linux-kernel - Re: A further though on /proc/PID/timens

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Date:   Thu, 9 Apr 2020 22:23:49 -0700
From:   Andrey Vagin <avagin@...nvz.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Dmitry Safonov <dima@...sta.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>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Adrian Reber <adrian@...as.de>
Subject: Re: A further though on /proc/PID/timens_offsets

On Thu, Apr 9, 2020 at 2:35 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> Andrey Vagin <avagin@...nvz.org> writes:
>
> > On Tue, Apr 7, 2020 at 6:24 AM Michael Kerrisk (man-pages)
> > <mtk.manpages@...il.com> wrote:
> >
> >>        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.
> >>
> >> What was the reason for exposing numeric clock IDs in the
> >> timens_offsets file? In API terms, that seems a little ugly.
> >>
> >> I think it would have been much nicer if the clocks were defined
> >> symbolically in this file. I.e., that reading the file would have
> >> shown something like
> >>
> >> monotonic    x    y
> >> boottime     x    y
> >>
> >> And that records similarly with symbolic clock names could have
> >> been written to the file. Was there a reason not to do this?
> >
> > No, there was not except that I haven't thought about this. I agree
> > that symbolic clock names looks nicer for humans, but numeric clock
> > IDs are a bit more convenient when we  need to set/read offsets from
> > code. This interface is in the released kernel, so I think we can't
> > change the format of the content of this file. But we can add support
> > of symbolic clock names for setting clock offsets. What do you think?
>
> The rule is we can change things as long as userspace doesn't care.  For
> very new interfaces like this it is possible there are few enough
> userspace programs that nothing cares.
>
> Do you know if someone is using this interface yet?

CRIU has the support of time namespace, but these changes are in the
devel branch and have not been released yet.

I know two more project:
* The util-linux (unshare and nsenter tools):
https://github.com/karelzak/util-linux/commit/7f1f0584c24a77909a7c96e62e30f63f4c1b10ad
https://github.com/karelzak/util-linux/commit/ef0341c9be441b834848d260ba0dbeb47a20f7a3

The last release of util-linux was at the end of January, so these
changes have not been released.

* crun
https://github.com/containers/crun/commit/a669dc64f70f71423a0ee1bb977f2d77e473649a

These changes have been released in the crun v0.13.

All these projects only set offsets, so I think following the rule
that you described, we can start showing symbolic clock names and
accept both variants for setting offsets. If everyone agrees with
this, I can prepare a patch tomorrow.

Thanks,
Andrei