[PATCH v5 3/4] vdso/vsyscall: Keep the CLOCK_AUX base scaled

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

The vDSO basetime of a clock is stored in the scaled nanoseconds of
tkr_mono, so that the reader can floor the base and the cycle delta
together in vdso_calc_ns().

vdso_time_update_aux() instead shifts the base down to nanoseconds, adds
the offset, and shifts it back up, which zeroes the fractional nanoseconds
of xtime_nsec.  The reader then floors the base and the delta separately:

  ktime_get_aux():  base + ((delta * mult + xtime_nsec) >> shift)
  vdso:             base + (xtime_nsec >> shift)
                         + ((delta * mult) >> shift)

Since floor(a) + floor(b) <= floor(a + b), the vDSO reports 0 or 1 ns
below the syscall for the same clock.  It is not a monotonicity problem:
across an update the step is floor(a + d) - floor(a) - floor(d), which is
0 or 1, never negative.

Add the offset in scaled nanoseconds as the other high resolution clocks
do, and normalise with __iter_div64_u64_rem() so that the stored base
stays below one second and the userspace fast-path does not iterate more
in __iter_div_u64_rem().

Only the sub-second field changes.  (a + (b << shift)) >> shift is exactly
(a >> shift) + b, so the seconds carried out of the normalisation are the
same as before; what the old form dropped was the low shift bits of the
remainder.

monotonic_to_aux.tv_nsec is a normalised timespec64 fraction, so it stays
below NSEC_PER_SEC even for a negative offset, and the sum stays below
2 * (NSEC_PER_SEC << shift).  The largest shift clocks_calc_mult_shift()
can pick is 32, which makes that 8.6e18 against a u64 limit of 1.8e19.

Fixes: 380b84e168e5 ("vdso/vsyscall: Update auxiliary clock data in the datapage")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 kernel/time/vsyscall.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/time/vsyscall.c b/kernel/time/vsyscall.c
index f43dd3f4744b..0e4b499328c0 100644
--- a/kernel/time/vsyscall.c
+++ b/kernel/time/vsyscall.c
@@ -155,11 +155,11 @@ void vdso_time_update_aux(struct timekeeper *tk)
 
 		vdso_ts->sec = tk->xtime_sec + tk->monotonic_to_aux.tv_sec;
 
-		nsec = tk->tkr_mono.xtime_nsec >> tk->tkr_mono.shift;
-		nsec += tk->monotonic_to_aux.tv_nsec;
-		vdso_ts->sec += __iter_div_u64_rem(nsec, NSEC_PER_SEC, &nsec);
-		nsec = nsec << tk->tkr_mono.shift;
-		vdso_ts->nsec = nsec;
+		nsec = tk->tkr_mono.xtime_nsec;
+		nsec += (u64)tk->monotonic_to_aux.tv_nsec << tk->tkr_mono.shift;
+		vdso_ts->sec += __iter_div64_u64_rem(nsec,
+						     (u64)NSEC_PER_SEC << tk->tkr_mono.shift,
+						     &vdso_ts->nsec);
 	}
 
 	__arch_update_vdso_clock(vc);
-- 
2.43.0