Properly enforce the far end max datagram when constructing UDPTL packets

src/mod/applications/mod_spandsp/udptl.c

@@ -349,6 +349,8 @@ int udptl_build_packet(udptl_state_t *s, uint8_t buf[], const uint8_t msg[], int
 	int len;
 	int limit;
 	int high_tide;
+	int len_before_entries;
+	int previous_len;
 
 	/* UDPTL cannot cope with zero length messages, and our buffering for redundancy limits their
 	   maximum length. */
@@ -392,15 +394,26 @@ int udptl_build_packet(udptl_state_t *s, uint8_t buf[], const uint8_t msg[], int
 			entries = s->error_correction_entries;
 		else
 			entries = s->tx_seq_no;
+		len_before_entries = len;
 		/* The number of entries will always be small, so it is pointless allowing
 		   for the fragmented case here. */
 		if (encode_length(buf, &len, entries) < 0)
 			return -1;
 		/* Encode the elements */
-		for (i = 0; i < entries; i++) {
-			j = (entry - i - 1) & UDPTL_BUF_MASK;
+		for (m = 0; m < entries; m++) {
+			previous_len = len;
+			j = (entry - m - 1) & UDPTL_BUF_MASK;
 			if (encode_open_type(buf, &len, s->tx[j].buf, s->tx[j].buf_len) < 0)
 				return -1;
+
+			/* If we have exceeded the far end's max datagram size, don't include this last chunk,
+			   and stop trying to add more. */
+			if (len > s->far_max_datagram_size) {
+				len = previous_len;
+				if (encode_length(buf, &len_before_entries, m) < 0)
+					return -1;
+				break;
+            }
 		}
 		break;
 	case UDPTL_ERROR_CORRECTION_FEC:
@@ -417,11 +430,13 @@ int udptl_build_packet(udptl_state_t *s, uint8_t buf[], const uint8_t msg[], int
 		/* Span is defined as an inconstrained integer, which it dumb. It will only
 		   ever be a small value. Treat it as such. */
 		buf[len++] = 1;
-		buf[len++] = (uint8_t)span;
+		buf[len++] = (uint8_t) span;
+		len_before_entries = len;
 		/* The number of entries is defined as a length, but will only ever be a small
 		   value. Treat it as such. */
-		buf[len++] = (uint8_t)entries;
+		buf[len++] = (uint8_t) entries;
 		for (m = 0; m < entries; m++) {
+			previous_len = len;
 			/* Make an XOR'ed entry the maximum length */
 			limit = (entry + m) & UDPTL_BUF_MASK;
 			high_tide = 0;
@@ -439,6 +454,14 @@ int udptl_build_packet(udptl_state_t *s, uint8_t buf[], const uint8_t msg[], int
 			}
 			if (encode_open_type(buf, &len, fec, high_tide) < 0)
 				return -1;
+
+			/* If we have exceeded the far end's max datagram size, don't include this last chunk,
+			   and stop trying to add more. */
+			if (len > s->far_max_datagram_size) {
+				len = previous_len;
+				buf[len_before_entries] = (uint8_t) m;
+				break;
+			}
 		}
 		break;
 	}