icmp: Don't discard first reply sequence for a given echo ID

In pasta mode, ICMP and ICMPv6 echo sockets relay back to us any
reply we send: we're on the same host as the target, after all. We
discard them by comparing the last sequence we sent with the sequence
we receive.

However, on the first reply for a given identifier, the sequence
might be zero, depending on the implementation of ping(8): we need
another value to indicate we haven't sent any sequence number, yet.

Use -1 as initialiser in the echo identifier map.

This is visible with Busybox's ping, and was reported by Paul on the
integration at https://github.com/containers/podman/pull/16141, with:

  $ podman run --net=pasta alpine ping -c 2 192.168.188.1

...where only the second reply would be routed back.

Reported-by: Paul Holzinger <pholzing@redhat.com>
Fixes: 33482d5bf2 ("passt: Add PASTA mode, major rework")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

icmp.c

@@ -44,12 +44,12 @@
 /**
  * struct icmp_id_sock - Tracking information for single ICMP echo identifier
  * @sock:	Bound socket for identifier
- * @seq:	Last sequence number sent to tap, host order
+ * @seq:	Last sequence number sent to tap, host order, -1: not sent yet
  * @ts:		Last associated activity from tap, seconds
  */
 struct icmp_id_sock {
 	int sock;
-	uint16_t seq;
+	int seq;
 	time_t ts;
 };
 
@@ -273,6 +273,7 @@ static void icmp_timer_one(const struct ctx *c, int v6, uint16_t id,
 	epoll_ctl(c->epollfd, EPOLL_CTL_DEL, id_map->sock, NULL);
 	close(id_map->sock);
 	id_map->sock = 0;
+	id_map->seq = -1;
 }
 
 /**
@@ -301,3 +302,14 @@ v6:
 		goto v6;
 	}
 }
+
+/**
+ * icmp_init() - Initialise sequences in ID map to -1 (no sequence sent yet)
+ */
+void icmp_init(void)
+{
+	unsigned i;
+
+	for (i = 0; i < ICMP_NUM_IDS; i++)
+		icmp_id_map[V4][i].seq = icmp_id_map[V6][i].seq = -1;
+}

icmp.h

@@ -15,6 +15,7 @@ void icmp_sock_handler(const struct ctx *c, union epoll_ref ref,
 int icmp_tap_handler(const struct ctx *c, int af, const void *addr,
 		     const struct pool *p, const struct timespec *now);
 void icmp_timer(const struct ctx *c, const struct timespec *ts);
+void icmp_init(void);
 
 /**
  * union icmp_epoll_ref - epoll reference portion for ICMP tracking

passt.c

@@ -256,6 +256,9 @@ int main(int argc, char **argv)
 	if ((!c.no_udp && udp_init(&c)) || (!c.no_tcp && tcp_init(&c)))
 		exit(EXIT_FAILURE);
 
+	if (!c.no_icmp)
+		icmp_init();
+
 	proto_update_l2_buf(c.mac_guest, c.mac, &c.ip4.addr);
 
 	if (c.ifi4 && !c.no_dhcp)