[PATCH 27/32] tcp: NAT IPv4-mapped IPv6 addresses like IPv4 addresses

[David Gibson <david@gibson.dropbear.id.au>]

passt usually doesn't NAT, but it does do so for the remapping of the
gateway address to refer to the host.  Currently we perform this NAT with
slightly different rules on both IPv4 addresses and IPv6 addresses, but not
on IPv4-mapped IPv6 addresses.  This means we won't correctly handle the
case of an IPv4 connection over an IPv6 socket, which is possible on Linux
(and probably other platforms).

Refactor tcp_conn_from_sock() to perform the NAT after converting either
address family into an inany_addr, so IPv4 and and IPv4-mapped addresses
have the same representation.

With two new helpers this lets us remove the IPv4 and IPv6 specific paths
from tcp_conn_from_sock().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 inany.h | 30 ++++++++++++++++++++++++++--
 tcp.c   | 62 ++++++++++++++++++++++++---------------------------------
 2 files changed, 54 insertions(+), 38 deletions(-)

diff --git a/inany.h b/inany.h
index 4e53da9..a677aa7 100644
--- a/inany.h
+++ b/inany.h
@@ -30,11 +30,11 @@ union inany_addr {
  *
  * Return: IPv4 address if @addr is IPv4, NULL otherwise
  */
-static inline const struct in_addr *inany_v4(const union inany_addr *addr)
+static inline struct in_addr *inany_v4(const union inany_addr *addr)
 {
 	if (!IN6_IS_ADDR_V4MAPPED(&addr->a6))
 		return NULL;
-	return &addr->_v4mapped.a4;
+	return (struct in_addr *)&addr->_v4mapped.a4;
 }
 
 /** inany_equals - Compare two IPv[46] addresses
@@ -66,3 +66,29 @@ static inline void inany_from_af(union inany_addr *aa, int af, const void *addr)
 		assert(0);
 	}
 }
+
+/** inany_from_sockaddr - Extract IPv[46] address and port number from sockaddr
+ * @a:		Pointer to store IPv[46] address
+ * @port:	Pointer to store port number, host order
+ * @addr:	struct sockaddr_in (IPv4) or struct sockaddr_in6 (IPv6)
+ */
+static inline void inany_from_sockaddr(union inany_addr *aa, in_port_t *port,
+				       const void *sa_)
+{
+	const struct sockaddr *sa = (const struct sockaddr *)sa_;
+
+	if (sa->sa_family == AF_INET6) {
+		struct sockaddr_in6 *sa6 = (struct sockaddr_in6 *)sa;
+
+		inany_from_af(aa, AF_INET6, &sa6->sin6_addr);
+		*port = ntohs(sa6->sin6_port);
+	} else if (sa->sa_family == AF_INET) {
+		struct sockaddr_in *sa4 = (struct sockaddr_in *)sa;
+
+		inany_from_af(aa, AF_INET, &sa4->sin_addr);
+		*port = ntohs(sa4->sin_port);
+	} else {
+		/* Not valid to call with other address families */
+		assert(0);
+	}
+}
diff --git a/tcp.c b/tcp.c
index b05ed6c..fca5df4 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2724,6 +2724,29 @@ static void tcp_connect_finish(struct ctx *c, struct tcp_tap_conn *conn)
 	conn_flag(c, conn, ACK_FROM_TAP_DUE);
 }
 
+static void tcp_snat_inbound(const struct ctx *c, union inany_addr *addr)
+{
+	struct in_addr *addr4 = inany_v4(addr);
+
+	if (addr4) {
+		if (IN4_IS_ADDR_LOOPBACK(addr4) ||
+		    IN4_IS_ADDR_UNSPECIFIED(addr4) ||
+		    IN4_ARE_ADDR_EQUAL(addr4, &c->ip4.addr_seen))
+			*addr4 = c->ip4.gw;
+	} else {
+		struct in6_addr *addr6 = &addr->a6;
+
+		if (IN6_IS_ADDR_LOOPBACK(addr6) ||
+		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr_seen) ||
+		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr)) {
+			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
+				*addr6 = c->ip6.gw;
+			else
+				*addr6 = c->ip6.addr_ll;
+		}
+	}
+}
+
 /**
  * tcp_tap_conn_from_sock() - Initialize state for non-spliced connection
  * @c:		Execution context
@@ -2744,43 +2767,10 @@ static void tcp_tap_conn_from_sock(struct ctx *c, union epoll_ref ref,
 	conn->ws_to_tap = conn->ws_from_tap = 0;
 	conn_event(c, conn, SOCK_ACCEPTED);
 
-	if (sa->sa_family == AF_INET6) {
-		struct sockaddr_in6 sa6;
-
-		memcpy(&sa6, sa, sizeof(sa6));
-
-		if (IN6_IS_ADDR_LOOPBACK(&sa6.sin6_addr) ||
-		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr_seen) ||
-		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr)) {
-			struct in6_addr *src;
+	inany_from_sockaddr(&conn->addr, &conn->sock_port, sa);
+	conn->tap_port = ref.r.p.tcp.tcp.index;
 
