inany: Helper functions for handling addresses which could be IPv4 or IPv6

struct tcp_conn stores an address which could be IPv6 or IPv4 using a
union.  We can do this without an additional tag by encoding IPv4 addresses
as IPv4-mapped IPv6 addresses.

This approach is useful wider than the specific place in tcp_conn, so
expose a new 'union inany_addr' like this from a new inany.h.  Along with
that create a number of helper functions to make working with these "inany"
addresses easier.

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

5 files changed, 117 insertions, 61 deletions

diff --git a/Makefile b/Makefile
index a9b4f79..ce159e5 100644
--- a/Makefile
+++ b/Makefile
@@ -46,9 +46,9 @@ SRCS = $(PASST_SRCS) $(QRAP_SRCS)
 MANPAGES = passt.1 pasta.1 qrap.1
 
 PASST_HEADERS = arch.h arp.h checksum.h conf.h dhcp.h dhcpv6.h icmp.h \
-	isolation.h lineread.h log.h ndp.h netlink.h packet.h passt.h pasta.h \
-	pcap.h port_fwd.h siphash.h tap.h tcp.h tcp_conn.h tcp_splice.h udp.h \
-	util.h
+	inany.h isolation.h lineread.h log.h ndp.h netlink.h packet.h passt.h \
+	pasta.h pcap.h port_fwd.h siphash.h tap.h tcp.h tcp_conn.h \
+	tcp_splice.h udp.h util.h
 HEADERS = $(PASST_HEADERS) seccomp.h
 
 # On gcc 11 and 12, with -O2 and -flto, tcp_hash() and siphash_20b(), if
diff --git a/inany.h b/inany.h
new file mode 100644
index 0000000..03e3e3f
--- /dev/null
+++ b/inany.h
@@ -0,0 +1,68 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ *
+ * inany.h - Types and helpers for handling addresses which could be
+ *           IPv6 or IPv4 (encoded as IPv4-mapped IPv6 addresses)
+ */
+
+#include <assert.h>
+
+/** union inany_addr - Represents either an IPv4 or IPv6 address
+ * @a6:			Address as an IPv6 address, may be IPv4-mapped
+ * @v4mapped.zero:	All zero-bits for an IPv4 address
+ * @v4mapped.one:	All one-bits for an IPv4 address
+ * @v4mapped.a4:	If @a6 is an IPv4 mapped address, the IPv4 address
+ *
+ * @v4mapped shouldn't be accessed except via helpers.
+ */
+union inany_addr {
+	struct in6_addr a6;
+	struct {
+		uint8_t zero[10];
+		uint8_t one[2];
+		struct in_addr a4;
+	} v4mapped;
+};
+
+/** inany_v4 - Extract IPv4 address, if present, from IPv[46] address
+ * @addr:	IPv4 or IPv6 address
+ *
+ * Return: IPv4 address if @addr is IPv4, NULL otherwise
+ */
+static inline const struct in_addr *inany_v4(const union inany_addr *addr)
+{
+	if (!IN6_IS_ADDR_V4MAPPED(&addr->a6))
+		return NULL;
+	return &addr->v4mapped.a4;
+}
+
+/** inany_equals - Compare two IPv[46] addresses
+ * @a, @b:	IPv[46] addresses
+ *
+ * Return: true if @a and @b are the same address
+ */
+static inline bool inany_equals(const union inany_addr *a,
+				const union inany_addr *b)
+{
+	return IN6_ARE_ADDR_EQUAL(&a->a6, &b->a6);
+}
+
+/** inany_from_af - Set IPv[46] address from IPv4 or IPv6 address
+ * @aa:		Pointer to store IPv[46] address
+ * @af:		Address family of @addr
+ * @addr:	struct in_addr (IPv4) or struct in6_addr (IPv6)
+ */
+static inline void inany_from_af(union inany_addr *aa, int af, const void *addr)
+{
+	if (af == AF_INET6) {
+		aa->a6 = *((struct in6_addr *)addr);
+	} else if (af == AF_INET) {
+		memset(&aa->v4mapped.zero, 0, sizeof(aa->v4mapped.zero));
+		memset(&aa->v4mapped.one, 0xff, sizeof(aa->v4mapped.one));
+		aa->v4mapped.a4 = *((struct in_addr *)addr);
+	} else {
+		/* Not valid to call with other address families */
+		assert(0);
+	}
+}
diff --git a/tcp.c b/tcp.c
index e9803aa..d1008ae 100644
--- a/tcp.c
+++ b/tcp.c
@@ -301,6 +301,7 @@
 #include "conf.h"
 #include "tcp_splice.h"
 #include "log.h"
+#include "inany.h"
 
