Re: [PATCH v11 21/40] virtio_ring: packed: introduce virtqueue_resize_packed()

Xuan Zhuo
2022-07-04T02:20:54Z

On Fri, 1 Jul 2022 17:27:48 +0800, Jason Wang <jasowang@redhat.com> wrote:
>
> 在 2022/6/29 14:56, Xuan Zhuo 写道:
> > virtio ring packed supports resize.
> >
> > Only after the new vring is successfully allocated based on the new num,
> > we will release the old vring. In any case, an error is returned,
> > indicating that the vring still points to the old vring.
> >
> > In the case of an error, re-initialize(by virtqueue_reinit_packed()) the
> > virtqueue to ensure that the vring can be used.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> > drivers/virtio/virtio_ring.c | 29 +++++++++++++++++++++++++++++
> > 1 file changed, 29 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 650f701a5480..4860787286db 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -2042,6 +2042,35 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > return NULL;
> > }
> >
> > +static int virtqueue_resize_packed(struct virtqueue *_vq, u32 num)
> > +{
> > + struct vring_virtqueue_packed vring = {};
> > + struct vring_virtqueue *vq = to_vvq(_vq);
> > + struct virtio_device *vdev = _vq->vdev;
> > + int err;
> > +
> > + if (vring_alloc_queue_packed(&vring, vdev, num))
> > + goto err_ring;
> > +
> > + err = vring_alloc_state_extra_packed(&vring);
> > + if (err)
> > + goto err_state_extra;
> > +
> > + vring_free(&vq->vq);
> > +
> > + virtqueue_init(vq, vring.vring.num);
> > + virtqueue_vring_attach_packed(vq, &vring);
> > + virtqueue_vring_init_packed(vq);
> > +
> > + return 0;
> > +
> > +err_state_extra:
> > + vring_free_packed(&vring, vdev);
> > +err_ring:
> > + virtqueue_reinit_packed(vq);
>
>
> So desc_state and desc_extra has been freed vring_free_packed() when
> vring_alloc_state_extra_packed() fails. We might get use-after-free here?

vring_free_packed() frees the temporary structure vring. It does not affect
desc_state and desc_extra of vq. So it is safe.

>
> Actually, I think for resize we need
>
> 1) detach old
> 2) allocate new
> 3) if 2) succeed, attach new otherwise attach old

The implementation is now:

1. allocate new
2. free old (detach old)
3. attach new

error:
1. free temporary
2. reinit old

Do you think this is ok? We need to add a new variable to save the old vring in
the process you mentioned, there is not much difference in other.

Thanks.

>
> This seems more clearer than the current logic?
>
> Thanks
>
>
> > + return -ENOMEM;
> > +}
> > +
> >
> > /*
> > * Generic functions and exported symbols.
>