On 12/18/2018 11:13 PM, Roger Pau Monné wrote:
> 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 :).
However, for the "ring-page-order" case, the err used in xenbus_dev_fatal() is
not set as xenbus_read_unsigned() does not return any err?
For "ring-page-order", I would still need to set err = -EINVAL with extra one
line of code?
>
>> + 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!
>
Dongli Zhang
