Messages in this thread | | | Subject | Re: [PATCH v2 3/5] ALSA: xen-front: Implement Xen event channel handling | From | Oleksandr Andrushchenko <> | Date | Tue, 24 Apr 2018 17:58:43 +0300 |
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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
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