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

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Message-ID: <20161115005636.GF24747@umbus.fritz.box>
Date:   Tue, 15 Nov 2016 11:56:36 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Chris Metcalf <cmetcalf@...lanox.com>
Cc:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Laurent Vivier <lvivier@...hat.com>,
        "Christopher S . Hall" <christopher.s.hall@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] time: Avoid signed overflow in timekeeping_delta_to_ns()

On Mon, Nov 14, 2016 at 02:42:49PM -0500, Chris Metcalf 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 This bugfix was originally made in commit 35a4933a8959 ("time: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>

Reviewed-by: David Gibson <david@...son.dropbear.id.au>

> ---
> 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;
>  
>  	/* If arch requires, add in get_arch_timeoffset() */
>  	return nsec + arch_gettimeoffset();

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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