tools: libbpf: allow map reuse

More advanced applications may want to only replace programs without
destroying associated maps.  Allow libbpf users to achieve that.
Instead of always creating all of the maps at load time, expose to
users an API to reconstruct the map object from already existing
map.

The map parameters are read from the kernel and replace the parameters
of the ELF map.  libbpf does not restrict the map replacement, i.e.
the reused map does not have to be compatible with the ELF map
definition.  We relay on the verifier for checking the compatibility
between maps and programs.  The ELF map definition is completely
overwritten by the information read from the kernel, to make sure
libbpf's view of map object corresponds to the actual map.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -1035,6 +1035,53 @@ static int bpf_map_find_btf_info(struct bpf_map *map, const struct btf *btf)
 	return 0;
 }
 
+int bpf_map__reuse_fd(struct bpf_map *map, int fd)
+{
+	struct bpf_map_info info = {};
+	__u32 len = sizeof(info);
+	int new_fd, err;
+	char *new_name;
+
+	err = bpf_obj_get_info_by_fd(fd, &info, &len);
+	if (err)
+		return err;
+
+	new_name = strdup(info.name);
+	if (!new_name)
+		return -errno;
+
+	new_fd = open("/", O_RDONLY | O_CLOEXEC);
+	if (new_fd < 0)
+		goto err_free_new_name;
+
+	new_fd = dup3(fd, new_fd, O_CLOEXEC);
+	if (new_fd < 0)
+		goto err_close_new_fd;
+
+	err = zclose(map->fd);
+	if (err)
+		goto err_close_new_fd;
+	free(map->name);
+
+	map->fd = new_fd;
+	map->name = new_name;
+	map->def.type = info.type;
+	map->def.key_size = info.key_size;
+	map->def.value_size = info.value_size;
+	map->def.max_entries = info.max_entries;
+	map->def.map_flags = info.map_flags;
+	map->btf_key_type_id = info.btf_key_type_id;
+	map->btf_value_type_id = info.btf_value_type_id;
+
+	return 0;
+
+err_close_new_fd:
+	close(new_fd);
+err_free_new_name:
+	free(new_name);
+	return -errno;
+}
+
 static int
 bpf_object__create_maps(struct bpf_object *obj)
 {
@@ -1047,6 +1094,12 @@ bpf_object__create_maps(struct bpf_object *obj)
 		struct bpf_map_def *def = &map->def;
 		int *pfd = &map->fd;
 
+		if (map->fd >= 0) {
+			pr_debug("skip map create (preset) %s: fd=%d\n",
+				 map->name, map->fd);
+			continue;
+		}
+
 		create_attr.name = map->name;
 		create_attr.map_ifindex = map->map_ifindex;
 		create_attr.map_type = def->type;

tools/lib/bpf/libbpf.h

@@ -261,6 +261,7 @@ typedef void (*bpf_map_clear_priv_t)(struct bpf_map *, void *);
 int bpf_map__set_priv(struct bpf_map *map, void *priv,
 		      bpf_map_clear_priv_t clear_priv);
 void *bpf_map__priv(struct bpf_map *map);
+int bpf_map__reuse_fd(struct bpf_map *map, int fd);
 bool bpf_map__is_offload_neutral(struct bpf_map *map);
 void bpf_map__set_ifindex(struct bpf_map *map, __u32 ifindex);
 int bpf_map__pin(struct bpf_map *map, const char *path);