[PATCH AUTOSEL 5.0 039/173] ntp: Allow TAI-UTC offset to be set to zero

[Sasha Levin]

From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>

[ Upstream commit fdc6bae940ee9eb869e493990540098b8c0fd6ab ]

The ADJ_TAI adjtimex mode sets the TAI-UTC offset of the system clock.
It is typically set by NTP/PTP implementations and it is automatically
updated by the kernel on leap seconds. The initial value is zero (which
applications may interpret as unknown), but this value cannot be set by
adjtimex. This limitation seems to go back to the original "nanokernel"
implementation by David Mills.

Change the ADJ_TAI check to accept zero as a valid TAI-UTC offset in
order to allow setting it back to the initial value.

Fixes: 153b5d054ac2 ("ntp: support for TAI")
Suggested-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
Signed-off-by: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Richard Cochran <richardcochran@xxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190417084833.7401-1-mlichvar@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/time/ntp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 36a2bef001253..2f11dc592eb3a 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -689,7 +689,7 @@ static inline void process_adjtimex_modes(const struct timex *txc, s32 *time_tai
 		time_constant = max(time_constant, 0l);
 	}
 
-	if (txc->modes & ADJ_TAI && txc->constant > 0)
+	if (txc->modes & ADJ_TAI && txc->constant >= 0)
 		*time_tai = txc->constant;
 
 	if (txc->modes & ADJ_OFFSET)
-- 
2.20.1