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    SubjectRe: [PATCH] virtio_ring: use u32 for virtio_max_dma_size
    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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