linux-kernel - Re: [PATCH 3/4] time: Avoid potential shift-overflow with large shift values

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Date:	Wed, 22 Aug 2012 10:11:11 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Schwab <schwab@...ux-m68k.org>
Subject: Re: [PATCH 3/4] time: Avoid potential shift-overflow with large shift values

Hi John,

On Wed, Aug 22, 2012 at 2:30 AM, John Stultz <john.stultz@...aro.org> wrote:
> Andreas Schwab noted that the 1 << tk->shift could overflow
> if the shift value was greater then 30, since 1 would be
> a 32bit long on 32bit architectures.

This comment is not entirely correct: "1" is not a long, but an
(32-bit signed) int,
so it can overflow on 64-bit platforms, too.

> This patch uses 1ULL instead to ensure we don't overflow on
> the shift.
>
> This issue was introduced by 1e75fa8be9fb61e1af46b5b3b176347a4c958ca1
>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Andreas Schwab <schwab@...ux-m68k.org>
> Reported-by: Andreas Schwab <schwab@...ux-m68k.org>
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  kernel/time/timekeeping.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 1dbf80e..a5a9389 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1184,9 +1184,9 @@ static void update_wall_time(void)
>         * the vsyscall implementations are converted to use xtime_nsec
>         * (shifted nanoseconds), this can be killed.
>         */
> -       remainder = tk->xtime_nsec & ((1 << tk->shift) - 1);
> +       remainder = tk->xtime_nsec & ((1ULL << tk->shift) - 1);
>         tk->xtime_nsec -= remainder;
> -       tk->xtime_nsec += 1 << tk->shift;
> +       tk->xtime_nsec += 1ULL << tk->shift;
>         tk->ntp_error += remainder << tk->ntp_error_shift;
>
>         /*

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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