Subject: Re: [PATCH RFC 01/13] vhost: option to fetch descriptors through an independent struct

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 7 Jun 2020 09:59:00 -0400

On Wed, Jun 03, 2020 at 08:04:45PM +0800, Jason Wang wrote:
> 
> On 2020/6/3 下午5:48, Michael S. Tsirkin wrote:
> > On Wed, Jun 03, 2020 at 03:13:56PM +0800, Jason Wang wrote:
> > > On 2020/6/2 下午9:05, Michael S. Tsirkin wrote:
> 
> 
> [...]
> 
> 
> > > > +
> > > > +static int fetch_indirect_descs(struct vhost_virtqueue *vq,
> > > > +				struct vhost_desc *indirect,
> > > > +				u16 head)
> > > > +{
> > > > +	struct vring_desc desc;
> > > > +	unsigned int i = 0, count, found = 0;
> > > > +	u32 len = indirect->len;
> > > > +	struct iov_iter from;
> > > > +	int ret;
> > > > +
> > > > +	/* Sanity check */
> > > > +	if (unlikely(len % sizeof desc)) {
> > > > +		vq_err(vq, "Invalid length in indirect descriptor: "
> > > > +		       "len 0x%llx not multiple of 0x%zx\n",
> > > > +		       (unsigned long long)len,
> > > > +		       sizeof desc);
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +	ret = translate_desc(vq, indirect->addr, len, vq->indirect,
> > > > +			     UIO_MAXIOV, VHOST_ACCESS_RO);
> > > > +	if (unlikely(ret < 0)) {
> > > > +		if (ret != -EAGAIN)
> > > > +			vq_err(vq, "Translation failure %d in indirect.\n", ret);
> > > > +		return ret;
> > > > +	}
> > > > +	iov_iter_init(&from, READ, vq->indirect, ret, len);
> > > > +
> > > > +	/* We will use the result as an address to read from, so most
> > > > +	 * architectures only need a compiler barrier here. */
> > > > +	read_barrier_depends();
> > > > +
> > > > +	count = len / sizeof desc;
> > > > +	/* Buffers are chained via a 16 bit next field, so
> > > > +	 * we can have at most 2^16 of these. */
> > > > +	if (unlikely(count > USHRT_MAX + 1)) {
> > > > +		vq_err(vq, "Indirect buffer length too big: %d\n",
> > > > +		       indirect->len);
> > > > +		return -E2BIG;
> > > > +	}
> > > > +	if (unlikely(vq->ndescs + count > vq->max_descs)) {
> > > > +		vq_err(vq, "Too many indirect + direct descs: %d + %d\n",
> > > > +		       vq->ndescs, indirect->len);
> > > > +		return -E2BIG;
> > > > +	}
> > > > +
> > > > +	do {
> > > > +		if (unlikely(++found > count)) {
> > > > +			vq_err(vq, "Loop detected: last one at %u "
> > > > +			       "indirect size %u\n",
> > > > +			       i, count);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +		if (unlikely(!copy_from_iter_full(&desc, sizeof(desc), &from))) {
> > > > +			vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
> > > > +			       i, (size_t)indirect->addr + i * sizeof desc);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +		if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) {
> > > > +			vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
> > > > +			       i, (size_t)indirect->addr + i * sizeof desc);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +
> > > > +		push_split_desc(vq, &desc, head);
> > > 
> > > The error is ignored.
> > See above:
> > 
> >       	if (unlikely(vq->ndescs + count > vq->max_descs))
> > 
> > So it can't fail here, we never fetch unless there's space.
> > 
> > I guess we can add a WARN_ON here.
> 
> 
> Yes.
> 
> 
> > 
> > > > +	} while ((i = next_desc(vq, &desc)) != -1);
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static int fetch_descs(struct vhost_virtqueue *vq)
> > > > +{
> > > > +	unsigned int i, head, found = 0;
> > > > +	struct vhost_desc *last;
> > > > +	struct vring_desc desc;
> > > > +	__virtio16 avail_idx;
> > > > +	__virtio16 ring_head;
> > > > +	u16 last_avail_idx;
> > > > +	int ret;
> > > > +
> > > > +	/* Check it isn't doing very strange things with descriptor numbers. */
> > > > +	last_avail_idx = vq->last_avail_idx;
> > > > +
> > > > +	if (vq->avail_idx == vq->last_avail_idx) {
> > > > +		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
> > > > +			vq_err(vq, "Failed to access avail idx at %p\n",
> > > > +				&vq->avail->idx);
> > > > +			return -EFAULT;
> > > > +		}
> > > > +		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> > > > +
> > > > +		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
> > > > +			vq_err(vq, "Guest moved used index from %u to %u",
> > > > +				last_avail_idx, vq->avail_idx);
> > > > +			return -EFAULT;
> > > > +		}
> > > > +
> > > > +		/* If there's nothing new since last we looked, return
> > > > +		 * invalid.
> > > > +		 */
> > > > +		if (vq->avail_idx == last_avail_idx)
> > > > +			return vq->num;
> > > > +
> > > > +		/* Only get avail ring entries after they have been
> > > > +		 * exposed by guest.
> > > > +		 */
> > > > +		smp_rmb();
> > > > +	}
> > > > +
> > > > +	/* Grab the next descriptor number they're advertising */
> > > > +	if (unlikely(vhost_get_avail_head(vq, &ring_head, last_avail_idx))) {
> > > > +		vq_err(vq, "Failed to read head: idx %d address %p\n",
> > > > +		       last_avail_idx,
> > > > +		       &vq->avail->ring[last_avail_idx % vq->num]);
> > > > +		return -EFAULT;
> > > > +	}
> > > > +
> > > > +	head = vhost16_to_cpu(vq, ring_head);
> > > > +
> > > > +	/* If their number is silly, that's an error. */
> > > > +	if (unlikely(head >= vq->num)) {
> > > > +		vq_err(vq, "Guest says index %u > %u is available",
> > > > +		       head, vq->num);
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +	i = head;
> > > > +	do {
> > > > +		if (unlikely(i >= vq->num)) {
> > > > +			vq_err(vq, "Desc index is %u > %u, head = %u",
> > > > +			       i, vq->num, head);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +		if (unlikely(++found > vq->num)) {
> > > > +			vq_err(vq, "Loop detected: last one at %u "
> > > > +			       "vq size %u head %u\n",
> > > > +			       i, vq->num, head);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +		ret = vhost_get_desc(vq, &desc, i);
> > > > +		if (unlikely(ret)) {
> > > > +			vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
> > > > +			       i, vq->desc + i);
> > > > +			return -EFAULT;
> > > > +		}
> > > > +		ret = push_split_desc(vq, &desc, head);
> > > > +		if (unlikely(ret)) {
> > > > +			vq_err(vq, "Failed to save descriptor: idx %d\n", i);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +	} while ((i = next_desc(vq, &desc)) != -1);
> > > > +
> > > > +	last = peek_split_desc(vq);
> > > > +	if (unlikely(last->flags & VRING_DESC_F_INDIRECT)) {
> > > > +		pop_split_desc(vq);
> > > > +		ret = fetch_indirect_descs(vq, last, head);
> > > 
> > > Note that this means we don't supported chained indirect descriptors which
> > > complies the spec but we support this in vhost_get_vq_desc().
> > Well the spec says:
> > 	A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT in flags.
> > 
> > Did I miss anything?
> > 
> 
> No, but I meant current vhost_get_vq_desc() supports chained indirect
> descriptor. Not sure if there's an application that depends on this
> silently.
> 
> Thanks
> 

I don't think we need to worry about that unless this actually
surfaces.

-- 
MST