selftests/bpf: Add connmark read test

Test that the prog can read from the connection mark. This test is nice
because it ensures progs can interact with netfilter subsystem
correctly.
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/d3bc620a491e4c626c20d80631063922cbe13e2b.1660254747.git.dxu@dxuuu.xyz

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

@@ -44,7 +44,7 @@ static int connect_to_server(int srv_fd)
 
 static void test_bpf_nf_ct(int mode)
 {
-	const char *iptables = "iptables -t raw %s PREROUTING -j CT";
+	const char *iptables = "iptables -t raw %s PREROUTING -j CONNMARK --set-mark 42/0";
 	int srv_fd = -1, client_fd = -1, srv_client_fd = -1;
 	struct sockaddr_in peer_addr = {};
 	struct test_bpf_nf *skel;
@@ -114,6 +114,7 @@ static void test_bpf_nf_ct(int mode)
 	/* expected status is IPS_SEEN_REPLY */
 	ASSERT_EQ(skel->bss->test_status, 2, "Test for ct status update ");
 	ASSERT_EQ(skel->data->test_exist_lookup, 0, "Test existing connection lookup");
+	ASSERT_EQ(skel->bss->test_exist_lookup_mark, 43, "Test existing connection lookup ctmark");
 end:
 	if (srv_client_fd != -1)
 		close(srv_client_fd);

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

@@ -28,6 +28,7 @@ __be16 sport = 0;
 __be32 daddr = 0;
 __be16 dport = 0;
 int test_exist_lookup = -ENOENT;
+u32 test_exist_lookup_mark = 0;
 
 struct nf_conn;
 
@@ -174,6 +175,8 @@ nf_ct_test(struct nf_conn *(*lookup_fn)(void *, struct bpf_sock_tuple *, u32,
 		       sizeof(opts_def));
 	if (ct) {
 		test_exist_lookup = 0;
+		if (ct->mark == 42)
+			test_exist_lookup_mark = 43;
 		bpf_ct_release(ct);
 	} else {
 		test_exist_lookup = opts_def.error;