 #include "tcp_conn.h"
 
@@ -404,7 +405,7 @@ struct tcp6_l2_head {	/* For MSS6 macro: keep in sync with tcp6_l2_buf_t */
 #define OPT_SACK	5
 #define OPT_TS		8
 
-#define CONN_V4(conn)		IN6_IS_ADDR_V4MAPPED(&conn->a.a6)
+#define CONN_V4(conn)		(!!inany_v4(&(conn)->addr))
 #define CONN_V6(conn)		(!CONN_V4(conn))
 #define CONN_IS_CLOSING(conn)						\
 	((conn->events & ESTABLISHED) &&				\
@@ -438,7 +439,7 @@ static int tcp_sock_init_ext	[NUM_PORTS][IP_VERSIONS];
 static int tcp_sock_ns		[NUM_PORTS][IP_VERSIONS];
 
 /* Table of destinations with very low RTT (assumed to be local), LRU */
-static struct in6_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
+static union inany_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
 
 /* Static buffers */
 
@@ -861,7 +862,7 @@ static int tcp_rtt_dst_low(const struct tcp_tap_conn *conn)
 	int i;
 
 	for (i = 0; i < LOW_RTT_TABLE_SIZE; i++)
-		if (IN6_ARE_ADDR_EQUAL(&conn->a.a6, low_rtt_dst + i))
+		if (inany_equals(&conn->addr, low_rtt_dst + i))
 			return 1;
 
 	return 0;
@@ -883,7 +884,7 @@ static void tcp_rtt_dst_check(const struct tcp_tap_conn *conn,
 		return;
 
 	for (i = 0; i < LOW_RTT_TABLE_SIZE; i++) {
-		if (IN6_ARE_ADDR_EQUAL(&conn->a.a6, low_rtt_dst + i))
+		if (inany_equals(&conn->addr, low_rtt_dst + i))
 			return;
 		if (hole == -1 && IN6_IS_ADDR_UNSPECIFIED(low_rtt_dst + i))
 			hole = i;
@@ -895,10 +896,10 @@ static void tcp_rtt_dst_check(const struct tcp_tap_conn *conn,
 	if (hole == -1)
 		return;
 
-	memcpy(low_rtt_dst + hole++, &conn->a.a6, sizeof(conn->a.a6));
+	low_rtt_dst[hole++] = conn->addr;
 	if (hole == LOW_RTT_TABLE_SIZE)
 		hole = 0;
-	memcpy(low_rtt_dst + hole, &in6addr_any, sizeof(conn->a.a6));
+	inany_from_af(low_rtt_dst + hole, AF_INET6, &in6addr_any);
 #else
 	(void)conn;
 	(void)tinfo;
@@ -1187,13 +1188,14 @@ static int tcp_hash_match(const struct tcp_tap_conn *conn,
 			  int af, const void *addr,
 			  in_port_t tap_port, in_port_t sock_port)
 {
-	if (af == AF_INET && CONN_V4(conn)			&&
-	    !memcmp(&conn->a.a4.a, addr, sizeof(conn->a.a4.a))	&&
+	const struct in_addr *a4 = inany_v4(&conn->addr);
+
+	if (af == AF_INET && a4	&& !memcmp(a4, addr, sizeof(*a4)) &&
 	    conn->tap_port == tap_port && conn->sock_port == sock_port)
 		return 1;
 
 	if (af == AF_INET6					&&
-	    IN6_ARE_ADDR_EQUAL(&conn->a.a6, addr)		&&
+	    IN6_ARE_ADDR_EQUAL(&conn->addr.a6, addr)		&&
 	    conn->tap_port == tap_port && conn->sock_port == sock_port)
 		return 1;
 
