[PATCH] block: bio: Use struct_size() in kmalloc()

["Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>]

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct bio_map_data {
	...
        struct iovec iov[];
};

instance = kmalloc(sizeof(sizeof(struct bio_map_data) + sizeof(struct iovec) *
                          count, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kmalloc(struct_size(instance, iov, count), GFP_KERNEL);

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
---
 block/bio.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 683cbb40f051..4bcdcd3f63f4 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1120,8 +1120,7 @@ static struct bio_map_data *bio_alloc_map_data(struct iov_iter *data,
 	if (data->nr_segs > UIO_MAXIOV)
 		return NULL;
 
-	bmd = kmalloc(sizeof(struct bio_map_data) +
-		       sizeof(struct iovec) * data->nr_segs, gfp_mask);
+	bmd = kmalloc(struct_size(bmd, iov, data->nr_segs), gfp_mask);
 	if (!bmd)
 		return NULL;
 	memcpy(bmd->iov, data->iov, sizeof(struct iovec) * data->nr_segs);
-- 
2.21.0