linux-kernel - Re: RFC: time_namespaces(7) manual page

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Message-ID: <87r1wzk2p5.fsf@nanos.tec.linutronix.de>
Date:   Tue, 07 Apr 2020 12:30:46 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andrei Vagin <avagin@...il.com>,
        "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Cc:     Andrei Vagin <avagin@...nvz.org>, Dmitry Safonov <dima@...sta.com>,
        linux-man <linux-man@...r.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux API <linux-api@...r.kernel.org>,
        Containers <containers@...ts.linux-foundation.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Adrian Reber <adrian@...as.de>
Subject: Re: RFC: time_namespaces(7) manual page

Andrei Vagin <avagin@...il.com> writes:
> On Sat, Apr 04, 2020 at 01:08:50PM +0200, Michael Kerrisk (man-pages) wrote:
>>    /proc/PID/timens_offsets
>>        Associated  with  each  time namespace are offsets, expressed with
>>        respect to the initial time namespace, that define the  values  of
>>        the  monotonic  and  boot clocks in that namespace.  These offsets
>>        are exposed via the file  /proc/PID/timens_offsets.   Within  this
>>        file,  the  offsets  are  expressed  as  lines consisting of three
>>        space-delimited fields:
>> 
>>            <clock-id> <offset-secs> <offset-nanosecs>
>> 
>>        The clock-id identifies the clock whose offsets are  being  shown.
>>        This field is either 1, for CLOCK_MONOTONIC, or 7, for CLOCK_BOOT‐
>>        TIME.  The remaining  fields  express  the  offset  (seconds  plus
>>        nanoseconds)  for the clock in this time namespace.  These offsets
>>        are expressed relative to the clock values  in  the  initial  time
>>        namespace.   In  the  initial time namespace, the contents of this
>>        file are as follows:
>
> I think we can mention that offset-secs can be negative, but
> offset-nanosleep has to be 0 or positive.

I assume you meant offset-nanosecs :)

That aside, there are also limitations in place.

  1) Negative offsets which would offset time into negative space are
     rejected, i.e. its enforced that

          now(CLOCK) + offset[CLOCK] >= 0

     This is necessary as the kernel expects and also enforces that time
     cannot be negative.

  2) Positive offsets which would offset time above KTTIME_SEC_MAX / 2 are
     rejected, i.e. it's enforced that

          now(CLOCK) + offset[CLOCK] <= KTIME_SEC_MAX / 2

     That is done to prevent that clocks wrap around if the offset would
     bring it close enough to the wrap around point.

     The cutoff value is a pretty arbitrary choice (~146 years). So to
     hit this you'd need a system which has an uptime of > 146 years,
     which is pretty much unrealistic.

Thanks,

        tglx