net: tcp: Add support for TCP options parsing

Add a generic function for TCP option parsing. So far we're
interested only in MSS option value, so that's what it handles.
Use it to parse MSS value in net_context incoming SYN packet
handler.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>

subsys/net/ip/net_context.c

@@ -1613,10 +1613,22 @@ NET_CONN_CB(tcp_syn_rcvd)
 	 */
 	if (NET_TCP_FLAGS(tcp_hdr) == NET_TCP_SYN) {
 		int r;
-		u16_t send_mss = NET_TCP_DEFAULT_MSS;
+		int opt_totlen;
+		struct net_tcp_options tcp_opts = {
+			.mss = NET_TCP_DEFAULT_MSS,
+		};
 
 		net_tcp_print_recv_info("SYN", pkt, tcp_hdr->src_port);
 
+		opt_totlen = NET_TCP_HDR_LEN(tcp_hdr)
+			     - sizeof(struct net_tcp_hdr);
+		/* We expect MSS option to be present (opt_totlen > 0),
+		 * so call unconditionally.
+		 */
+		if (net_tcp_parse_opts(pkt, opt_totlen, &tcp_opts) < 0) {
+			return NET_DROP;
+		}
+
 		net_tcp_change_state(tcp, NET_TCP_SYN_RCVD);
 
 		/* Set TCP seq and ack which are then stored in the backlog */
@@ -1627,7 +1639,7 @@ NET_CONN_CB(tcp_syn_rcvd)
 
 		/* Get MSS from TCP options here*/
 
-		r = tcp_backlog_syn(pkt, context, send_mss);
+		r = tcp_backlog_syn(pkt, context, tcp_opts.mss);
 		if (r < 0) {
 			if (r == -EADDRINUSE) {
 				NET_DBG("TCP connection already exists");

subsys/net/ip/tcp.c

@@ -600,7 +600,7 @@ u16_t net_tcp_get_recv_mss(const struct net_tcp *tcp)
 static void net_tcp_set_syn_opt(struct net_tcp *tcp, u8_t *options,
 				u8_t *optionlen)
 {
-	u16_t recv_mss;
+	u32_t recv_mss;
 
 	*optionlen = 0;
 
@@ -611,7 +611,8 @@ static void net_tcp_set_syn_opt(struct net_tcp *tcp, u8_t *options,
 		recv_mss = 0;
 	}
 
-	UNALIGNED_PUT(htonl((u32_t)recv_mss | NET_TCP_MSS_HEADER),
+	recv_mss |= (NET_TCP_MSS_OPT << 24) | (NET_TCP_MSS_SIZE << 16);
+	UNALIGNED_PUT(htonl(recv_mss),
 		      (u32_t *)(options + *optionlen));
 
 	*optionlen += NET_TCP_MSS_SIZE;
@@ -1235,3 +1236,77 @@ struct net_buf *net_tcp_set_chksum(struct net_pkt *pkt, struct net_buf *frag)
 
 	return frag;
 }
+
+int net_tcp_parse_opts(struct net_pkt *pkt, int opt_totlen,
+		       struct net_tcp_options *opts)
+{
+	struct net_buf *frag = pkt->frags;
+	u16_t pos = net_pkt_ip_hdr_len(pkt)
+		  + net_pkt_ipv6_ext_len(pkt)
+		  + sizeof(struct net_tcp_hdr);
+	u8_t opt, optlen;
+
+	/* TODO: this should be done for each TCP pkt, on reception */
+	if (pos + opt_totlen > net_pkt_get_len(pkt)) {
+		NET_ERR("Truncated pkt len: %d, expected: %d",
+			(int)net_pkt_get_len(pkt), pos + opt_totlen);
+		return -EINVAL;
+	}
+
+	while (opt_totlen) {
+		frag = net_frag_read(frag, pos, &pos, sizeof(opt), &opt);
+		opt_totlen--;
+
+		/* https://www.iana.org/assignments/tcp-parameters/tcp-parameters.xhtml#tcp-parameters-1 */
+		/* "Options 0 and 1 are exactly one octet which is their
+		 * kind field.  All other options have their one octet
+		 * kind field, followed by a one octet length field,
+		 * followed by length-2 octets of option data."
+		 */
+		if (opt == NET_TCP_END_OPT) {
+			break;
+		} else if (opt == NET_TCP_NOP_OPT) {
+			continue;
+		}
+
+		if (!opt_totlen) {
+			optlen = 0;
+			goto error;
+		}
+
+		frag = net_frag_read(frag, pos, &pos, sizeof(optlen), &optlen);
+		opt_totlen--;
+		if (optlen < 2) {
+			goto error;
+		}
+
+		/* Subtract opt/optlen size now to avoid doing this
+		 * repeatedly.
+		 */
+		optlen -= 2;
+		if (opt_totlen < optlen) {
+			goto error;
+		}
+
+		switch (opt) {
+		case NET_TCP_MSS_OPT:
+			if (optlen != 2) {
+				goto error;
+			}
+			frag = net_frag_read(frag, pos, &pos,
+					     optlen, (u8_t *)&opts->mss);
+			break;
+		default:
+			frag = net_frag_skip(frag, pos, &pos, optlen);
+			break;
+		}
+
+		opt_totlen -= optlen;
+	}
+
+	return 0;
+
+error:
+	NET_ERR("Invalid TCP opt: %d len: %d", opt, optlen);
+	return -EINVAL;
+}

subsys/net/ip/tcp.h

@@ -89,11 +89,22 @@ enum net_tcp_state {
 
 #define NET_TCP_MAX_OPT_SIZE  8
 
-#define NET_TCP_MSS_HEADER    0x02040000 /* MSS option */
-#define NET_TCP_WINDOW_HEADER 0x30300    /* Window scale option */
-
-#define NET_TCP_MSS_SIZE      4          /* MSS option size */
-#define NET_TCP_WINDOW_SIZE   3          /* Window scale option size */
+/* TCP Option codes */
+#define NET_TCP_END_OPT          0
+#define NET_TCP_NOP_OPT          1
+#define NET_TCP_MSS_OPT          2
+#define NET_TCP_WINDOW_SCALE_OPT 3
+
+/* TCP Option sizes */
+#define NET_TCP_END_SIZE          1
+#define NET_TCP_NOP_SIZE          1
+#define NET_TCP_MSS_SIZE          4
+#define NET_TCP_WINDOW_SCALE_SIZE 3
+
+/** Parsed TCP option values for net_tcp_parse_opts()  */
+struct net_tcp_options {
+	u16_t mss;
+};
 
 /* Max received bytes to buffer internally */
 #define NET_TCP_BUF_MAX_LEN 1280
@@ -443,6 +454,23 @@ struct net_buf *net_tcp_set_chksum(struct net_pkt *pkt, struct net_buf *frag);
  */
 u16_t net_tcp_get_chksum(struct net_pkt *pkt, struct net_buf *frag);
 
+/**
+ * @brief Parse TCP options from network packet.
+ *
+ * Parse TCP options, returning MSS value (as that the only one we
+ * handle so far).
+ *
+ * @param pkt Network packet
+ * @param opt_totlen Total length of options to parse
+ * @param opts Pointer to TCP options structure. (Each option is updated
+ * only if present, so the structure must be initialized with the default
+ * values.)
+ *
+ * @return 0 if no error, <0 in case of error
+ */
+int net_tcp_parse_opts(struct net_pkt *pkt, int opt_totlen,
+		       struct net_tcp_options *opts);
+
 #else
 
 static inline u16_t net_tcp_get_chksum(struct net_pkt *pkt,