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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

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
> thanks,
>
> Takashi
Thank you,
Oleksandr
>>> Takashi
>> Thank you,
>> Oleksandr
>>
>> [1]
>> https://elixir.bootlin.com/linux/v4.17-rc2/source/drivers/block/xen-blkback/blkback.c#L1127
>> [2]
>> https://elixir.bootlin.com/linux/v4.17-rc2/source/drivers/block/xen-blkback/blkback.c#L1135
>>

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