[PATCH 2/2] arm64: VDSO: fix coarse clock monotonicity regression

[Nathan Lynch]

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 at mentor.com>
---
 arch/arm64/kernel/vdso.c | 7 +++----
 1 file 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 @@ int arch_setup_additional_pages(struct linux_binprm *bprm,
  */
 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;
 
-- 
2.1.0