libmsc/ussd.c: FIX: properly indicate errors

There are error and problem codes defined by GSM TS 04.80:

  - Error codes are used when a message is structured correctly,
    but something is wrong in context of the current operation.
    Usually they are carried by 'Return Error' component.

  - Problem codes are used when something is wrong with the
    message structure, or with carried values. They are
    carried by 'Reject' component.

    There are three groups of them (see table 3.13):

      - General Problem Codes (table 3.14),
      - Invoke Problem Codes (table 3.15),
      - Return Result Problem Codes (table 3.16),
      - Return Error Problem Codes (table 3.17).

    The first group is general purpose, and can be sent in
    response to any kind of message, excluding 'Reject' itself.
    Other ones are bound to specific component types, such as
    'Invoke', 'Return Result' and 'Return Error'.

For some reason, a 'Reject' component with the general problem
code 'GSM_0480_GEN_PROB_CODE_UNRECOGNISED' was always used in
OsmoMSC. Even when the message structure is correct.

Let's properly indicate errors in the following way:

  - 'Reject' with GSM_0480_GEN_PROB_CODE_UNRECOGNISED
    when the gsm0480_decode_ss_request() fails to decode
    a message. It can only return 0 or 1, so it's hard to
    guess which exact part of message caused the error.

  - 'Return Error' with GSM0480_ERR_CODE_ILLEGAL_SS_OPERATION
    when the operation code is not related to USSD.

  - 'Return Error' with GSM0480_ERR_CODE_UNEXPECTED_DATA_VALUE
    when the requested USSD code is unhandled (not supported).

There is a TTCN-3 testcase for this:

https://gerrit.osmocom.org/9470/

Change-Id: I800e7ec98dc9d0bca2d45a8b8255d60253d63e14

include/osmocom/msc/gsm_04_80.h

@@ -9,8 +9,12 @@ struct gsm_subscriber_connection;
 int gsm0480_send_ussd_response(struct gsm_subscriber_connection *conn,
 			       const char* response_text,
 			       const struct ss_request *req);
+int gsm0480_send_ussd_return_error(struct gsm_subscriber_connection *conn,
+				   const struct ss_request *req,
+				   uint8_t error_code);
 int gsm0480_send_ussd_reject(struct gsm_subscriber_connection *conn,
-			     const struct ss_request *request);
+			     const struct ss_request *req,
+			     uint8_t error_tag, uint8_t error_code);
 
 int msc_send_ussd_notify(struct gsm_subscriber_connection *conn, int level,
 			 const char *text);

src/libmsc/gsm_04_80.c

@@ -108,15 +108,42 @@ int gsm0480_send_ussd_response(struct gsm_subscriber_connection *conn,
 	return msc_tx_dtap(conn, msg);
 }
 
+int gsm0480_send_ussd_return_error(struct gsm_subscriber_connection *conn,
+	const struct ss_request *req, uint8_t error_code)
+{
+	struct msgb *msg = gsm48_msgb_alloc_name("GSM 04.08 USSD ERR");
+	struct gsm48_hdr *gh;
+
+	/* First insert the problem code */
+	msgb_push_TLV1(msg, GSM_0480_ERROR_CODE_TAG, error_code);
+
+	/* Before it insert the invoke ID */
+	msgb_push_TLV1(msg, GSM0480_COMPIDTAG_INVOKE_ID, req->invoke_id);
+
+	/* Wrap this up as a Reject component */
+	msgb_wrap_with_TL(msg, GSM0480_CTYPE_RETURN_ERROR);
+
+	/* Wrap the component in a Facility message */
+	msgb_wrap_with_TL(msg, GSM0480_IE_FACILITY);
+
+	/* And finally pre-pend the L3 header */
+	gh = (struct gsm48_hdr *) msgb_push(msg, sizeof(*gh));
+	gh->proto_discr = GSM48_PDISC_NC_SS;
+	gh->proto_discr |= req->transaction_id | (1<<7);  /* TI direction = 1 */
+	gh->msg_type = GSM0480_MTYPE_RELEASE_COMPLETE;
+
+	return msc_tx_dtap(conn, msg);
+}
+
 int gsm0480_send_ussd_reject(struct gsm_subscriber_connection *conn,
-			     const struct ss_request *req)
+			     const struct ss_request *req,
+			     uint8_t error_tag, uint8_t error_code)
 {
 	struct msgb *msg = gsm48_msgb_alloc_name("GSM 04.08 USSD REJ");
 	struct gsm48_hdr *gh;
 
 	/* First insert the problem code */
-	msgb_push_TLV1(msg, GSM_0480_PROBLEM_CODE_TAG_GENERAL,
-			GSM_0480_GEN_PROB_CODE_UNRECOGNISED);
+	msgb_push_TLV1(msg, error_tag, error_code);
 
 	/* Before it insert the invoke ID */
 	msgb_push_TLV1(msg, GSM0480_COMPIDTAG_INVOKE_ID, req->invoke_id);

src/libmsc/ussd.c

@@ -74,7 +74,8 @@ int handle_rcv_ussd(struct gsm_subscriber_connection *conn, struct msgb *msg)
 	if (!rc) {
 		LOGP(DMM, LOGL_ERROR, "SS/USSD message parsing error, "
 			"rejecting request...\n");
-		gsm0480_send_ussd_reject(conn, &req);
+		gsm0480_send_ussd_reject(conn, &req, GSM_0480_PROBLEM_CODE_TAG_GENERAL,
+			GSM_0480_GEN_PROB_CODE_UNRECOGNISED);
 		/* The GSM 04.80 API uses inverted codes (0 means error) */
 		return -EPROTO;
 	}
@@ -83,8 +84,8 @@ int handle_rcv_ussd(struct gsm_subscriber_connection *conn, struct msgb *msg)
 	if (req.ussd_text[0] == '\0' || req.ussd_text[0] == 0xFF) {
 		if (req.ss_code > 0) {
 			/* Assume interrogateSS or modification of it and reject */
-			rc = gsm0480_send_ussd_reject(conn, &req);
-			return rc;
+			return gsm0480_send_ussd_return_error(conn, &req,
+				GSM0480_ERR_CODE_ILLEGAL_SS_OPERATION);
 		}
 		/* Still assuming a Release-Complete and returning */
 		return 0;
@@ -96,7 +97,8 @@ int handle_rcv_ussd(struct gsm_subscriber_connection *conn, struct msgb *msg)
 		rc = send_own_number(conn, &req);
 	} else {
 		DEBUGP(DMM, "Unhandled USSD %s\n", req.ussd_text);
-		rc = gsm0480_send_ussd_reject(conn, &req);
+		rc = gsm0480_send_ussd_return_error(conn, &req,
+			GSM0480_ERR_CODE_UNEXPECTED_DATA_VALUE);
 	}
 
 	return rc;