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

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 60ef4c5..4763260 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/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;