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Subject: Re: [PATCH 1/3] virtio_ring: always warn when descriptor chain
 exceeds queue size
To:     Connor Kuehl <ckuehl@redhat.com>, virtio-fs@redhat.com
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, stefanha@redhat.com,
        vgoyal@redhat.com, miklos@szeredi.hu, mst@redhat.com
References: <20210318135223.1342795-1-ckuehl@redhat.com>
 <20210318135223.1342795-2-ckuehl@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <fa4988fa-a671-0abf-f922-6b362faf10d5@redhat.com>
Date:   Mon, 22 Mar 2021 11:22:15 +0800
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?? 2021/3/18 ????9:52, Connor Kuehl д??:
>  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;