Re: [PATCH] virtio_ring: use u32 for virtio_max_dma_size

["Michael S. Tsirkin" <mst@redhat.com>]

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@suse.de>
> Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
> ---
>  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);
>  #endif
>  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)
> -- 
> 2.20.1

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