[PATCH][next] btrfs: Add __counted_by for struct btrfs_delayed_item and use struct_size()

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version, to calculate the size for the allocation of the whole
flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and
fixed manually.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 fs/btrfs/delayed-inode.c | 2 +-
 fs/btrfs/delayed-inode.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
index 35d7616615c1..4f364e242341 100644
--- a/fs/btrfs/delayed-inode.c
+++ b/fs/btrfs/delayed-inode.c
@@ -313,7 +313,7 @@ static struct btrfs_delayed_item *btrfs_alloc_delayed_item(u16 data_len,
 {
 	struct btrfs_delayed_item *item;
 
-	item = kmalloc(sizeof(*item) + data_len, GFP_NOFS);
+	item = kmalloc(struct_size(item, data, data_len), GFP_NOFS);
 	if (item) {
 		item->data_len = data_len;
 		item->type = type;
diff --git a/fs/btrfs/delayed-inode.h b/fs/btrfs/delayed-inode.h
index d050e572c7f9..5cceb31bbd16 100644
--- a/fs/btrfs/delayed-inode.h
+++ b/fs/btrfs/delayed-inode.h
@@ -95,7 +95,7 @@ struct btrfs_delayed_item {
 	bool logged;
 	/* The maximum leaf size is 64K, so u16 is more than enough. */
 	u16 data_len;
-	char data[];
+	char data[] __counted_by(data_len);
 };
 
 static inline void btrfs_init_delayed_root(
-- 
2.34.1