netdev - Re: [PATCH v11 21/40] virtio_ring: packed: introduce virtqueue_resize

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Date:   Mon, 4 Jul 2022 10:13:32 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <markgross@...nel.org>,
        Vadim Pasternak <vadimp@...dia.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Eric Farman <farman@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        linux-um@...ts.infradead.org, netdev@...r.kernel.org,
        platform-driver-x86@...r.kernel.org,
        linux-remoteproc@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, bpf@...r.kernel.org,
        kangjie.xu@...ux.alibaba.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v11 21/40] virtio_ring: packed: introduce virtqueue_resize_packed()

On Fri, 1 Jul 2022 17:27:48 +0800, Jason Wang <jasowang@...hat.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@...ux.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.
>