selftests/bpf: Make sure we trigger metadata kfuncs for dst 8080

xdp_metadata test is flaky sometimes:

  verify_xsk_metadata:FAIL:rx_hash_type unexpected rx_hash_type: actual 8 != expected 0

Where 8 means XDP_RSS_TYPE_L4_ANY and is exported from veth driver only when
'skb->l4_hash' condition is met. This makes me think that the program is
triggering again for some other packet.

Let's have a filter, similar to xdp_hw_metadata, where we trigger XDP kfuncs
only for UDP packets destined to port 8080.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231204174423.3460052-1-sdf@google.com

tools/testing/selftests/bpf/progs/xdp_metadata.c

@@ -27,11 +27,40 @@ extern int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, __u32 *hash,
 SEC("xdp")
 int rx(struct xdp_md *ctx)
 {
-	void *data, *data_meta;
+	void *data, *data_meta, *data_end;
+	struct ipv6hdr *ip6h = NULL;
+	struct ethhdr *eth = NULL;
+	struct udphdr *udp = NULL;
+	struct iphdr *iph = NULL;
 	struct xdp_meta *meta;
 	u64 timestamp = -1;
 	int ret;
 
+	data = (void *)(long)ctx->data;
+	data_end = (void *)(long)ctx->data_end;
+	eth = data;
+	if (eth + 1 < data_end) {
+		if (eth->h_proto == bpf_htons(ETH_P_IP)) {
+			iph = (void *)(eth + 1);
+			if (iph + 1 < data_end && iph->protocol == IPPROTO_UDP)
+				udp = (void *)(iph + 1);
+		}
+		if (eth->h_proto == bpf_htons(ETH_P_IPV6)) {
+			ip6h = (void *)(eth + 1);
+			if (ip6h + 1 < data_end && ip6h->nexthdr == IPPROTO_UDP)
+				udp = (void *)(ip6h + 1);
+		}
+		if (udp && udp + 1 > data_end)
+			udp = NULL;
+	}
+
+	if (!udp)
+		return XDP_PASS;
+
+	/* Forwarding UDP:8080 to AF_XDP */
+	if (udp->dest != bpf_htons(8080))
+		return XDP_PASS;
+
 	/* Reserve enough for all custom metadata. */
 
 	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));