] = len << 1 | EA;		/* Data bytes */
-	modembits[1] = dlci->addr << 2 | 3;	/* DLCI, EA, 1 */
-	modembits[2] = gsm_encode_modem(dlci) << 1 | EA;
-	if (brk)
-		modembits[3] = brk << 4 | 2 | EA;	/* Valid, EA */
-	ctrl = gsm_control_send(dlci->gsm, CMD_MSC, modembits, len + 1);
+	}
+	ctrl = gsm_control_send(dlci->gsm, CMD_MSC, modembits, len);
 	if (ctrl == NULL)
 		return -ENOMEM;
 	return gsm_control_wait(dlci->gsm, ctrl);