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>

1 files changed, 3 insertions, 14 deletions

diff --git a/tcp.c b/tcp.c
index b2155ab..0d22e07 100644
--- a/tcp.c
+++ b/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;
-		*data++ = OPT_WS;
-		*data++ = OPT_WS_LEN;
-		*data++ = conn->ws_to_tap;
+		*opts = TCP_SYN_OPTS(mss, conn->ws_to_tap);
+		*optlen = sizeof(*opts);
 	} else if (!(flags & RST)) {
 		flags |= ACK;
 	}