tcp: Use structures to construct initial TCP options

As a rule, we prefer constructing packets with matching C structures,
rather than building them byte by byte.  However, one case we still build
byte by byte is the TCP options we include in SYN packets (in fact the only
time we generate TCP options on the tap interface).

Rework this to use a structure and initialisers which make it a bit
clearer what's going on.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by; Stefano Brivio <sbrivio@redhat.com>

tcp.c

@@ -1232,7 +1232,7 @@ static void tcp_update_seqack_from_tap(const struct ctx *c,
  *	     1 otherwise
  */
 int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
-		      int flags, struct tcphdr *th, char *data,
+		      int flags, struct tcphdr *th, struct tcp_syn_opts *opts,
 		      size_t *optlen)
 {
 	struct tcp_info tinfo = { 0 };
@@ -1258,12 +1258,6 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
 	if (flags & SYN) {
 		int mss;
 
-		/* Options: MSS, NOP and window scale (8 bytes) */
-		*optlen = OPT_MSS_LEN + 1 + OPT_WS_LEN;
-
-		*data++ = OPT_MSS;
-		*data++ = OPT_MSS_LEN;
-
 		if (c->mtu == -1) {
 			mss = tinfo.tcpi_snd_mss;
 		} else {
@@ -1279,16 +1273,11 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
 			else if (mss > PAGE_SIZE)
 				mss = ROUND_DOWN(mss, PAGE_SIZE);
 		}
-		*(uint16_t *)data = htons(MIN(USHRT_MAX, mss));
-
-		data += OPT_MSS_LEN - 2;
 
 		conn->ws_to_tap = MIN(MAX_WS, tinfo.tcpi_snd_wscale);
 
-		*data++ = OPT_NOP;
+		*opts = TCP_SYN_OPTS(mss, conn->ws_to_tap);
-		*data++ = OPT_WS;
+		*optlen = sizeof(*opts);
-		*data++ = OPT_WS_LEN;
-		*data++ = conn->ws_to_tap;
 	} else if (!(flags & RST)) {
 		flags |= ACK;
 	}

tcp_buf.c

@@ -282,7 +282,7 @@ int tcp_buf_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 
 	seq = conn->seq_to_tap;
 	ret = tcp_prepare_flags(c, conn, flags, &payload->th,
-				payload->opts, &optlen);
+				&payload->opts, &optlen);
 	if (ret <= 0) {
 		if (CONN_V4(conn))
 			tcp4_flags_used--;

tcp_internal.h

@@ -33,9 +33,7 @@
 #define OPT_EOL		0
 #define OPT_NOP		1
 #define OPT_MSS		2
-#define OPT_MSS_LEN	4
 #define OPT_WS		3
-#define OPT_WS_LEN	3
 #define OPT_SACKP	4
 #define OPT_SACK	5
 #define OPT_TS		8
@@ -77,6 +75,65 @@ struct tcp_payload_t {
 } __attribute__ ((packed, aligned(__alignof__(unsigned int))));
 #endif
 
+/** struct tcp_opt_nop - TCP NOP option
+ * @kind:	Option kind (OPT_NOP = 1)
+ */
+struct tcp_opt_nop {
+	uint8_t kind;
+} __attribute__ ((packed));
+#define TCP_OPT_NOP		((struct tcp_opt_nop){ .kind = OPT_NOP, })
+
+/** struct tcp_opt_mss - TCP MSS option
+ * @kind:	Option kind (OPT_MSS == 2)
+ * @len:	Option length (4)
+ * @mss:	Maximum Segment Size
+ */
+struct tcp_opt_mss {
+	uint8_t kind;
+	uint8_t len;
+	uint16_t mss;
+} __attribute__ ((packed));
+#define TCP_OPT_MSS(mss_)				\
+	((struct tcp_opt_mss) {				\
+		.kind = OPT_MSS,			\
+		.len = sizeof(struct tcp_opt_mss),	\
+		.mss = htons(mss_),			\
+	})
+
+/** struct tcp_opt_ws - TCP Window Scaling option
+ * @kind:	Option kind (OPT_WS == 3)
+ * @len:	Option length (3)
+ * @shift:	Window scaling shift
+ */
+struct tcp_opt_ws {
+	uint8_t kind;
+	uint8_t len;
+	uint8_t shift;
+} __attribute__ ((packed));
+#define TCP_OPT_WS(shift_)				\
+	((struct tcp_opt_ws) {				\
+		.kind = OPT_WS,				\
+		.len = sizeof(struct tcp_opt_ws),	\
+		.shift = (shift_),			\
+	})
+
+/** struct tcp_syn_opts - TCP options we apply to SYN packets
+ * @mss:	Maximum Segment Size (MSS) option
+ * @nop:	NOP opt (for alignment)
+ * @ws:		Window Scaling (WS) option
+ */
+struct tcp_syn_opts {
+	struct tcp_opt_mss mss;
+	struct tcp_opt_nop nop;
+	struct tcp_opt_ws ws;
+} __attribute__ ((packed));
+#define TCP_SYN_OPTS(mss_, ws_)				\
+	((struct tcp_syn_opts){				\
+		.mss = TCP_OPT_MSS(mss_),		\
+		.nop = TCP_OPT_NOP,			\
+		.ws = TCP_OPT_WS(ws_),			\
+	})
+
 /**
  * struct tcp_flags_t - TCP header and data to send zero-length
  *                      segments (flags)
@@ -85,7 +142,7 @@ struct tcp_payload_t {
  */
 struct tcp_flags_t {
 	struct tcphdr th;
-	char opts[OPT_MSS_LEN + OPT_WS_LEN + 1];
+	struct tcp_syn_opts opts;
 #ifdef __AVX2__
 } __attribute__ ((packed, aligned(32)));
 #else
@@ -124,7 +181,8 @@ size_t tcp_l2_buf_fill_headers(const struct tcp_tap_conn *conn,
 			       bool no_tcp_csum);
 int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
 			  bool force_seq, struct tcp_info *tinfo);
-int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn, int flags,
+int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
-		      struct tcphdr *th, char *data, size_t *optlen);
+		      int flags, struct tcphdr *th, struct tcp_syn_opts *opts,
+		      size_t *optlen);
 
 #endif /* TCP_INTERNAL_H */