libbpf: Ensure FD >= 3 during bpf_map__reuse_fd()

Improve bpf_map__reuse_fd() logic and ensure that dup'ed map FD is
"good" (>= 3) and has O_CLOEXEC flags. Use fcntl(F_DUPFD_CLOEXEC) for
that, similarly to ensure_good_fd() helper we already use in low-level
APIs that work with bpf() syscall.
Suggested-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230525221311.2136408-2-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -4414,18 +4414,17 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
 	if (!new_name)
 		return libbpf_err(-errno);
 
-	new_fd = open("/", O_RDONLY | O_CLOEXEC);
+	/*
+	 * Like dup(), but make sure new FD is >= 3 and has O_CLOEXEC set.
+	 * This is similar to what we do in ensure_good_fd(), but without
+	 * closing original FD.
+	 */
+	new_fd = fcntl(fd, F_DUPFD_CLOEXEC, 3);
 	if (new_fd < 0) {
 		err = -errno;
 		goto err_free_new_name;
 	}
 
-	new_fd = dup3(fd, new_fd, O_CLOEXEC);
-	if (new_fd < 0) {
-		err = -errno;
-		goto err_close_new_fd;
-	}
-
 	err = zclose(map->fd);
 	if (err) {
 		err = -errno;