linux-kernel - Re: [PATCH] drm/etnaviv: only reject timeouts with tv

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Date:   Tue, 21 Jan 2020 20:05:27 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     Guido Günther <agx@...xcpu.org>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Sam Ravnborg <sam@...nborg.org>, Rob Herring <robh@...nel.org>,
        Emil Velikov <emil.velikov@...labora.com>,
        The etnaviv authors <etnaviv@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/etnaviv: only reject timeouts with tv_nsec >= 2 seconds

On Tue, Jan 21, 2020 at 5:10 PM Lucas Stach <l.stach@...gutronix.de> wrote:
>
> Hi Guido,
>
> On Di, 2020-01-21 at 13:55 +0100, Guido Günther wrote:
> > Hi,
> > On Tue, Jan 21, 2020 at 12:45:25PM +0100, Arnd Bergmann wrote:
> > > As Guido Günther reported, get_abs_timeout() in the etnaviv user space
> > > sometimes passes timeouts with nanosecond values larger than 1000000000,
> > > which gets rejected after my first patch.
> > >
> > > To avoid breaking this, while also not allowing completely arbitrary
> > > values, set the limit to 1999999999 and use set_normalized_timespec64()
> > > to get the correct format before comparing it.
> >
> > I'm seeing values up to 5 seconds so I need
> >
> >      if (args->timeout.tv_nsec > (5 * NSEC_PER_SEC))
> >
> > to unbreak rendering. Which seems to match what mesa's get_abs_timeout()
> > does and how it's invoked.
>
> I have not tested this myself yet, only looked at the code. From the
> code I quoted earlier, I don't see how we end up with 5 * NSEC_PER_SEC
> in the tv_nsec member, even if the timeout passed to get_abs_timeout()
> is 5 seconds.

I can think of two different ways you'd end up with around five seconds here:

a) you have a completely arbitrary 32-bit number through truncation,
    which is up to 4.2 seconds
b) you have the same kind of 32-bit number, but add up to another 999999999
    nanoseconds, so you get up to 5.2 seconds in the 64-bit field.

It could of course be something completely different. If this works correctly
today, we may need to allow any 64-bit input for the nanoseconds and do
an expensive 64-bit div/mod in the kernel for normalization rather than the
cheaper set_normalized_timespec64() from my patch.

        Arnd