From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751779AbeDYJEm (ORCPT ); Wed, 25 Apr 2018 05:04:42 -0400 Received: from mail-lf0-f50.google.com ([209.85.215.50]:42799 "EHLO mail-lf0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750882AbeDYJEj (ORCPT ); Wed, 25 Apr 2018 05:04:39 -0400 X-Google-Smtp-Source: AB8JxZrp8hnQyVXcfEv0kIAuIHb/lAH5VrsASBxB7vZx+Lpj802YJn2ejhCMHr4RDFWxF0mgpLuqFQ== Subject: Re: [PATCH v2 3/5] ALSA: xen-front: Implement Xen event channel handling To: Takashi Iwai Cc: alsa-devel@alsa-project.org, xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, perex@perex.cz, Juergen Gross , linux-kernel@vger.kernel.org, Oleksandr Andrushchenko References: <20180416062453.24743-1-andr2000@gmail.com> <20180416062453.24743-4-andr2000@gmail.com> <04e313e5-4fdb-9f7b-43e3-f1551f582db2@gmail.com> From: Oleksandr Andrushchenko Message-ID: <72af8fdb-586f-6c5c-72af-071682ceab7d@gmail.com> Date: Wed, 25 Apr 2018 12:04:35 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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