linux-kernel - Re: [PATCH] pstore: use ktime_get_real_fast_ns() instead of _

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Date:   Wed, 8 Nov 2017 16:55:40 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: use ktime_get_real_fast_ns() instead of __getnstimeofday()

On Wed, Nov 8, 2017 at 8:00 AM, Arnd Bergmann <arnd@...db.de> wrote:
> I noticed that __getnstimeofday() is a rather odd interface, with
> a number of quirks:
>
> - The caller may come from NMI context, but the implementation is not NMI safe
> - The calling conventions are different from any other timekeeping functions
> - The naming doesn't fit into the 'ktime_get_*()' scheme
>
> This addresses the above issues by using a completely different
> method to get the time: ktime_get_real_fast_ns() is NMI safe and
> doesn't print a warning when called with the timekeeping suspended,
> and we can easily transform the result into a timespec structure. Since
> ktime_get_real_fast_ns() was not exported to modules, I also add the
> export.
>
> The behavior for the suspended case changes, we now return the time
> if we can find it out rather than setting it to zero. We could still
> change that but this would require exporting the 'timekeeping_suspended'
> to modules.
>
> I'm not trying to address y2038-safety at this point, but plan to
> do that later with a more complex patch.
>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> It would be helpful to get this merged into v4.15-rc1, since I have
> some other cleanups that depend on this. Please have a look to see if
> this makes sense to you.

As long as this is sane to call when timekeeping is not running, I'm
happy to take the patch. Would you prefer I take this via pstore or do
you have another tree it should go through?

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  fs/pstore/platform.c      | 5 +----
>  kernel/time/timekeeping.c | 1 +
>  2 files changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
> index ec7199e859d2..086e491faf04 100644
> --- a/fs/pstore/platform.c
> +++ b/fs/pstore/platform.c
> @@ -482,10 +482,7 @@ void pstore_record_init(struct pstore_record *record,
>         record->psi = psinfo;
>
>         /* Report zeroed timestamp if called before timekeeping has resumed. */
> -       if (__getnstimeofday(&record->time)) {
> -               record->time.tv_sec = 0;
> -               record->time.tv_nsec = 0;
> -       }
> +       record->time = ns_to_timespec(ktime_get_real_fast_ns());
>  }
>
>  /*
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 353f7bd1eeb0..198afa78bf69 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -528,6 +528,7 @@ u64 ktime_get_real_fast_ns(void)
>  {
>         return __ktime_get_real_fast_ns(&tk_fast_mono);
>  }
> +EXPORT_SYMBOL_GPL(ktime_get_real_fast_ns);
>
>  /**
>   * halt_fast_timekeeper - Prevent fast timekeeper from accessing clocksource.
> --
> 2.9.0
>



-- 
Kees Cook
Pixel Security