Re: [PATCH v2 3/5] ALSA: xen-front: Implement Xen event channel handling

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 04/25/2018 12:02 PM, Takashi Iwai wrote:
> On Wed, 25 Apr 2018 10:26:34 +0200,
> Oleksandr Andrushchenko wrote:
>> On 04/24/2018 07:23 PM, Oleksandr Andrushchenko wrote:
>>> On 04/24/2018 06:02 PM, Takashi Iwai wrote:
>>>> On Tue, 24 Apr 2018 16:58:43 +0200,
>>>> Oleksandr Andrushchenko wrote:
>>>>> On 04/24/2018 05:35 PM, Takashi Iwai wrote:
>>>>>> On Tue, 24 Apr 2018 16:29:15 +0200,
>>>>>> Oleksandr Andrushchenko wrote:
>>>>>>> On 04/24/2018 05:20 PM, Takashi Iwai wrote:
>>>>>>>> On Mon, 16 Apr 2018 08:24:51 +0200,
>>>>>>>> Oleksandr Andrushchenko wrote:
>>>>>>>>> +static irqreturn_t evtchnl_interrupt_req(int irq, void *dev_id)
>>>>>>>>> +{
>>>>>>>>> +    struct xen_snd_front_evtchnl *channel = dev_id;
>>>>>>>>> +    struct xen_snd_front_info *front_info = channel->front_info;
>>>>>>>>> +    struct xensnd_resp *resp;
>>>>>>>>> +    RING_IDX i, rp;
>>>>>>>>> +    unsigned long flags;
>>>>>>>>> +
>>>>>>>>> +    if (unlikely(channel->state != EVTCHNL_STATE_CONNECTED))
>>>>>>>>> +        return IRQ_HANDLED;
>>>>>>>>> +
>>>>>>>>> +    spin_lock_irqsave(&front_info->io_lock, flags);
>>>>>>>>> +
>>>>>>>>> +again:
>>>>>>>>> +    rp = channel->u.req.ring.sring->rsp_prod;
>>>>>>>>> +    /* ensure we see queued responses up to rp */
>>>>>>>>> +    rmb();
>>>>>>>>> +
>>>>>>>>> +    for (i = channel->u.req.ring.rsp_cons; i != rp; i++) {
>>>>>>>> I'm not familiar with Xen stuff in general, but through a quick
>>>>>>>> glance, this kind of code worries me a bit.
>>>>>>>>
>>>>>>>> If channel->u.req.ring.rsp_cons has a bogus number, this may
>>>>>>>> lead to a
>>>>>>>> very long loop, no?  Better to have a sanity check of the ring
>>>>>>>> buffer
>>>>>>>> size.
>>>>>>> In this loop I have:
>>>>>>> resp = RING_GET_RESPONSE(&channel->u.req.ring, i);
>>>>>>> and the RING_GET_RESPONSE macro is designed in the way that
>>>>>>> it wraps around when *i* in the question gets bigger than
>>>>>>> the ring size:
>>>>>>>
>>>>>>> #define RING_GET_REQUEST(_r, _idx)                    \
>>>>>>>        (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
>>>>>>>
>>>>>>> So, even if the counter has a bogus number it will not last long
>>>>>> Hm, this prevents from accessing outside the ring buffer, but does it
>>>>>> change the loop behavior?
>>>>> no, it doesn't
>>>>>> Suppose channel->u.req.ring_rsp_cons = 1, and rp = 0, the loop below
>>>>>> would still consume the whole 32bit counts, no?
>>>>>>
>>>>>>      for (i = channel->u.req.ring.rsp_cons; i != rp; i++) {
>>>>>>          resp = RING_GET_RESPONSE(&channel->u.req.ring, i);
>>>>>>          ...
>>>>>>      }
>>>>> You are right here and the comment is totally valid.
>>>>> I'll put an additional check like here [1] and here [2]
>>>>> Will this address your comment?
>>>> Yep, this kind of sanity checks should work.
>>>>
>>> Great, will implement the checks this way then
>> Well, after thinking a bit more on that and chatting on #xendevel IRC
>> with Juergen (he is on CC list), it seems that the way the code is now
>> it is all fine without the checks: the assumption here is that
>> the backend is trusted to always write sane values to the ring counters,
>> thus no overflow checks on frontend side are required.
>> Even if I implement the checks then I have no means to recover, but
>> just print
>> an error message and bail out not handling any responses.
>> This is probably why the checks [1] and [2] are only implemented for the
>> backend side and there are no such macros for the frontend side.
>>
>> Takashi, please let me know if the above sounds reasonable and
>> addresses your comments.
> If it's guaranteed to work, that's OK.
> But maybe it's worth to comment for readers.
ok, will put a comment on that
>
> thanks,
>
> Takashi
Thank you,
Oleksandr
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