@@ -1253,11 +1255,13 @@ static unsigned int tcp_hash(const struct ctx *c, int af, const void *addr,
 static unsigned int tcp_conn_hash(const struct ctx *c,
 				  const struct tcp_tap_conn *conn)
 {
-	if (CONN_V6(conn))
-		return tcp_hash(c, AF_INET6, &conn->a.a6,
+	const struct in_addr *a4 = inany_v4(&conn->addr);
+
+	if (a4)
+		return tcp_hash(c, AF_INET, a4,
 				conn->tap_port, conn->sock_port);
 	else
-		return tcp_hash(c, AF_INET, &conn->a.a4.a,
+		return tcp_hash(c, AF_INET6, &conn->addr.a6,
 				conn->tap_port, conn->sock_port);
 }
 
@@ -1582,6 +1586,7 @@ static size_t tcp_l2_buf_fill_headers(const struct ctx *c,
 				      void *p, size_t plen,
 				      const uint16_t *check, uint32_t seq)
 {
+	const struct in_addr *a4 = inany_v4(&conn->addr);
 	size_t ip_len, eth_len;
 
 #define SET_TCP_HEADER_COMMON_V4_V6(b, conn, seq)			\
@@ -1599,13 +1604,33 @@ do {									\
 	}								\
 } while (0)
 
-	if (CONN_V6(conn)) {
+	if (a4) {
+		struct tcp4_l2_buf_t *b = (struct tcp4_l2_buf_t *)p;
+
+		ip_len = plen + sizeof(struct iphdr) + sizeof(struct tcphdr);
+		b->iph.tot_len = htons(ip_len);
+		b->iph.saddr = a4->s_addr;
+		b->iph.daddr = c->ip4.addr_seen.s_addr;
+
+		if (check)
+			b->iph.check = *check;
+		else
+			tcp_update_check_ip4(b);
+
+		SET_TCP_HEADER_COMMON_V4_V6(b, conn, seq);
+
+		tcp_update_check_tcp4(b);
+
+		eth_len = ip_len + sizeof(struct ethhdr);
+		if (c->mode == MODE_PASST)
+			b->vnet_len = htonl(eth_len);
+	} else {
 		struct tcp6_l2_buf_t *b = (struct tcp6_l2_buf_t *)p;
 
 		ip_len = plen + sizeof(struct ipv6hdr) + sizeof(struct tcphdr);
 
 		b->ip6h.payload_len = htons(plen + sizeof(struct tcphdr));
-		b->ip6h.saddr = conn->a.a6;
+		b->ip6h.saddr = conn->addr.a6;
 		if (IN6_IS_ADDR_LINKLOCAL(&b->ip6h.saddr))
 			b->ip6h.daddr = c->ip6.addr_ll_seen;
 		else
@@ -1624,26 +1649,6 @@ do {									\
 		eth_len = ip_len + sizeof(struct ethhdr);
 		if (c->mode == MODE_PASST)
 			b->vnet_len = htonl(eth_len);
-	} else {
-		struct tcp4_l2_buf_t *b = (struct tcp4_l2_buf_t *)p;
-
-		ip_len = plen + sizeof(struct iphdr) + sizeof(struct tcphdr);
-		b->iph.tot_len = htons(ip_len);
-		b->iph.saddr = conn->a.a4.a.s_addr;
-		b->iph.daddr = c->ip4.addr_seen.s_addr;
-
-		if (check)
-			b->iph.check = *check;
-		else
-			tcp_update_check_ip4(b);
-
-		SET_TCP_HEADER_COMMON_V4_V6(b, conn, seq);
-
-		tcp_update_check_tcp4(b);
-
-		eth_len = ip_len + sizeof(struct ethhdr);
-		if (c->mode == MODE_PASST)
-			b->vnet_len = htonl(eth_len);
 	}
 