-			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
-				src = &c->ip6.gw;
-			else
-				src = &c->ip6.addr_ll;
-
-			memcpy(&sa6.sin6_addr, src, sizeof(*src));
-		}
-
-		inany_from_af(&conn->addr, AF_INET6, &sa6.sin6_addr);
-
-		conn->sock_port = ntohs(sa6.sin6_port);
-		conn->tap_port = ref.r.p.tcp.tcp.index;
-	} else {
-		struct sockaddr_in sa4;
-
-		memcpy(&sa4, sa, sizeof(sa4));
-
-		if (IN4_IS_ADDR_LOOPBACK(&sa4.sin_addr) ||
-		    IN4_IS_ADDR_UNSPECIFIED(&sa4.sin_addr) ||
-		    IN4_ARE_ADDR_EQUAL(&sa4.sin_addr, &c->ip4.addr_seen))
-			sa4.sin_addr = c->ip4.gw;
-
-		inany_from_af(&conn->addr, AF_INET, &sa4.sin_addr);
-
-		conn->sock_port = ntohs(sa4.sin_port);
-		conn->tap_port = ref.r.p.tcp.tcp.index;
-	}
+	tcp_snat_inbound(c, &conn->addr);
 
 	tcp_seq_init(c, conn, now);
 	tcp_hash_insert(c, conn);
-- 
@@ -2724,6 +2724,29 @@ static void tcp_connect_finish(struct ctx *c, struct tcp_tap_conn *conn)
 	conn_flag(c, conn, ACK_FROM_TAP_DUE);
 }
 
+static void tcp_snat_inbound(const struct ctx *c, union inany_addr *addr)
+{
+	struct in_addr *addr4 = inany_v4(addr);
+
+	if (addr4) {
+		if (IN4_IS_ADDR_LOOPBACK(addr4) ||
+		    IN4_IS_ADDR_UNSPECIFIED(addr4) ||
+		    IN4_ARE_ADDR_EQUAL(addr4, &c->ip4.addr_seen))
+			*addr4 = c->ip4.gw;
+	} else {
+		struct in6_addr *addr6 = &addr->a6;
+
+		if (IN6_IS_ADDR_LOOPBACK(addr6) ||
+		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr_seen) ||
+		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr)) {
+			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
+				*addr6 = c->ip6.gw;
+			else
+				*addr6 = c->ip6.addr_ll;
+		}
+	}
+}
+
 /**
  * tcp_tap_conn_from_sock() - Initialize state for non-spliced connection
  * @c:		Execution context
@@ -2744,43 +2767,10 @@ static void tcp_tap_conn_from_sock(struct ctx *c, union epoll_ref ref,
 	conn->ws_to_tap = conn->ws_from_tap = 0;
 	conn_event(c, conn, SOCK_ACCEPTED);
 
-	if (sa->sa_family == AF_INET6) {
-		struct sockaddr_in6 sa6;
-
-		memcpy(&sa6, sa, sizeof(sa6));
-
-		if (IN6_IS_ADDR_LOOPBACK(&sa6.sin6_addr) ||
-		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr_seen) ||
-		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr)) {
-			struct in6_addr *src;
+	inany_from_sockaddr(&conn->addr, &conn->sock_port, sa);
+	conn->tap_port = ref.r.p.tcp.tcp.index;
 
-			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
-				src = &c->ip6.gw;
-			else
-				src = &c->ip6.addr_ll;
-
-			memcpy(&sa6.sin6_addr, src, sizeof(*src));
-		}
-
-		inany_from_af(&conn->addr, AF_INET6, &sa6.sin6_addr);
-
-		conn->sock_port = ntohs(sa6.sin6_port);
-		conn->tap_port = ref.r.p.tcp.tcp.index;
-	} else {
-		struct sockaddr_in sa4;
-
-		memcpy(&sa4, sa, sizeof(sa4));
-
-		if (IN4_IS_ADDR_LOOPBACK(&sa4.sin_addr) ||
-		    IN4_IS_ADDR_UNSPECIFIED(&sa4.sin_addr) ||
-		    IN4_ARE_ADDR_EQUAL(&sa4.sin_addr, &c->ip4.addr_seen))
-			sa4.sin_addr = c->ip4.gw;
-
-		inany_from_af(&conn->addr, AF_INET, &sa4.sin_addr);
-
-		conn->sock_port = ntohs(sa4.sin_port);
-		conn->tap_port = ref.r.p.tcp.tcp.index;
-	}
+	tcp_snat_inbound(c, &conn->addr);
 
 	tcp_seq_init(c, conn, now);
 	tcp_hash_insert(c, conn);
-- 
2.38.1