[CRIU] Time Namespaces: CLONE_NEWTIME and clone3()?

[Michael Kerrisk (man-pages)]

Hello Christian,

On Mon, 17 Feb 2020 at 16:15, Christian Brauner
<christian.brauner at ubuntu.com> wrote:
>
> 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
>
> s/CLONE_NEWSIGNAL/CLONE_NEWTIME/
>
> > 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
> think.)
> 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.

Actually, I think the alternative you propose just here is better. I
imagine there are times when one will want to create multiple
namespaces with a single call to clone3(), including a time namespace.
I think this should be allowed by the API. And, otherwise, clone3()
becomes something of a second-class citizen for creating namespaces.
(I don't really get the "less invasive" argument. Implementing this is
just a piece of kernel to code to make user-space's life a bit simpler
and more consistent.)

Thanks,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/