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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 13 Mar 2019 14:24:35 +0100
Message-ID: <CAK8P3a0A9dbneHo7_CrgiPS8ROaV1bWrYWJUh5Zo2FfEuABfhw@mail.gmail.com>
Subject: Re: [PATCH] powerpc/vdso64: Fix CLOCK_MONOTONIC inconsistencies
 across Y2038
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     linuxppc-dev@ozlabs.org, Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        John Stultz <john.stultz@linaro.org>, jaydee@email.cz,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>
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On Wed, Mar 13, 2019 at 2:14 PM Michael Ellerman <mpe@ellerman.id.au> wrote:

> That causes CLOCK_MONOTONIC to jump ahead by ~4 billion seconds which
> it is not meant to do. Worse, if the time is then set back before the
> Y2038 boundary CLOCK_MONOTONIC will jump backward.
>
> We can fix it simply by storing the full 64-bit offset in the
> vdso_data, and using that in the VDSO assembly code. We also shuffle
> some of the fields in vdso_data to avoid creating a hole.

I see nothing wrong with your patch, but I would point out that there is
a patch series [1] from Vincenzo Frascino to unify the vdso implementation
across architectures that I hope can make it into linux-5.2, and that will
resolve this issue, as well as allow 32-bit architectures to provide
a working interface with 64-bit time_t.

      Arnd

[1] https://lore.kernel.org/linux-arm-kernel/20190222122430.21180-1-vincenzo.frascino@arm.com/