selftests/bpf: Fix BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL for empty flow label

Kernel's flow dissector continues to parse the packet when
the (optional) IPv6 flow label is empty even when instructed
to stop (via BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL). Do
the same in our reference BPF reimplementation.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20230221180518.2139026-1-sdf@google.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/flow_dissector.c

@@ -345,6 +345,30 @@ struct test tests[] = {
 		.flags = BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL,
 		.retval = BPF_OK,
 	},
+	{
+		.name = "ipv6-empty-flow-label",
+		.pkt.ipv6 = {
+			.eth.h_proto = __bpf_constant_htons(ETH_P_IPV6),
+			.iph.nexthdr = IPPROTO_TCP,
+			.iph.payload_len = __bpf_constant_htons(MAGIC_BYTES),
+			.iph.flow_lbl = { 0x00, 0x00, 0x00 },
+			.tcp.doff = 5,
+			.tcp.source = 80,
+			.tcp.dest = 8080,
+		},
+		.keys = {
+			.flags = BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL,
+			.nhoff = ETH_HLEN,
+			.thoff = ETH_HLEN + sizeof(struct ipv6hdr),
+			.addr_proto = ETH_P_IPV6,
+			.ip_proto = IPPROTO_TCP,
+			.n_proto = __bpf_constant_htons(ETH_P_IPV6),
+			.sport = 80,
+			.dport = 8080,
+		},
+		.flags = BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL,
+		.retval = BPF_OK,
+	},
 	{
 		.name = "ipip-encap",
 		.pkt.ipip = {

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

@@ -337,7 +337,7 @@ PROG(IPV6)(struct __sk_buff *skb)
 	keys->ip_proto = ip6h->nexthdr;
 	keys->flow_label = ip6_flowlabel(ip6h);
 
-	if (keys->flags & BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL)
+	if (keys->flow_label && keys->flags & BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL)
 		return export_flow_keys(keys, BPF_OK);
 
 	return parse_ipv6_proto(skb, ip6h->nexthdr);