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

Jason Wang
2022-07-04T04:02:54Z

在 2022/7/4 10:13, Xuan Zhuo 写道:
> 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.

You are right.

>
>> 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.

Yes, I think the code is fine. But I'd suggest to rename "vring" to
"vring_packed", this simplify the reviewers.

Other than this, you can add:

Acked-by: Jason Wang <jasowang@redhat.com>

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