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 <nathan_ly...@mentor.com>
> ---
>
> Changes since v1:
> - Add u32 cast to nsec calculation.
Acked-by: Will Deacon <will.dea...@arm.com>
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
>
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