linux-kernel - Re: [PATCH] time: Avoid signed overflow in timekeeping_delta_to

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Date:   Mon, 14 Nov 2016 11:54:54 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Chris Metcalf <cmetcalf@...lanox.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Laurent Vivier <lvivier@...hat.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Christopher S . Hall" <christopher.s.hall@...el.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] time: Avoid signed overflow in timekeeping_delta_to_ns()

On Mon, Nov 14, 2016 at 11:42 AM, Chris Metcalf <cmetcalf@...lanox.com> wrote:
> This bugfix was originally made in commit 35a4933a8959 ("time:
> Avoid signed overflow in timekeeping_get_ns()").  When the code was
> refactored in commit 6bd58f09e1d8 ("time: Add cycles to nanoseconds
> translation") the signed overflow fix was lost.  Re-introduce it.
>
> Signed-off-by: Chris Metcalf <cmetcalf@...lanox.com>
> ---
> I happened to be looking for an unrelated fix, found this code,
> realized the tip code didn't match the fixed code, and
> backtracked to where it had gone away.
>
>  kernel/time/timekeeping.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 37dec7e3db43..57926bc7b7f3 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -304,8 +304,7 @@ static inline s64 timekeeping_delta_to_ns(struct tk_read_base *tkr,
>  {
>         s64 nsec;
>
> -       nsec = delta * tkr->mult + tkr->xtime_nsec;
> -       nsec >>= tkr->shift;
> +       nsec = (delta * tkr->mult + tkr->xtime_nsec) >> tkr->shift;

Ugh.

So... I think this proves the original fix was *far* too subtle to
maintain. So I think reintroducing it as-is doesn't protect us from
undoing it.  If the problem is really using the signed intermediate
nsec value, we should get rid of that.

thanks
-john