[PATCH v6 5/8] udp: forward external source MAC address through tap interface

[Jon Maloy <jmaloy@redhat.com>]

We forward the incoming MAC address through the tap interface when
receiving incoming packets from network local hosts.

This is a part of the solution to bug
https://bugs.passt.top/show_bug.cgi?id=120

Signed-off-by: Jon Maloy <jmaloy@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

---
v3: - Adapted to the move of external MAC address from struct flowside
      to struct flow_common
v4: - Changed signature of udp_tap_prepare() to take a MAC address
      instead of a flow.
    - Eliminated initialization of MAC source address in all frames,
      since those now are set per send occasion anyway.
v5: - Added lookup in ARP/NDP table on incoming messages in
      case flow->tap_omac wasn't initialized at flow creation,
      i.e., the flow was initiated from the guest.
v6: - Using MAC_ZERO
---
 passt.c |  2 +-
 udp.c   | 45 +++++++++++++++++++++++++--------------------
 udp.h   |  2 +-
 3 files changed, 27 insertions(+), 22 deletions(-)

diff --git a/passt.c b/passt.c
index a4ec115..2a28e20 100644
--- a/passt.c
+++ b/passt.c
@@ -154,7 +154,7 @@ static void timer_init(struct ctx *c, const struct timespec *now)
 void proto_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s)
 {
 	tcp_update_l2_buf(eth_d, eth_s);
-	udp_update_l2_buf(eth_d, eth_s);
+	udp_update_l2_buf(eth_d);
 }
 
 /**
diff --git a/udp.c b/udp.c
index 86585b7..eb57f05 100644
--- a/udp.c
+++ b/udp.c
@@ -133,11 +133,8 @@ static int udp_splice_init[IP_VERSIONS][NUM_PORTS];
 /* UDP header and data for inbound messages */
 static struct udp_payload_t udp_payload[UDP_MAX_FRAMES];
 
-/* Ethernet header for IPv4 frames */
-static struct ethhdr udp4_eth_hdr;
-
-/* Ethernet header for IPv6 frames */
-static struct ethhdr udp6_eth_hdr;
+/* Ethernet headers for IPv4 and IPv6 frames */
+static struct ethhdr udp_eth_hdr[UDP_MAX_FRAMES];
 
 /**
  * struct udp_meta_t - Pre-cooked headers for UDP packets
@@ -210,12 +207,13 @@ void udp_portmap_clear(void)
 /**
  * udp_update_l2_buf() - Update L2 buffers with Ethernet and IPv4 addresses
  * @eth_d:	Ethernet destination address, NULL if unchanged
- * @eth_s:	Ethernet source address, NULL if unchanged
  */
-void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s)
+void udp_update_l2_buf(const unsigned char *eth_d)
 {
-	eth_update_mac(&udp4_eth_hdr, eth_d, eth_s);
-	eth_update_mac(&udp6_eth_hdr, eth_d, eth_s);
+	int i;
+
+	for (i = 0; i < UDP_MAX_FRAMES; i++)
+		eth_update_mac(&udp_eth_hdr[i], eth_d, NULL);
 }
 
 /**
@@ -238,6 +236,7 @@ static void udp_iov_init_one(const struct ctx *c, size_t i)
 
 	*siov = IOV_OF_LVALUE(payload->data);
 
+	tiov[UDP_IOV_ETH] = IOV_OF_LVALUE(udp_eth_hdr[i]);
 	tiov[UDP_IOV_TAP] = tap_hdr_iov(c, &meta->taph);
 	tiov[UDP_IOV_PAYLOAD].iov_base = payload;
 
@@ -253,9 +252,6 @@ static void udp_iov_init(const struct ctx *c)
 {
 	size_t i;
 
