libbpf: don't rely on map->fd as an indicator of map being created

With the upcoming switch to preallocated placeholder FDs for maps,
switch various getters/setter away from checking map->fd. Use
map_is_created() helper that detect whether BPF map can be modified based
on map->obj->loaded state, with special provision for maps set up with
bpf_map__reuse_fd().

For backwards compatibility, we take map_is_created() into account in
bpf_map__fd() getter as well. This way before bpf_object__load() phase
bpf_map__fd() will always return -1, just as before the changes in
subsequent patches adding stable map->fd placeholders.

We also get rid of all internal uses of bpf_map__fd() getter, as it's
more oriented for uses external to libbpf. The above map_is_created()
check actually interferes with some of the internal uses, if map FD is
fetched through bpf_map__fd().
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240104013847.3875810-4-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -5200,6 +5200,11 @@ bpf_object__populate_internal_map(struct bpf_object *obj, struct bpf_map *map)
 
 static void bpf_map__destroy(struct bpf_map *map);
 
+static bool map_is_created(const struct bpf_map *map)
+{
+	return map->obj->loaded || map->reused;
+}
+
 static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, bool is_inner)
 {
 	LIBBPF_OPTS(bpf_map_create_opts, create_attr);
@@ -5231,7 +5236,7 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
 					map->name, err);
 				return err;
 			}
-			map->inner_map_fd = bpf_map__fd(map->inner_map);
+			map->inner_map_fd = map->inner_map->fd;
 		}
 		if (map->inner_map_fd >= 0)
 			create_attr.inner_map_fd = map->inner_map_fd;
@@ -5314,7 +5319,7 @@ static int init_map_in_map_slots(struct bpf_object *obj, struct bpf_map *map)
 			continue;
 
 		targ_map = map->init_slots[i];
-		fd = bpf_map__fd(targ_map);
+		fd = targ_map->fd;
 
 		if (obj->gen_loader) {
 			bpf_gen__populate_outer_map(obj->gen_loader,
@@ -7135,7 +7140,7 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
 				if (map->libbpf_type != LIBBPF_MAP_RODATA)
 					continue;
 
-				if (bpf_prog_bind_map(ret, bpf_map__fd(map), NULL)) {
+				if (bpf_prog_bind_map(ret, map->fd, NULL)) {
 					cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
 					pr_warn("prog '%s': failed to bind map '%s': %s\n",
 						prog->name, map->real_name, cp);
@@ -9601,7 +9606,11 @@ int libbpf_attach_type_by_name(const char *name,
 
 int bpf_map__fd(const struct bpf_map *map)
 {
-	return map ? map->fd : libbpf_err(-EINVAL);
+	if (!map)
+		return libbpf_err(-EINVAL);
+	if (!map_is_created(map))
+		return -1;
+	return map->fd;
 }
 
 static bool map_uses_real_name(const struct bpf_map *map)
@@ -9637,7 +9646,7 @@ enum bpf_map_type bpf_map__type(const struct bpf_map *map)
 
 int bpf_map__set_type(struct bpf_map *map, enum bpf_map_type type)
 {
-	if (map->fd >= 0)
+	if (map_is_created(map))
 		return libbpf_err(-EBUSY);
 	map->def.type = type;
 	return 0;
@@ -9650,7 +9659,7 @@ __u32 bpf_map__map_flags(const struct bpf_map *map)
 
 int bpf_map__set_map_flags(struct bpf_map *map, __u32 flags)
 {
-	if (map->fd >= 0)
+	if (map_is_created(map))
 		return libbpf_err(-EBUSY);
 	map->def.map_flags = flags;
 	return 0;
@@ -9663,7 +9672,7 @@ __u64 bpf_map__map_extra(const struct bpf_map *map)
 
 int bpf_map__set_map_extra(struct bpf_map *map, __u64 map_extra)
 {
-	if (map->fd >= 0)
+	if (map_is_created(map))
 		return libbpf_err(-EBUSY);
 	map->map_extra = map_extra;
 	return 0;
@@ -9676,7 +9685,7 @@ __u32 bpf_map__numa_node(const struct bpf_map *map)
 
 int bpf_map__set_numa_node(struct bpf_map *map, __u32 numa_node)
 {
-	if (map->fd >= 0)
+	if (map_is_created(map))
 		return libbpf_err(-EBUSY);
 	map->numa_node = numa_node;
 	return 0;
@@ -9689,7 +9698,7 @@ __u32 bpf_map__key_size(const struct bpf_map *map)
 
 int bpf_map__set_key_size(struct bpf_map *map, __u32 size)
 {
-	if (map->fd >= 0)
+	if (map_is_created(map))
 		return libbpf_err(-EBUSY);
 	map->def.key_size = size;
 	return 0;
@@ -9773,7 +9782,7 @@ static int map_btf_datasec_resize(struct bpf_map *map, __u32 size)
 
 int bpf_map__set_value_size(struct bpf_map *map, __u32 size)
 {
-	if (map->fd >= 0)
+	if (map->obj->loaded || map->reused)
 		return libbpf_err(-EBUSY);
 
 	if (map->mmaped) {
@@ -9814,8 +9823,11 @@ __u32 bpf_map__btf_value_type_id(const struct bpf_map *map)
 int bpf_map__set_initial_value(struct bpf_map *map,
 			       const void *data, size_t size)
 {
+	if (map->obj->loaded || map->reused)
+		return libbpf_err(-EBUSY);
+
 	if (!map->mmaped || map->libbpf_type == LIBBPF_MAP_KCONFIG ||
-	    size != map->def.value_size || map->fd >= 0)
+	    size != map->def.value_size)
 		return libbpf_err(-EINVAL);
 
 	memcpy(map->mmaped, data, size);
@@ -9842,7 +9854,7 @@ __u32 bpf_map__ifindex(const struct bpf_map *map)
 
 int bpf_map__set_ifindex(struct bpf_map *map, __u32 ifindex)
 {
-	if (map->fd >= 0)
+	if (map_is_created(map))
 		return libbpf_err(-EBUSY);
 	map->map_ifindex = ifindex;
 	return 0;
@@ -9947,7 +9959,7 @@ bpf_object__find_map_fd_by_name(const struct bpf_object *obj, const char *name)
 static int validate_map_op(const struct bpf_map *map, size_t key_sz,
 			   size_t value_sz, bool check_value_sz)
 {
-	if (map->fd <= 0)
+	if (!map_is_created(map)) /* map is not yet created */
 		return -ENOENT;
 
 	if (map->def.key_size != key_sz) {
@@ -12400,7 +12412,7 @@ 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->fd < 0)
+	if (!bpf_map__is_struct_ops(map) || !map_is_created(map))
 		return -EINVAL;
 
 	st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
@@ -13304,7 +13316,7 @@ int bpf_object__load_skeleton(struct bpf_object_skeleton *s)
 	for (i = 0; i < s->map_cnt; i++) {
 		struct bpf_map *map = *s->maps[i].map;
 		size_t mmap_sz = bpf_map_mmap_sz(map->def.value_size, map->def.max_entries);
-		int prot, map_fd = bpf_map__fd(map);
+		int prot, map_fd = map->fd;
 		void **mmaped = s->maps[i].mmaped;
 
 		if (!mmaped)