 #undef SET_TCP_HEADER_COMMON_V4_V6
@@ -2144,18 +2149,14 @@ static void tcp_conn_from_tap(struct ctx *c, int af, const void *addr,
 	if (!(conn->wnd_from_tap = (htons(th->window) >> conn->ws_from_tap)))
 		conn->wnd_from_tap = 1;
 
+	inany_from_af(&conn->addr, af, addr);
+
 	if (af == AF_INET) {
 		sa = (struct sockaddr *)&addr4;
 		sl = sizeof(addr4);
-
-		memset(&conn->a.a4.zero, 0,    sizeof(conn->a.a4.zero));
-		memset(&conn->a.a4.one,  0xff, sizeof(conn->a.a4.one));
-		memcpy(&conn->a.a4.a,    addr, sizeof(conn->a.a4.a));
 	} else {
 		sa = (struct sockaddr *)&addr6;
 		sl = sizeof(addr6);
-
-		memcpy(&conn->a.a6,      addr, sizeof(conn->a.a6));
 	}
 
 	conn->sock_port = ntohs(th->dest);
@@ -2808,7 +2809,7 @@ static void tcp_tap_conn_from_sock(struct ctx *c, union epoll_ref ref,
 			memcpy(&sa6.sin6_addr, src, sizeof(*src));
 		}
 
-		memcpy(&conn->a.a6, &sa6.sin6_addr, sizeof(conn->a.a6));
+		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;
@@ -2824,15 +2825,12 @@ static void tcp_tap_conn_from_sock(struct ctx *c, union epoll_ref ref,
 
 		memcpy(&sa4, sa, sizeof(sa4));
 
-		memset(&conn->a.a4.zero,   0, sizeof(conn->a.a4.zero));
-		memset(&conn->a.a4.one, 0xff, sizeof(conn->a.a4.one));
-
 		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;
 
-		conn->a.a4.a = sa4.sin_addr;
+		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;
diff --git a/tcp_conn.h b/tcp_conn.h
index 4bffe9a..bf50e1c 100644
--- a/tcp_conn.h
+++ b/tcp_conn.h
@@ -35,10 +35,7 @@ extern const char *tcp_common_flag_str[];
  * @ws_to_tap:		Window scaling factor advertised to tap/guest
  * @sndbuf:		Sending buffer in kernel, rounded to 2 ^ SNDBUF_BITS
  * @seq_dup_ack_approx:	Last duplicate ACK number sent to tap
- * @a.a6:		IPv6 remote address, can be IPv4-mapped
- * @a.a4.zero:		Zero prefix for IPv4-mapped, see RFC 6890, Table 20
- * @a.a4.one:		Ones prefix for IPv4-mapped
- * @a.a4.a:		IPv4 address
+ * @addr:		Remote address (IPv4 or IPv6)
  * @tap_port:		Guest-facing tap port
  * @sock_port:		Remote, socket-facing port
  * @wnd_from_tap:	Last window size from tap, unscaled (as received)
@@ -108,15 +105,7 @@ struct tcp_tap_conn {
 	uint8_t		seq_dup_ack_approx;
 
 
-	union {
-		struct in6_addr a6;
-		struct {
-			uint8_t zero[10];
-			uint8_t one[2];
-			struct in_addr a;
-		} a4;
-	} a;
-
+	union inany_addr addr;
 	in_port_t	tap_port;
 	in_port_t	sock_port;
 
diff --git a/tcp_splice.c b/tcp_splice.c
index bfcf63d..2be9b77 100644
--- a/tcp_splice.c
+++ b/tcp_splice.c
@@ -52,6 +52,7 @@
 #include "passt.h"
 #include "log.h"
 #include "tcp_splice.h"
+#include "inany.h"
 
 #include "tcp_conn.h"