selftests/bpf: expand bpf tunnel test with decap

The bpf tunnel test encapsulates using bpf, then decapsulates using
a standard tunnel device to verify correctness.

Once encap is verified, also test decap, by replacing the tunnel
device on decap with another bpf program.
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -80,4 +80,35 @@ int encap_f(struct __sk_buff *skb)
 	return TC_ACT_OK;
 }
 
+SEC("decap")
+int decap_f(struct __sk_buff *skb)
+{
+	struct iphdr iph_outer, iph_inner;
+
+	if (skb->protocol != __bpf_constant_htons(ETH_P_IP))
+		return TC_ACT_OK;
+
+	if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph_outer,
+			       sizeof(iph_outer)) < 0)
+		return TC_ACT_OK;
+
+	if (iph_outer.ihl != 5 || iph_outer.protocol != IPPROTO_IPIP)
+		return TC_ACT_OK;
+
+	if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(iph_outer),
+			       &iph_inner, sizeof(iph_inner)) < 0)
+		return TC_ACT_OK;
+
+	if (bpf_skb_adjust_room(skb, -(int)sizeof(iph_outer),
+				BPF_ADJ_ROOM_NET, 0))
+		return TC_ACT_SHOT;
+
+	/* bpf_skb_adjust_room has moved outer over inner header: restore */
+	if (bpf_skb_store_bytes(skb, ETH_HLEN, &iph_inner, sizeof(iph_inner),
+				BPF_F_INVALIDATE_HASH) < 0)
+		return TC_ACT_SHOT;
+
+	return TC_ACT_OK;
+}
+
 char __license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/test_tc_tunnel.sh

@@ -72,4 +72,13 @@ ip netns exec "${ns2}" ip link set dev testtun0 up
 echo "test bpf encap with tunnel device decap"
 client_connect
 
+# serverside, use BPF for decap
+ip netns exec "${ns2}" ip link del dev testtun0
+ip netns exec "${ns2}" tc qdisc add dev veth2 clsact
+ip netns exec "${ns2}" tc filter add dev veth2 ingress \
+	bpf direct-action object-file ./test_tc_tunnel.o section decap
+server_listen
+echo "test bpf encap with bpf decap"
+client_connect
+
 echo OK