linux-kernel - Re: [PATCH 1/2] ARM: VDSO: fix coarse clock monotonicity regression

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Date:	Mon, 10 Aug 2015 10:08:24 +0100
From:	Will Deacon <will.deacon@....com>
To:	Nathan Lynch <nathan_lynch@...tor.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] ARM: VDSO: fix coarse clock monotonicity regression

On Sat, Aug 08, 2015 at 03:03:22AM +0100, Nathan Lynch wrote:
> Since 906c55579a63 ("timekeeping: Copy the shadow-timekeeper over the
> real timekeeper last") it has become possible on ARM to:
> 
> - Obtain a CLOCK_MONOTONIC_COARSE or CLOCK_REALTIME_COARSE timestamp
>   via syscall.
> - Subsequently obtain a timestamp for the same clock ID via VDSO which
>   predates the first timestamp (by one jiffy).
> 
> This is because ARM's update_vsyscall is deriving the coarse time
> using the __current_kernel_time interface, when it should really be
> using the timekeeper object provided to it by the timekeeping core.
> It happened to work before only because __current_kernel_time would
> access the same timekeeper object which had been passed to
> update_vsyscall.  This is no longer the case.
> 
> Signed-off-by: Nathan Lynch <nathan_lynch@...tor.com>
> ---
>  arch/arm/kernel/vdso.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/kernel/vdso.c b/arch/arm/kernel/vdso.c
> index efe17dd9b921..c8b243c1aef8 100644
> --- a/arch/arm/kernel/vdso.c
> +++ b/arch/arm/kernel/vdso.c
> @@ -296,7 +296,6 @@ static bool tk_is_cntvct(const struct timekeeper *tk)
>   */
>  void update_vsyscall(struct timekeeper *tk)
>  {
> -	struct timespec xtime_coarse;
>  	struct timespec64 *wtm = &tk->wall_to_monotonic;
>  
>  	if (!cntvct_ok) {
> @@ -308,10 +307,10 @@ void update_vsyscall(struct timekeeper *tk)
>  
>  	vdso_write_begin(vdso_data);
>  
> -	xtime_coarse = __current_kernel_time();
>  	vdso_data->tk_is_cntvct			= tk_is_cntvct(tk);
> -	vdso_data->xtime_coarse_sec		= xtime_coarse.tv_sec;
> -	vdso_data->xtime_coarse_nsec		= xtime_coarse.tv_nsec;
> +	vdso_data->xtime_coarse_sec		= tk->xtime_sec;
> +	vdso_data->xtime_coarse_nsec		= tk->tkr_mono.xtime_nsec >>
> +							tk->tkr_mono.shift;

Do you need a cast to u32 here?

Will
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