Re: [RFC PATCH 02/12] virtio_ring: validate premapped addresses through the device's map

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

On Sun, Aug 09, 2026 at 06:20:00PM +0000, Alexander Graf wrote:
> Callers that forget to handle a mapping failure may accidentally pass
> DMA_MAPPING_ERROR as target map address and we don't error out for it.
> The mapped path in vring_map_one_sg() checks the result, but the
> premapped path takes the address as given. That makes it more difficult
> to identify accidental API misuse.


Nah.

We don't spread null checks all over the place and this is the same.


> 
> Run the address through the existing vring_mapping_error(), which asks
> the device's virtio_map_ops mapping_error op or dma_mapping_error(),
> warn once and return -ENOMEM when it rejects the address. Document
> where the premapped helpers expect their addresses to come from, since
> an address from any other source is indistinguishable from a valid one.
> That way an API misuse shows up at the call that made it.

Commit log does not match what doc patches do.

> Assisted-by: Kiro:claude-opus-5 checkpatch sparse
> Signed-off-by: Alexander Graf <graf@amazon.com>
> ---
>  drivers/virtio/virtio_ring.c | 40 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index b438dc2ce1b8..9caa4f96204f 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -499,6 +499,19 @@ static int vring_map_one_sg(const struct vring_virtqueue *vq, struct scatterlist
>  	if (premapped) {
>  		*addr = sg_dma_address(sg);
>  		*len = sg_dma_len(sg);
> +
> +		/*
> +		 * The caller mapped this itself, so the map it used is the
> +		 * only thing that can judge the result.  Ask it rather than
> +		 * skipping the check the mapped path performs: a caller that
> +		 * ignored a failed mapping would otherwise publish the
> +		 * reserved error value to the device.
> +		 */
> +		if (dev_WARN_ONCE(&vq->vq.vdev->dev,
> +				  vring_mapping_error(vq, *addr),
> +				  "premapped buffer holds no valid mapping\n"))
> +			return -ENOMEM;
> +
>  		return 0;
>  	}
>  
> @@ -2910,6 +2923,14 @@ EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
>   * @data: the token identifying the buffer.
>   * @gfp: how to do memory allocations (if necessary).
>   *
> + * Each entry of @sg must carry an address the caller obtained for this
> + * virtqueue: from the DMA API when virtqueue_dma_dev() returns a device, and
> + * from virtqueue_map_page_attrs() when it returns NULL, because the device
> + * then interprets every address published to it in its own terms.  Only an
> + * address the map itself rejects is caught here; an address from any other
> + * source is indistinguishable from a valid one and reaches the device
> + * unchanged.
> + *
>   * Caller must ensure we don't call this with other virtqueue operations
>   * at the same time (except where noted).
>   *
> @@ -3008,6 +3029,14 @@ EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_ctx);
>   * @ctx: extra context for the token
>   * @gfp: how to do memory allocations (if necessary).
>   *
> + * Each entry of @sg must carry an address the caller obtained for this
> + * virtqueue: from the DMA API when virtqueue_dma_dev() returns a device, and
> + * from virtqueue_map_page_attrs() when it returns NULL, because the device
> + * then interprets every address published to it in its own terms.  Only an
> + * address the map itself rejects is caught here; an address from any other
> + * source is indistinguishable from a valid one and reaches the device
> + * unchanged.

I don't see what this comment gets us.
And let's not do this half hearted validation.


> + *
>   * Caller must ensure we don't call this with other virtqueue operations
>   * at the same time (except where noted).
>   *
> @@ -3025,10 +3054,17 @@ int virtqueue_add_inbuf_premapped(struct virtqueue *vq,
>  EXPORT_SYMBOL_GPL(virtqueue_add_inbuf_premapped);
>  
>  /**
> - * virtqueue_dma_dev - get the dma dev
> + * virtqueue_dma_dev - get the device to use for DMA API calls
>   * @_vq: the struct virtqueue we're talking about.
>   *
> - * Returns the dma dev. That can been used for dma api.
> + * A NULL return means this virtqueue publishes no DMA addresses: either it
> + * needs no mapping at all, or the device supplies its own virtio_map_ops and
> + * interprets every address published to it in its own terms.  A caller that
> + * maps buffers itself must therefore check for NULL before using the DMA API
> + * on this virtqueue's behalf, and use virtqueue_map_page_attrs() when it is,
> + * which maps through whichever of the two the device uses.


Too verbose and repetetive. E.g. of course people who call virtqueue_dma_dev
do it to "map bufers itself".

> + *
> + * Return: the device to use for DMA API calls, or NULL when there is none.
>   */
>  struct device *virtqueue_dma_dev(struct virtqueue *_vq)
>  {