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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

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?
>
> 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