selftests/bpf: Verify first of struct mptcp_sock

This patch verifies the 'first' struct member of struct mptcp_sock, which
points to the first subflow of msk. Save 'sk' in mptcp_storage, and verify
it with 'first' in verify_msk().

v5:
- Use ASSERT_EQ() instead of a manual comparison + log (Andrii).

Signed-off-by: Geliang Tang <[e-mail]>
Signed-off-by: Mat Martineau <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Matthieu Baerts <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/bpf_tcp_helpers.h

···
 	struct inet_connection_sock	sk;
 
 	__u32		token;
+	struct sock	*first;
 	char		ca_name[TCP_CA_NAME_MAX];
 } __attribute__((preserve_access_index));
 

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

···
 struct mptcp_storage {
 	__u32 invoked;
 	__u32 is_mptcp;
+	struct sock *sk;
 	__u32 token;
+	struct sock *first;
 	char ca_name[TCP_CA_NAME_MAX];
 };
 
···
 		err++;
 
 	if (!ASSERT_EQ(val.token, token, "unexpected token"))
+		err++;
+
+	if (!ASSERT_EQ(val.first, val.sk, "unexpected first"))
 		err++;
 
 	if (!ASSERT_STRNEQ(val.ca_name, ca_name, TCP_CA_NAME_MAX, "unexpected ca_name"))

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

···
 struct mptcp_storage {
 	__u32 invoked;
 	__u32 is_mptcp;
+	struct sock *sk;
 	__u32 token;
+	struct sock *first;
 	char ca_name[TCP_CA_NAME_MAX];
 };
 
···
 
 		storage->token = 0;
 		__builtin_memset(storage->ca_name, 0, TCP_CA_NAME_MAX);
+		storage->first = NULL;
 	} else {
 		msk = bpf_skc_to_mptcp_sock(sk);
 		if (!msk)
···
 
 		storage->token = msk->token;
 		__builtin_memcpy(storage->ca_name, msk->ca_name, TCP_CA_NAME_MAX);
+		storage->first = msk->first;
 	}
 	storage->invoked++;
 	storage->is_mptcp = is_mptcp;
+	storage->sk = (struct sock *)sk;
 
 	return 1;
 }