[PATCH][next] iommu/virtio: Add __counted_by for struct viommu_request 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/iommu/virtio-iommu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
index 17dcd826f5c2..379ebe03efb6 100644
--- a/drivers/iommu/virtio-iommu.c
+++ b/drivers/iommu/virtio-iommu.c
@@ -85,7 +85,7 @@ struct viommu_request {
 	void				*writeback;
 	unsigned int			write_offset;
 	unsigned int			len;
-	char				buf[];
+	char				buf[] __counted_by(len);
 };
 
 #define VIOMMU_FAULT_RESV_MASK		0xffffff00
@@ -230,7 +230,7 @@ static int __viommu_add_req(struct viommu_dev *viommu, void *buf, size_t len,
 	if (write_offset <= 0)
 		return -EINVAL;
 
-	req = kzalloc(sizeof(*req) + len, GFP_ATOMIC);
+	req = kzalloc(struct_size(req, buf, len), GFP_ATOMIC);
 	if (!req)
 		return -ENOMEM;
 
-- 
2.34.1