linux-kernel - Re: [PATCH] mac80211: fix race in ieee80211_register

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Date:   Mon, 6 Apr 2020 19:25:30 +0530
From:   Krishna Chaitanya <chaitanya.mgit@...il.com>
To:     Kalle Valo <kvalo@...eaurora.org>
Cc:     Sumit Garg <sumit.garg@...aro.org>,
        Johannes Berg <johannes@...solutions.net>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>, kuba@...nel.org,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthias-Peter Schöpfer 
        <matthias.schoepfer@...inx.io>,
        "Berg Philipp (HAU-EDS)" <Philipp.Berg@...bherr.com>,
        "Weitner Michael (HAU-EDS)" <Michael.Weitner@...bherr.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Loic Poulain <loic.poulain@...aro.org>, stable@...r.kernel.org
Subject: Re: [PATCH] mac80211: fix race in ieee80211_register_hw()

On Mon, Apr 6, 2020 at 6:57 PM Kalle Valo <kvalo@...eaurora.org> wrote:
>
> Sumit Garg <sumit.garg@...aro.org> writes:
>
> > On Mon, 6 Apr 2020 at 18:38, Johannes Berg <johannes@...solutions.net> wrote:
> >>
> >> On Mon, 2020-04-06 at 16:04 +0300, Kalle Valo wrote:
> >> > Johannes Berg <johannes@...solutions.net> writes:
> >> >
> >> > > On Mon, 2020-04-06 at 15:52 +0300, Kalle Valo wrote:
> >> > > > Johannes Berg <johannes@...solutions.net> writes:
> >> > > >
> >> > > > > On Mon, 2020-04-06 at 15:44 +0300, Kalle Valo wrote:
> >> > > > > > >     user-space  ieee80211_register_hw()  RX IRQ
> >> > > > > > >     +++++++++++++++++++++++++++++++++++++++++++++
> >> > > > > > >        |                    |             |
> >> > > > > > >        |<---wlan0---wiphy_register()      |
> >> > > > > > >        |----start wlan0---->|             |
> >> > > > > > >        |                    |<---IRQ---(RX packet)
> >> > > > > > >        |              Kernel crash        |
> >> > > > > > >        |              due to unallocated  |
> >> > > > > > >        |              workqueue.          |
> >> > > > >
> >> > > > > [snip]
> >> > > > >
> >> > > > > > I have understood that no frames should be received until mac80211 calls
> >> > > > > > struct ieee80211_ops::start:
> >> > > > > >
> >> > > > > >  * @start: Called before the first netdevice attached to the hardware
> >> > > > > >  *         is enabled. This should turn on the hardware and must turn on
> >> > > > > >  *         frame reception (for possibly enabled monitor interfaces.)
> >> > > > >
> >> > > > > True, but I think he's saying that you can actually add and configure an
> >> > > > > interface as soon as the wiphy is registered?
> >> > > >
> >> > > > With '<---IRQ---(RX packet)' I assumed wcn36xx is delivering a frame to
> >> > > > mac80211 using ieee80211_rx(), but of course I'm just guessing here.
> >> > >
> >> > > Yeah, but that could be legitimate?
> >> >
> >> > Ah, I misunderstood then. The way I have understood is that no rx frames
> >> > should be delivered (= calling ieee80211_rx()_ before start() is called,
> >> > but if that's not the case please ignore me :)
> >>
> >> No no, that _is_ the case. But I think the "start wlan0" could end up
> >> calling it?
> >>
> >
> > Sorry if I wasn't clear enough via the sequence diagram. It's a common
> > RX packet that arrives via ieee80211_tasklet_handler() which is
> > enabled via call to "struct ieee80211_ops::start" api.
>
> Ah sorry, I didn't realise that. So wcn36xx is not to be blamed then,
> thanks for the clarification.
>
> --
> https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches
I am still confused, without ieee80211_if_add how can the userspace
bring up the interface?
(there by calling drv_start())?