Messages in this thread | | | Subject | Re: [PATCH v2 3/5] ALSA: xen-front: Implement Xen event channel handling | From | Oleksandr Andrushchenko <> | Date | Wed, 25 Apr 2018 12:04:35 +0300 |
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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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