[PATCH bpf-next] selftests/bpf: Fix BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL for empty flow label

[Stanislav Fomichev <sdf@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 .../selftests/bpf/prog_tests/flow_dissector.c | 24 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/bpf_flow.c  |  2 +-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/flow_dissector.c b/tools/testing/selftests/bpf/prog_tests/flow_dissector.c
index 7acca37a3d2b..c4773173a4e4 100644
--- a/tools/testing/selftests/bpf/prog_tests/flow_dissector.c
+++ b/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 = {
diff --git a/tools/testing/selftests/bpf/progs/bpf_flow.c b/tools/testing/selftests/bpf/progs/bpf_flow.c
index a20c5ed5e454..b04e092fac94 100644
--- a/tools/testing/selftests/bpf/progs/bpf_flow.c
+++ b/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);
-- 
2.39.2.637.g21b0678d19-goog