linux-kernel - Re: [PATCH RFC 04/12] vdpa: add vdpa simulator for block device

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Date:   Wed, 18 Nov 2020 11:23:55 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        Laurent Vivier <lvivier@...hat.com>,
        linux-kernel@...r.kernel.org, Eli Cohen <elic@...dia.com>,
        Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Max Gurtovoy <mgurtovoy@...dia.com>
Subject: Re: [PATCH RFC 04/12] vdpa: add vdpa simulator for block device

On Tue, Nov 17, 2020 at 06:38:11PM +0100, Stefano Garzarella wrote:
> On Tue, Nov 17, 2020 at 04:43:42PM +0000, Stefan Hajnoczi wrote:
> > On Tue, Nov 17, 2020 at 03:16:20PM +0100, Stefano Garzarella wrote:
> > > On Tue, Nov 17, 2020 at 11:11:21AM +0000, Stefan Hajnoczi wrote:
> > > > On Fri, Nov 13, 2020 at 02:47:04PM +0100, Stefano Garzarella wrote:
> > > > > +static void vdpasim_blk_work(struct work_struct *work)
> > > > > +{
> > > > > +	struct vdpasim *vdpasim = container_of(work, struct vdpasim, work);
> > > > > +	u8 status = VIRTIO_BLK_S_OK;
> > > > > +	int i;
> > > > > +
> > > > > +	spin_lock(&vdpasim->lock);
> > > > > +
> > > > > +	if (!(vdpasim->status & VIRTIO_CONFIG_S_DRIVER_OK))
> > > > > +		goto out;
> > > > > +
> > > > > +	for (i = 0; i < VDPASIM_BLK_VQ_NUM; i++) {
> > > > > +		struct vdpasim_virtqueue *vq = &vdpasim->vqs[i];
> > > > > +
> > > > > +		if (!vq->ready)
> > > > > +			continue;
> > > > > +
> > > > > +		while (vringh_getdesc_iotlb(&vq->vring, &vq->iov, &vq->iov,
> > > > > +					    &vq->head, GFP_ATOMIC) > 0) {
> > > > > +
> > > > > +			int write;
> > > > > +
> > > > > +			vq->iov.i = vq->iov.used - 1;
> > > > > +			write = vringh_iov_push_iotlb(&vq->vring, &vq->iov, &status, 1);
> > > > > +			if (write <= 0)
> > > > > +				break;
> > > >
> > > > We're lucky the guest driver doesn't crash after VIRTIO_BLK_T_GET_ID? :)
> > > 
> > > The crash could happen if the simulator doesn't put the string terminator,
> > > but in virtio_blk.c, the serial_show() initialize the buffer putting the
> > > string terminator in the VIRTIO_BLK_ID_BYTES element:
> > > 
> > >     buf[VIRTIO_BLK_ID_BYTES] = '\0';
> > >     err = virtblk_get_id(disk, buf);
> > > 
> > > This should prevent the issue, right?
> > > 
> > > However in the last patch of this series I implemented VIRTIO_BLK_T_GET_ID
> > > support :-)
> > 
> > Windows, BSD, macOS, etc guest drivers aren't necessarily going to
> > terminate or initialize the serial string buffer.
> 
> Unfortunately I discovered that VIRTIO_BLK_T_GET_ID is not in the VIRTIO
> specs, so, just for curiosity, I checked the QEMU code and I found this:
> 
>     case VIRTIO_BLK_T_GET_ID:
>     {
>         /*
>          * NB: per existing s/n string convention the string is
>          * terminated by '\0' only when shorter than buffer.
>          */
>         const char *serial = s->conf.serial ? s->conf.serial : "";
>         size_t size = MIN(strlen(serial) + 1,
>                           MIN(iov_size(in_iov, in_num),
>                               VIRTIO_BLK_ID_BYTES));
>         iov_from_buf(in_iov, in_num, 0, serial, size);
>         virtio_blk_req_complete(req, VIRTIO_BLK_S_OK);
>         virtio_blk_free_request(req);
>         break;
>     }
> 
> It seems that the device emulation in QEMU expects that the driver
> terminates the serial string buffer.
> 
> Do you know why VIRTIO_BLK_T_GET_ID is not in the specs?
> Should we add it?

It's about to be merged into the VIRTIO spec:
https://github.com/oasis-tcs/virtio-spec/issues/63

Stefan

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