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Date:   Mon, 17 Feb 2020 15:59:08 +0100
From:   Christian Brauner <>
To:     Michael Kerrisk <>
Cc:     Dmitry Safonov <>, Andrei Vagin <>,
        Linux Kernel <>,
        Dmitry Safonov <>,
        Adrian Reber <>,
        Andy Lutomirski <>,
        Arnd Bergmann <>,
        Cyrill Gorcunov <>,
        "Eric W. Biederman" <>,
        "H. Peter Anvin" <>, Ingo Molnar <>,
        Jann Horn <>, Jeff Dike <>,
        Oleg Nesterov <>,
        Pavel Emelyanov <>,
        Shuah Khan <>,
        Thomas Gleixner <>,
        Vincenzo Frascino <>,
        containers <>,, Linux API <>,, Andrei Vagin <>
Subject: Re: Time Namespaces: CLONE_NEWTIME and clone3()?

On Mon, Feb 17, 2020 at 03:20:55PM +0100, Michael Kerrisk wrote:
> Hello Dmitry, Andrei,
> Is the CLONE_NEWTIME flag intended to be usable with clone3()? The
> mail quoted below implies (in my reading) that this should be possible
> once clone3() is available, which it is by now. (See also [1].)
> If the answer is yes, CLONE_NEWTIME  should be usable with clone3(),
> then I have a bug report and a question.
> I successfully used CLONE_NEWTIME with unshare(). But if I try to use
> CLONE_NEWSIGNAL with clone3(), it errors out with EINVAL, because of


> the following check in clone3_args_valid():
>         /*
>          * - make the CLONE_DETACHED bit reuseable for clone3
>          * - make the CSIGNAL bits reuseable for clone3
>          */
>         if (kargs->flags & (CLONE_DETACHED | CSIGNAL))
>                 return false;
> The problem is that CLONE_NEWTIME matches one of the bits in the
> CSIGNAL mask. If the intention is to allow CLONE_NEWTIME with
> clone3(), then either the bit needs to be redefined, or the error
> checking in clone3_args_valid() needs to be reworked.

If this is intended to be useable with clone3() the check should be
adapted to allow for CLONE_NEWTIME. (I asked about this a while ago I
But below rather sounds like it should simply be an unshare() flag. The
code seems to set frozen_offsets to true right after copy_namespaces()
in timens_on_fork(new_ns, tsk) and so the offsets can't be changed
anymore unless I'm reading this wrong.
Alternatives seem to either make timens_offsets writable once after fork
and before exec, I guess - though that's probably not going to work
with the vdso judging from timens_on_fork().

The other alternative is that Andrei and Dmitry send me a patch to
enable CLONE_NEWTIME with clone3() by exposing struct timens_offsets (or
a version of it) in the uapi and extend struct clone_args to include a
pointer to a struct timens_offset that is _only_ set when CLONE_NEWTIME
is set.
Though the unshare() way sounds way less invasive simpler.


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