arm64: VDSO: fix coarse clock monotonicity regression

Since 906c55579a63 ("timekeeping: Copy the shadow-timekeeper over the
real timekeeper last") it has become possible on arm64 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 arm64'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@mentor.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index ec37ab3f524f..97bc68f4c689 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -199,16 +199,15 @@ up_fail:
  */
 void update_vsyscall(struct timekeeper *tk)
 {
-        struct timespec xtime_coarse;
         u32 use_syscall = strcmp(tk->tkr_mono.clock->name, "arch_sys_counter");
 
         ++vdso_data->tb_seq_count;
         smp_wmb();
 
-        xtime_coarse = __current_kernel_time();
         vdso_data->use_syscall                  = use_syscall;
-        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;
         vdso_data->wtm_clock_sec                = tk->wall_to_monotonic.tv_sec;
         vdso_data->wtm_clock_nsec               = tk->wall_to_monotonic.tv_nsec;