[PATCH][next] xen/xenbus: Add __counted_by for struct read_buffer 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>
---
 drivers/xen/xenbus/xenbus_dev_frontend.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_dev_frontend.c b/drivers/xen/xenbus/xenbus_dev_frontend.c
index 0792fda49a15..6f56640092a9 100644
--- a/drivers/xen/xenbus/xenbus_dev_frontend.c
+++ b/drivers/xen/xenbus/xenbus_dev_frontend.c
@@ -82,7 +82,7 @@ struct read_buffer {
 	struct list_head list;
 	unsigned int cons;
 	unsigned int len;
-	char msg[];
+	char msg[] __counted_by(len);
 };
 
 struct xenbus_file_priv {
@@ -195,7 +195,7 @@ static int queue_reply(struct list_head *queue, const void *data, size_t len)
 	if (len > XENSTORE_PAYLOAD_MAX)
 		return -EINVAL;
 
-	rb = kmalloc(sizeof(*rb) + len, GFP_KERNEL);
+	rb = kmalloc(struct_size(rb, msg, len), GFP_KERNEL);
 	if (rb == NULL)
 		return -ENOMEM;
 
-- 
2.34.1