bpf: check bpf_map/bpf_program fd validity

libbpf creates bpf_program/bpf_map structs for each program/map that
user defines, but it allows to disable creating/loading those objects in
kernel, in that case they won't have associated file descriptor
(fd < 0). Such functionality is used for backward compatibility
with some older kernels.

Nothing prevents users from passing these maps or programs with no
kernel counterpart to libbpf APIs. This change introduces explicit
checks for kernel objects existence, aiming to improve visibility of
those edge cases and provide meaningful warnings to users.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>

tools/lib/bpf/libbpf.c

@@ -8572,6 +8572,12 @@ int bpf_map__pin(struct bpf_map *map, const char *path)
 		return libbpf_err(-EINVAL);
 	}
 
+	if (map->fd < 0) {
+		pr_warn("map '%s': can't pin BPF map without FD (was it created?)\n",
+			bpf_map__name(map));
+		return libbpf_err(-EINVAL);
+	}
+
 	if (map->pin_path) {
 		if (path && strcmp(path, map->pin_path)) {
 			pr_warn("map '%s' already has pin path '%s' different from '%s'\n",
@@ -10316,6 +10322,11 @@ static int validate_map_op(const struct bpf_map *map, size_t key_sz,
 		return -EINVAL;
 	}
 
+	if (map->fd < 0) {
+		pr_warn("map '%s': can't use BPF map without FD (was it created?)\n", map->name);
+		return -EINVAL;
+	}
+
 	if (!check_value_sz)
 		return 0;
 
@@ -10428,8 +10439,15 @@ long libbpf_get_error(const void *ptr)
 int bpf_link__update_program(struct bpf_link *link, struct bpf_program *prog)
 {
 	int ret;
+	int prog_fd = bpf_program__fd(prog);
 
-	ret = bpf_link_update(bpf_link__fd(link), bpf_program__fd(prog), NULL);
+	if (prog_fd < 0) {
+		pr_warn("prog '%s': can't use BPF program without FD (was it created?)\n",
+			prog->name);
+		return libbpf_err(-EINVAL);
+	}
+
+	ret = bpf_link_update(bpf_link__fd(link), prog_fd, NULL);
 	return libbpf_err_errno(ret);
 }
 
@@ -11347,6 +11365,13 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 	if (!OPTS_VALID(opts, bpf_kprobe_multi_opts))
 		return libbpf_err_ptr(-EINVAL);
 
+	prog_fd = bpf_program__fd(prog);
+	if (prog_fd < 0) {
+		pr_warn("prog '%s': can't attach BPF program without FD (was it created?)\n",
+			prog->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
 	syms    = OPTS_GET(opts, syms, false);
 	addrs   = OPTS_GET(opts, addrs, false);
 	cnt     = OPTS_GET(opts, cnt, false);
@@ -11387,7 +11412,6 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 	}
 	link->detach = &bpf_link__detach_fd;
 
-	prog_fd = bpf_program__fd(prog);
 	link_fd = bpf_link_create(prog_fd, 0, BPF_TRACE_KPROBE_MULTI, &lopts);
 	if (link_fd < 0) {
 		err = -errno;
@@ -11770,6 +11794,13 @@ bpf_program__attach_uprobe_multi(const struct bpf_program *prog,
 	if (!OPTS_VALID(opts, bpf_uprobe_multi_opts))
 		return libbpf_err_ptr(-EINVAL);
 
+	prog_fd = bpf_program__fd(prog);
+	if (prog_fd < 0) {
+		pr_warn("prog '%s': can't attach BPF program without FD (was it created?)\n",
+			prog->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
 	syms = OPTS_GET(opts, syms, NULL);
 	offsets = OPTS_GET(opts, offsets, NULL);
 	ref_ctr_offsets = OPTS_GET(opts, ref_ctr_offsets, NULL);
@@ -11845,7 +11876,6 @@ bpf_program__attach_uprobe_multi(const struct bpf_program *prog,
 	}
 	link->detach = &bpf_link__detach_fd;
 
-	prog_fd = bpf_program__fd(prog);
 	link_fd = bpf_link_create(prog_fd, 0, BPF_TRACE_UPROBE_MULTI, &lopts);
 	if (link_fd < 0) {
 		err = -errno;
@@ -12671,6 +12701,12 @@ struct bpf_link *bpf_program__attach(const struct bpf_program *prog)
 	if (!prog->sec_def || !prog->sec_def->prog_attach_fn)
 		return libbpf_err_ptr(-EOPNOTSUPP);
 
+	if (bpf_program__fd(prog) < 0) {
+		pr_warn("prog '%s': can't attach BPF program w/o FD (did you load it?)\n",
+			prog->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
 	err = prog->sec_def->prog_attach_fn(prog, prog->sec_def->cookie, &link);
 	if (err)
 		return libbpf_err_ptr(err);
@@ -12711,9 +12747,14 @@ struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
 	__u32 zero = 0;
 	int err, fd;
 
-	if (!bpf_map__is_struct_ops(map) || map->fd == -1)
+	if (!bpf_map__is_struct_ops(map))
 		return libbpf_err_ptr(-EINVAL);
 
+	if (map->fd < 0) {
+		pr_warn("map '%s': can't attach BPF map w/o FD (did you load it?)\n", map->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
 	link = calloc(1, sizeof(*link));
 	if (!link)
 		return libbpf_err_ptr(-EINVAL);
@@ -12760,8 +12801,13 @@ int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map)
 	__u32 zero = 0;
 	int err;
 
-	if (!bpf_map__is_struct_ops(map) || !map_is_created(map))
+	if (!bpf_map__is_struct_ops(map))
+		return -EINVAL;
+
+	if (map->fd < 0) {
+		pr_warn("map '%s': can't use BPF map w/o FD (did you load it?)\n", map->name);
 		return -EINVAL;
+	}
 
 	st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
 	/* Ensure the type of a link is correct */