-	udp4_eth_hdr.h_proto = htons_constant(ETH_P_IP);
-	udp6_eth_hdr.h_proto = htons_constant(ETH_P_IPV6);
-
 	for (i = 0; i < UDP_MAX_FRAMES; i++)
 		udp_iov_init_one(c, i);
 }
@@ -352,31 +348,34 @@ size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
  * udp_tap_prepare() - Convert one datagram into a tap frame
  * @mmh:	Receiving mmsghdr array
  * @idx:	Index of the datagram to prepare
+ * @tap_omac:	MAC address of remote endpoint as seen from the guest
  * @toside:	Flowside for destination side
  * @no_udp_csum: Do not set UDP checksum
  */
 static void udp_tap_prepare(const struct mmsghdr *mmh,
-			    unsigned idx, const struct flowside *toside,
+			    unsigned int idx,
+			    const uint8_t *tap_omac,
+			    const struct flowside *toside,
 			    bool no_udp_csum)
 {
 	struct iovec (*tap_iov)[UDP_NUM_IOVS] = &udp_l2_iov[idx];
 	struct udp_payload_t *bp = &udp_payload[idx];
 	struct udp_meta_t *bm = &udp_meta[idx];
+	struct ethhdr *eh = (*tap_iov)[UDP_IOV_ETH].iov_base;
 	size_t l4len;
 
+	eth_update_mac(eh, NULL, tap_omac);
 	if (!inany_v4(&toside->eaddr) || !inany_v4(&toside->oaddr)) {
 		l4len = udp_update_hdr6(&bm->ip6h, bp, toside,
 					mmh[idx].msg_len, no_udp_csum);
-		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip6h) +
-			       sizeof(udp6_eth_hdr));
-		(*tap_iov)[UDP_IOV_ETH] = IOV_OF_LVALUE(udp6_eth_hdr);
+		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip6h) + ETH_HLEN);
+		eh->h_proto = htons_constant(ETH_P_IPV6);
 		(*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip6h);
 	} else {
 		l4len = udp_update_hdr4(&bm->ip4h, bp, toside,
 					mmh[idx].msg_len, no_udp_csum);
-		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip4h) +
-			       sizeof(udp4_eth_hdr));
-		(*tap_iov)[UDP_IOV_ETH] = IOV_OF_LVALUE(udp4_eth_hdr);
+		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip4h) + ETH_HLEN);
+		eh->h_proto = htons_constant(ETH_P_IP);
 		(*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip4h);
 	}
 	(*tap_iov)[UDP_IOV_PAYLOAD].iov_len = l4len;
@@ -801,13 +800,19 @@ static void udp_buf_sock_to_tap(const struct ctx *c, int s, int n,
 				flow_sidx_t tosidx)
 {
 	const struct flowside *toside = flowside_at_sidx(tosidx);
+	struct udp_flow *uflow = udp_at_sidx(tosidx);
+	uint8_t *omac = uflow->f.tap_omac;
 	int i;
 
 	if ((n = udp_sock_recv(c, s, udp_mh_recv, n)) <= 0)
 		return;
 
+	/* Make one attempt to find true MAC address in ARP/NDP table */
+	if (MAC_IS_ZERO(omac))
+		fwd_neigh_mac_get(c, &toside->oaddr, omac);
+
 	for (i = 0; i < n; i++)
-		udp_tap_prepare(udp_mh_recv, i, toside, false);
+		udp_tap_prepare(udp_mh_recv, i, omac, toside, false);
 
 	tap_send_frames(c, &udp_l2_iov[0][0], UDP_NUM_IOVS, n);
 }
diff --git a/udp.h b/udp.h
index 8f8531a..dd6e5ad 100644
--- a/udp.h
+++ b/udp.h
@@ -21,7 +21,7 @@ int udp_sock_init(const struct ctx *c, int ns, const union inany_addr *addr,
 		  const char *ifname, in_port_t port);
 int udp_init(struct ctx *c);
 void udp_timer(struct ctx *c, const struct timespec *now);
-void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s);
+void udp_update_l2_buf(const unsigned char *eth_d);
 
 /**
  * union udp_listen_epoll_ref - epoll reference for "listening" UDP sockets
-- 
@@ -21,7 +21,7 @@ int udp_sock_init(const struct ctx *c, int ns, const union inany_addr *addr,
 		  const char *ifname, in_port_t port);
 int udp_init(struct ctx *c);
 void udp_timer(struct ctx *c, const struct timespec *now);
-void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s);
+void udp_update_l2_buf(const unsigned char *eth_d);
 
 /**
  * union udp_listen_epoll_ref - epoll reference for "listening" UDP sockets
-- 
2.50.1