conf: Use 0 instead of -1 as "unassigned" mtu value

On the command line -m 0 means "don't assign an MTU" (letting the guest use
its default.  However, internally we use (c->mtu == -1) to represent that
state.  We use (c->mtu == 0) to represent "the user didn't specify on the
command line, so use the default" - but this is only used during conf(),
never afterwards.

This is unnecessarily confusing.  We can instead just initialise c->mtu to
its default (65520) before parsing options and use 0 on both the command
line and internally to represent the "don't assign" special case.  This
ensures that c->mtu is always 0..65535, so we can store it in a uint16_t
which is more natural.

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

conf.c

@@ -1413,6 +1413,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		optstring = "+dqfel:hs:F:p:P:m:a:n:M:g:i:o:D:S:H:461t:u:";
 	}
 
+	c->mtu = ROUND_DOWN(ETH_MAX_MTU - ETH_HLEN, sizeof(uint32_t));
 	c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = FWD_UNSET;
 	c->udp.fwd_in.mode = c->udp.fwd_out.mode = FWD_UNSET;
 	memcpy(c->our_tap_mac, MAC_OUR_LAA, ETH_ALEN);
@@ -1662,12 +1663,7 @@ void conf(struct ctx *c, int argc, char **argv)
 			if (errno || *e)
 				die("Invalid MTU: %s", optarg);
 
-			if (!mtu) {
+			if (mtu && (mtu < ETH_MIN_MTU || mtu > ETH_MAX_MTU)) {
-				c->mtu = -1;
-				break;
-			}
-
-			if (mtu < ETH_MIN_MTU || mtu > ETH_MAX_MTU) {
 				die("MTU %lu out of range (%u..%u)", mtu,
 				    ETH_MIN_MTU, ETH_MAX_MTU);
 			}
@@ -1980,9 +1976,6 @@ void conf(struct ctx *c, int argc, char **argv)
 		c->no_dhcpv6 = 1;
 	}
 
-	if (!c->mtu)
-		c->mtu = ROUND_DOWN(ETH_MAX_MTU - ETH_HLEN, sizeof(uint32_t));
-
 	get_dns(c);
 
 	if (!*c->pasta_ifn) {

dhcp.c

@@ -417,7 +417,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
 		       &c->ip4.guest_gw, sizeof(c->ip4.guest_gw));
 	}
 
-	if (c->mtu != -1) {
+	if (c->mtu) {
 		opts[26].slen = 2;
 		opts[26].s[0] = c->mtu / 256;
 		opts[26].s[1] = c->mtu % 256;

ndp.c

@@ -256,7 +256,7 @@ static void ndp_ra(const struct ctx *c, const struct in6_addr *dst)
 
 	ptr = &ra.var[0];
 
-	if (c->mtu != -1) {
+	if (c->mtu) {
 		struct opt_mtu *mtu = (struct opt_mtu *)ptr;
 		*mtu = (struct opt_mtu) {
 			.header = {

passt.h

@@ -274,6 +274,8 @@ struct ctx {
 	int fd_repair;
 	unsigned char our_tap_mac[ETH_ALEN];
 	unsigned char guest_mac[ETH_ALEN];
+	uint16_t mtu;
+
 	uint64_t hash_secret[2];
 
 	int ifi4;
@@ -298,7 +300,6 @@ struct ctx {
 	int no_icmp;
 	struct icmp_ctx icmp;
 
-	int mtu;
 	int no_dns;
 	int no_dns_search;
 	int no_dhcp_dns;

pasta.c

@@ -319,7 +319,7 @@ void pasta_ns_conf(struct ctx *c)
 	if (c->pasta_conf_ns) {
 		unsigned int flags = IFF_UP;
 
-		if (c->mtu != -1)
+		if (c->mtu)
 			nl_link_set_mtu(nl_sock_ns, c->pasta_ifi, c->mtu);
 
 		if (c->ifi6) /* Avoid duplicate address detection on link up */

tcp.c

@@ -1139,7 +1139,7 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
 	if (flags & SYN) {
 		int mss;
 
-		if (c->mtu == -1) {
+		if (!c->mtu) {
 			mss = tinfo.tcpi_snd_mss;
 		} else {
 			mss = c->mtu - sizeof(struct tcphdr);