On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote:
> Hi Roger,
>
> On 12/18/2018 05:33 PM, Roger Pau Monné wrote:
> > On Tue, Dec 18, 2018 at 08:55:38AM +0800, Dongli Zhang wrote:
> >> The xenstore 'ring-page-order' is used globally for each blkback queue and
> >> therefore should be read from xenstore only once. However, it is obtained
> >> in read_per_ring_refs() which might be called multiple times during the
> >> initialization of each blkback queue.
> >>
> >> If the blkfront is malicious and the 'ring-page-order' is set in different
> >> value by blkfront every time before blkback reads it, this may end up at
> >> the "WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));" in
> >> xen_blkif_disconnect() when frontend is destroyed.
> >>
> >> This patch reworks connect_ring() to read xenstore 'ring-page-order' only
> >> once.
> >>
> >> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> >> ---
> >> Changed since v1:
> >> * change the order of xenstore read in read_per_ring_refs(suggested by Roger Pau Monne)
> >> * use xenbus_read_unsigned() in connect_ring() (suggested by Roger Pau Monne)
> >>
> >> drivers/block/xen-blkback/xenbus.c | 70 ++++++++++++++++++++++----------------
> >> 1 file changed, 40 insertions(+), 30 deletions(-)
> >>
> >> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> >> index a4bc74e..7178f0f 100644
> >> --- a/drivers/block/xen-blkback/xenbus.c
> >> +++ b/drivers/block/xen-blkback/xenbus.c
> >> @@ -926,7 +926,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
> >> int err, i, j;
> >> struct xen_blkif *blkif = ring->blkif;
> >> struct xenbus_device *dev = blkif->be->dev;
> >> - unsigned int ring_page_order, nr_grefs, evtchn;
> >> + unsigned int nr_grefs, evtchn;
> >>
> >> err = xenbus_scanf(XBT_NIL, dir, "event-channel", "%u",
> >> &evtchn);
> >> @@ -936,43 +936,38 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
> >> return err;
> >> }
> >>
> >> - err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
> >> - &ring_page_order);
> >> - if (err != 1) {
> >> - err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", &ring_ref[0]);
> >> - if (err != 1) {
> >> + nr_grefs = blkif->nr_ring_pages;
> >> + WARN_ON(!nr_grefs);
> >> +
> >> + for (i = 0; i < nr_grefs; i++) {
> >> + char ring_ref_name[RINGREF_NAME_LEN];
> >> +
> >> + snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> >> + err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
> >> + "%u", &ring_ref[i]);
> >> +
> >> + if (err != 1 && (i || (!i && nr_grefs > 1))) {
> >
> > AFAICT the above condition can be simplified as "err != 1 &&
> > nr_grefs".
> >
> >> err = -EINVAL;
> >
> > There's no point in setting err here...
> >
> >> - xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
> >> + xenbus_dev_fatal(dev, err, "reading %s/%s",
> >> + dir, ring_ref_name);
> >> return err;
> >
> > ...since you can just return -EINVAL (same applies to the other
> > instance below).
>
> I would like to confirm if I would keep the err = -EINVAL in below because most
> of the below code is copied from original implementation without modification.
>
> There is no err set by xenbus_read_unsigned().
Right, but instead of doing:
err = -EINVAL;
return err;
You can just do:
return -EINVAL;
Which is one line shorter :).
> + ring_page_order = xenbus_read_unsigned(dev->otherend,
> + "ring-page-order", 0);
> +
> + if (ring_page_order > xen_blkif_max_ring_order) {
> + err = -EINVAL;
> + xenbus_dev_fatal(dev, err,
> + "requested ring page order %d exceed max:%d",
> + ring_page_order,
> + xen_blkif_max_ring_order);
> + return err;
> + }
> +
> + be->blkif->nr_ring_pages = 1 << ring_page_order;
>
>
> For the rest, I would do something like:
>
> + err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
> + "%u", &ring_ref[i]);
> +
> + if (err != 1 && nr_grefs > 1) {
> + xenbus_dev_fatal(dev, err, "reading %s/%s",
> + dir, ring_ref_name);
> + return -EINVAL;
> + }
>
>
> Thank you very much!
Thanks!
