Re: [PATCH] drm/etnaviv: only reject timeouts with tv_nsec >= 2 seconds

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Wed, Jan 22, 2020 at 11:30:34AM +0100, Guido Günther wrote:
> Hi,
> On Tue, Jan 21, 2020 at 08:05:27PM +0100, Arnd Bergmann wrote:
> > On Tue, Jan 21, 2020 at 5:10 PM Lucas Stach <l.stach@pengutronix.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.
> 
> I've dumped out some values tv_nsec values with current mesa git on arm64:
> 
> [   33.699652] etnaviv_ioctl_gem_cpu_prep: 4990449401
> [   33.813081] etnaviv_ioctl_gem_cpu_prep: 5103872445
> [   33.822936] etnaviv_ioctl_gem_cpu_prep: 5113731286
> [   33.840963] etnaviv_ioctl_gem_cpu_prep: 5131762726
> [   33.854120] etnaviv_ioctl_gem_cpu_prep: 5144920127
> [   33.861426] etnaviv_ioctl_gem_cpu_prep: 5152227527
> [   33.872666] etnaviv_ioctl_gem_cpu_prep: 5163466968
> [   33.879485] etnaviv_ioctl_gem_cpu_prep: 5170286808
> 
> The problem is that in mesa/libdrm
> 
> static inline void get_abs_timeout(struct drm_etnaviv_timespec *tv, uint64_t ns)
> {
>         struct timespec t;
>         uint32_t s = ns / 1000000000;
>         clock_gettime(CLOCK_MONOTONIC, &t);
>         tv->tv_sec = t.tv_sec + s;
>         tv->tv_nsec = t.tv_nsec + ns - (s * 1000000000);
>                                         ^^^^^^^^^^^^^^^
>    this overflows (since `s` is `uint_32t` and hence we substract a way
>    too small value with ns = 5000000000 which mesa uses in
>    etna_bo_cpu_prep.
> }
> 
> So with current mesa/libdrm (which needs to be fixed) we'd have a maximum
> 
>       t.tv_nsec + ns         - (s_max * 1000000000)
> 
>       999999999 + 5000000000 - 705032704            = 5294967295
> 
> Does that make sense? If so that'd be the possible upper bound for the
> kernel. Note that this only applies to etnaviv_ioctl_gem_cpu_prep. While
> etnaviv_ioctl_wait_fence and etnaviv_ioctl_gem_wait are affected too
> i've not yet seen user space passing in larger values.

Except the fact that the calculation being done above is buggy.
Not only do we end up with tv_sec incremented by 5 seconds, but
we also end up with tv_nsec containing around 5 seconds in
nanoseconds, which means we end up with about a 10 second timeout.

I think it would probably be better for the kernel to print a
warning once when noticing over-large nsec values, suggesting a
userspace upgrade is in order, but continue the existing behaviour.

-- 
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