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SubjectRe: [PATCH 1/3] virtio_ring: always warn when descriptor chain exceeds queue size
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在 2021/3/18 下午9:52, Connor Kuehl 写道:
> From section 2.6.5.3.1 (Driver Requirements: Indirect Descriptors)
> of the virtio spec:
>
> "A driver MUST NOT create a descriptor chain longer than the Queue
> Size of the device."
>
> This text suggests that the warning should trigger even if
> indirect descriptors are in use.


So I think at least the commit log needs some tweak.

For split virtqueue. We had:

2.6.5.2 Driver Requirements: The Virtqueue Descriptor Table

Drivers MUST NOT add a descriptor chain longer than 2^32 bytes in total;
this implies that loops in the descriptor chain are forbidden!

2.6.5.3.1 Driver Requirements: Indirect Descriptors

A driver MUST NOT create a descriptor chain longer than the Queue Size
of the device.

If I understand the spec correctly, the check is only needed for a
single indirect descriptor table?

For packed virtqueue. We had:

2.7.17 Driver Requirements: Scatter-Gather Support

A driver MUST NOT create a descriptor list longer than allowed by the
device.

A driver MUST NOT create a descriptor list longer than the Queue Size.

2.7.19 Driver Requirements: Indirect Descriptors

A driver MUST NOT create a descriptor chain longer than allowed by the
device.

So it looks to me the packed part is fine.

Note that if I understand the spec correctly 2.7.17 implies 2.7.19.

Thanks


>
> Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
> ---
> drivers/virtio/virtio_ring.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 71e16b53e9c1..1bc290f9ba13 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -444,11 +444,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>
> head = vq->free_head;
>
> + WARN_ON_ONCE(total_sg > vq->split.vring.num);
> +
> if (virtqueue_use_indirect(_vq, total_sg))
> desc = alloc_indirect_split(_vq, total_sg, gfp);
> else {
> desc = NULL;
> - WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);
> }
>
> if (desc) {
> @@ -1118,6 +1119,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>
> BUG_ON(total_sg == 0);
>
> + WARN_ON_ONCE(total_sg > vq->packed.vring.num);
> +
> if (virtqueue_use_indirect(_vq, total_sg))
> return virtqueue_add_indirect_packed(vq, sgs, total_sg,
> out_sgs, in_sgs, data, gfp);
> @@ -1125,8 +1128,6 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> head = vq->packed.next_avail_idx;
> avail_used_flags = vq->packed.avail_used_flags;
>
> - WARN_ON_ONCE(total_sg > vq->packed.vring.num && !vq->indirect);
> -
> desc = vq->packed.vring.desc;
> i = head;
> descs_used = total_sg;

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