Subject: [PATCH v2] ARM: VDSO: fix coarse clock monotonicity regression

From: [email-masked] (Will Deacon)
Date: Mon, 10 Aug 2015 16:48:16 +0100

On Mon, Aug 10, 2015 at 04:46:32PM +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 <[email-masked]>
> ---
> 
> Changes since v1:
> - Add u32 cast to nsec calculation.

Acked-by: Will Deacon <[email-masked]>

Probably best sticking it into Russell's patch system with a Cc stable
in case it doesn't make it for 4.2.

Will

>  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..54a5aeab988d 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		= (u32)(tk->tkr_mono.xtime_nsec >>
> +							tk->tkr_mono.shift);
>  	vdso_data->wtm_clock_sec		= wtm->tv_sec;
>  	vdso_data->wtm_clock_nsec		= wtm->tv_nsec;
>  
> -- 
> 2.1.0
>