selftests/bpf: Add helper to get port using getsockname

The helper will be used to programmatically retrieve
and pass ports in userspace and kernel selftest programs.

Suggested-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Aditi Ghag <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

tools/testing/selftests/bpf/network_helpers.c

···
 	close(token->orig_netns_fd);
 	free(token);
 }
+
+int get_socket_local_port(int sock_fd)
+{
+	struct sockaddr_storage addr;
+	socklen_t addrlen = sizeof(addr);
+	int err;
+
+	err = getsockname(sock_fd, (struct sockaddr *)&addr, &addrlen);
+	if (err < 0)
+		return err;
+
+	if (addr.ss_family == AF_INET) {
+		struct sockaddr_in *sin = (struct sockaddr_in *)&addr;
+
+		return sin->sin_port;
+	} else if (addr.ss_family == AF_INET6) {
+		struct sockaddr_in6 *sin = (struct sockaddr_in6 *)&addr;
+
+		return sin->sin6_port;
+	}
+
+	return -1;
+}

tools/testing/selftests/bpf/network_helpers.h

···
 int make_sockaddr(int family, const char *addr_str, __u16 port,
 		  struct sockaddr_storage *addr, socklen_t *len);
 char *ping_command(int family);
+int get_socket_local_port(int sock_fd);
 
 struct nstoken;
 /**