[tip:timers/urgent] ntp: Allow TAI-UTC offset to be set to zero

[tip-bot for Miroslav Lichvar]

Commit-ID:  fdc6bae940ee9eb869e493990540098b8c0fd6ab
Gitweb:     https://git.kernel.org/tip/fdc6bae940ee9eb869e493990540098b8c0fd6ab
Author:     Miroslav Lichvar <mlichvar@xxxxxxxxxx>
AuthorDate: Wed, 17 Apr 2019 10:48:33 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 9 May 2019 10:46:58 +0200

ntp: Allow TAI-UTC offset to be set to zero

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

---
 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 92a90014a925..f43d47c8c3b6 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -690,7 +690,7 @@ static inline void process_adjtimex_modes(const struct __kernel_timex *txc,
 		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)