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

[Takashi Iwai]

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)ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ \
> >>  ÂÂÂ (&((_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.


thanks,

Takashi

> >
> > 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
>