Re: [chrony-dev] "leapsectz" and leapsecond announce

[FUSTE Emmanuel <emmanuel.fuste@xxxxxxxxxxxxxxx>]

Le 12/10/2017 à 15:46, Miroslav Lichvar a écrit :
> On Thu, Oct 12, 2017 at 03:12:19PM +0200, FUSTE Emmanuel wrote:
>> Le 12/10/2017 à 13:21, Miroslav Lichvar a écrit :
>>> The TAI-UTC offset of the system clock will be wrong if the timezone
>>> is missing a (genuine) leap second. If it is a fake leap second, the
>>> clock will be twice (wrongly) corrected by one second.
>>>
>> So in the presence of a refclock with the "tai" option and after a leap
>> not present in the tz data but accepted because of a majority of other
>> sources /servers have announced it, the TAI-UTC offset will be wrong.
>> So what will be done with the refclock ?
>> - de-selected / ignored ?
> The refclock will be off by one seconds. If there are at least two
> good sources, it will be marked as a falseticker and ignored.
OK
>> - as it has a lower stratum, it could take precedence over the other ntp
>> sources and system time and served time will be re-offset-ed by 1s ?
>> Or (after re-reading your answer and what I write) the TAI-UTC offset
>> was run-time adjusted and is not wrong.
> The offset will be wrong. There is no adjustment of the TAI-UTC offset
> which is used for correcting refclock offset. The local TAI-UTC offset
> is adjusted after a leap second, but it will be overwritten on the
> next clock update with the incorrect value from tzdata.
OK
>> A complete solution could be to save the "not present in the leapsectz
>> zone" leap event in a journal and increment/decrement the TAI-UTC offset..
>> At startup, or at next runtime check
>> - clear the journal if the event was added in the leapsectz zone by an
>> update to the local TZ database and reset TAI-UTC offset with the one
>> given by leapsectz zone
>> - or replay the event on top of the offset given by leapsectz zone to
>> get the right TAI-UTC offset
> What if a fake leap second was accepted? This sounds too complicated
> and fragile to me.
I was confident that all necessary counter measures/dispositions are 
taken on Chrony to minimize false leap second.
Anyway, if accepted/played, damage is done TAI offset updated or not. 
(In this case, in my previous "plan" zap the journal created at "leap 
event play time" manually)
>
> If using leapsectz and refclock with tai option, I think the
> requirement should be to keep tzdata up to date.
>
Fair enough ! (a little note should be added too to the tai option).

Thank you.
Emmanuel.
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