Re: Re: [PATCH] virtio_ring: use u32 for virtio_max_dma_size

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On 7/4/23 14:21, Michael S. Tsirkin wrote:
On Wed, May 10, 2023 at 10:54:37AM +0800, zhenwei pi wrote:
Both split ring and packed ring use 32bits to describe the length of
a descriptor: see struct vring_desc and struct vring_packed_desc.
This means the max segment size supported by virtio is U32_MAX.

An example of virtio_max_dma_size in virtio_blk.c:
   u32 v, max_size;

   max_size = virtio_max_dma_size(vdev);  -> implicit convert
   err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX,
                              struct virtio_blk_config, size_max, &v);
   max_size = min(max_size, v);

There is a risk during implicit convert here, once virtio_max_dma_size
returns 4G, max_size becomes 0.

Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()")
Cc: Joerg Roedel <jroedel@xxxxxxx>
Signed-off-by: zhenwei pi <pizhenwei@xxxxxxxxxxxxx>
  drivers/virtio/virtio_ring.c | 12 ++++++++----
  include/linux/virtio.h       |  2 +-
  2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c5310eaf8b46..55cfecf030a1 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -289,12 +289,16 @@ static bool vring_use_dma_api(const struct virtio_device *vdev)
  	return false;
-size_t virtio_max_dma_size(const struct virtio_device *vdev)
+u32 virtio_max_dma_size(const struct virtio_device *vdev)
-	size_t max_segment_size = SIZE_MAX;
+	u32 max_segment_size = U32_MAX;
- if (vring_use_dma_api(vdev))
-		max_segment_size = dma_max_mapping_size(vdev->dev.parent);
+	if (vring_use_dma_api(vdev)) {
+		size_t max_dma_size = dma_max_mapping_size(vdev->dev.parent);
+		if (max_dma_size < max_segment_size)
+			max_segment_size = max_dma_size;
+	}
return max_segment_size;

Took a while for me to get this, it's confusing.  I think the issue is
really in virtio blk, so I would just change max_size there to size_t
and be done with it.


diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index b93238db94e3..1a605f408329 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -147,7 +147,7 @@ int virtio_device_restore(struct virtio_device *dev);
  void virtio_reset_device(struct virtio_device *dev);
-size_t virtio_max_dma_size(const struct virtio_device *vdev);
+u32 virtio_max_dma_size(const struct virtio_device *vdev);
#define virtio_device_for_each_vq(vdev, vq) \
  	list_for_each_entry(vq, &vdev->vqs, list)

zhenwei pi
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