selftests/bpf: Introduce __attribute__((cleanup)) in create_pair()

Rewrite function to have (unneeded) socket descriptors automatically
close()d when leaving the scope. Make sure the "ownership" of fds is
correctly passed via take_fd(); i.e. descriptor returned to caller will
remain valid.

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20240731-selftest-sockmap-fixes-v2-6-08a0c73abed2@rbox.co
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/prog_tests/sockmap_helpers.h

@@ -17,6 +17,17 @@
 
 #define __always_unused	__attribute__((__unused__))
 
+/* include/linux/cleanup.h */
+#define __get_and_null(p, nullvalue)                                           \
+	({                                                                     \
+		__auto_type __ptr = &(p);                                      \
+		__auto_type __val = *__ptr;                                    \
+		*__ptr = nullvalue;                                            \
+		__val;                                                         \
+	})
+
+#define take_fd(fd) __get_and_null(fd, -EBADF)
+
 #define _FAIL(errnum, fmt...)                                                  \
 	({                                                                     \
 		error_at_line(0, (errnum), __func__, __LINE__, fmt);           \
@@ -182,6 +193,14 @@
 		__ret;                                                         \
 	})
 
+static inline void close_fd(int *fd)
+{
+	if (*fd >= 0)
+		xclose(*fd);
+}
+
+#define __close_fd __attribute__((cleanup(close_fd)))
+
 static inline int poll_connect(int fd, unsigned int timeout_sec)
 {
 	struct timeval timeout = { .tv_sec = timeout_sec };
@@ -369,9 +388,10 @@ static inline int socket_loopback(int family, int sotype)
 
 static inline int create_pair(int family, int sotype, int *p0, int *p1)
 {
+	__close_fd int s, c = -1, p = -1;
 	struct sockaddr_storage addr;
 	socklen_t len = sizeof(addr);
-	int s, c, p, err;
+	int err;
 
 	s = socket_loopback(family, sotype);
 	if (s < 0)
@@ -379,25 +399,23 @@ static inline int create_pair(int family, int sotype, int *p0, int *p1)
 
 	err = xgetsockname(s, sockaddr(&addr), &len);
 	if (err)
-		goto close_s;
+		return err;
 
 	c = xsocket(family, sotype, 0);
-	if (c < 0) {
+	if (c < 0)
-		err = c;
+		return c;
-		goto close_s;
-	}
 
 	err = connect(c, sockaddr(&addr), len);
 	if (err) {
 		if (errno != EINPROGRESS) {
 			FAIL_ERRNO("connect");
-			goto close_c;
+			return err;
 		}
 
 		err = poll_connect(c, IO_TIMEOUT_SEC);
 		if (err) {
 			FAIL_ERRNO("poll_connect");
-			goto close_c;
+			return err;
 		}
 	}
 
@@ -405,36 +423,29 @@ static inline int create_pair(int family, int sotype, int *p0, int *p1)
 	case SOCK_DGRAM:
 		err = xgetsockname(c, sockaddr(&addr), &len);
 		if (err)
-			goto close_c;
+			return err;
 
 		err = xconnect(s, sockaddr(&addr), len);
-		if (!err) {
+		if (err)
-			*p0 = s;
-			*p1 = c;
 			return err;
-		}
+
+		*p0 = take_fd(s);
 		break;
 	case SOCK_STREAM:
 	case SOCK_SEQPACKET:
 		p = xaccept_nonblock(s, NULL, NULL);
-		if (p >= 0) {
+		if (p < 0)
-			*p0 = p;
+			return p;
-			*p1 = c;
-			goto close_s;
-		}
 
-		err = p;
+		*p0 = take_fd(p);
 		break;
 	default:
 		FAIL("Unsupported socket type %#x", sotype);
-		err = -EOPNOTSUPP;
+		return -EOPNOTSUPP;
 	}
 
-close_c:
+	*p1 = take_fd(c);
-	close(c);
+	return 0;
-close_s:
-	close(s);
-	return err;
 }
 
 static inline int create_socket_pairs(int family, int sotype, int *c0, int *c1,