[PATCH] vhost: use struct_size and size_add to compute flex array sizes

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The vhost_get_avail_size and vhost_get_used_size functions compute the size
of structures with flexible array members with an additional 2 bytes if the
VIRTIO_RING_F_EVENT_IDX feature flag is set. Convert these functions to use
struct_size() and size_add() instead of coding the calculation by hand.

This ensures that the calculations will saturate at SIZE_MAX rather than
overflowing.

Signed-off-by: Jacob Keller <jacob.e.keller@xxxxxxxxx>
Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
---

I found this using a coccinelle patch I developed and submitted at [1].

[1]: https://lore.kernel.org/all/20230227202428.3657443-1-jacob.e.keller@xxxxxxxxx/

 drivers/vhost/vhost.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index f11bdbe4c2c5..43fa626d4e44 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -436,8 +436,7 @@ static size_t vhost_get_avail_size(struct vhost_virtqueue *vq,
 	size_t event __maybe_unused =
 	       vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;
 
-	return sizeof(*vq->avail) +
-	       sizeof(*vq->avail->ring) * num + event;
+	return size_add(struct_size(vq->avail, ring, num), event);
 }
 
 static size_t vhost_get_used_size(struct vhost_virtqueue *vq,
@@ -446,8 +445,7 @@ static size_t vhost_get_used_size(struct vhost_virtqueue *vq,
 	size_t event __maybe_unused =
 	       vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;
 
-	return sizeof(*vq->used) +
-	       sizeof(*vq->used->ring) * num + event;
+	return size_add(struct_size(vq->used, ring, num), event);
 }
 
 static size_t vhost_get_desc_size(struct vhost_virtqueue *vq,
-- 
2.39.1.405.gd4c25cc71f83




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