bpf: Use struct_size() helper

In an effort to avoid open-coded arithmetic in the kernel, use the
struct_size() helper instead of open-coded calculation.
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://github.com/KSPP/linux/issues/160
Link: https://lore.kernel.org/bpf/20211220113048.2859-1-xiujianfeng@huawei.com

kernel/bpf/local_storage.c

@@ -163,8 +163,7 @@ static int cgroup_storage_update_elem(struct bpf_map *map, void *key,
 		return 0;
 	}
 
-	new = bpf_map_kmalloc_node(map, sizeof(struct bpf_storage_buffer) +
-				   map->value_size,
+	new = bpf_map_kmalloc_node(map, struct_size(new, data, map->value_size),
 				   __GFP_ZERO | GFP_ATOMIC | __GFP_NOWARN,
 				   map->numa_node);
 	if (!new)

kernel/bpf/reuseport_array.c

@@ -152,16 +152,12 @@ static struct bpf_map *reuseport_array_alloc(union bpf_attr *attr)
 {
 	int numa_node = bpf_map_attr_numa_node(attr);
 	struct reuseport_array *array;
-	u64 array_size;
 
 	if (!bpf_capable())
 		return ERR_PTR(-EPERM);
 
-	array_size = sizeof(*array);
-	array_size += (u64)attr->max_entries * sizeof(struct sock *);
-
 	/* allocate all map elements and zero-initialize them */
-	array = bpf_map_area_alloc(array_size, numa_node);
+	array = bpf_map_area_alloc(struct_size(array, ptrs, attr->max_entries), numa_node);
 	if (!array)
 		return ERR_PTR(-ENOMEM);