[PATCH][next] afs: Add __counted_by for struct afs_acl 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/afs/internal.h | 2 +-
 fs/afs/xattr.c    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index 469a717467a4..e263a58b82ba 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -1116,7 +1116,7 @@ extern void afs_fs_inline_bulk_status(struct afs_operation *);
 
 struct afs_acl {
 	u32	size;
-	u8	data[];
+	u8	data[] __counted_by(size);
 };
 
 extern void afs_fs_fetch_acl(struct afs_operation *);
diff --git a/fs/afs/xattr.c b/fs/afs/xattr.c
index 9048d8ccc715..8b42cac51660 100644
--- a/fs/afs/xattr.c
+++ b/fs/afs/xattr.c
@@ -75,7 +75,7 @@ static bool afs_make_acl(struct afs_operation *op,
 {
 	struct afs_acl *acl;
 
-	acl = kmalloc(sizeof(*acl) + size, GFP_KERNEL);
+	acl = kmalloc(struct_size(acl, data, size), GFP_KERNEL);
 	if (!acl) {
 		afs_op_nomem(op);
 		return false;
-- 
2.34.1