linux-kernel - Re: Setting mount propagation type in new mount API

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Date:   Fri, 15 May 2020 15:18:59 +0200
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     David Howells <dhowells@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Petr Vorel <pvorel@...e.cz>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: Setting mount propagation type in new mount API

On Fri, May 15, 2020 at 3:04 PM Miklos Szeredi <mszeredi@...hat.com> wrote:
>
> On Fri, May 15, 2020 at 1:40 PM Michael Kerrisk (man-pages)
> <mtk.manpages@...il.com> wrote:
> >
> > Hello David, Miklos,
> >
> > I've been looking at the new mount API (fsopen(), fsconfig(),
> > fsmount(), move_mount(), etc.) and among the details that remain
> > mysterious to me is this: how does one set the propagation type
> > (private/shared/slave/unbindable) of a new mount and change the
> > propagation type of an existing mount?
>
> Existing mount can be chaged with mount(NULL, path, NULL, MS_$(propflag), NULL).
>
> To do that with a detached mount created by fsmount(2) the
> "/proc/self/fd/$fd" trick can be used.
>
> The plan was to introduce a mount_setattr(2) syscall, but that hasn't
> happened yet...  I'm not sure we should be adding propagation flags to
> fsmount(2), since that is a less generic mechanism than
> mount_setattr(2) or just plain mount(2) as shown above.

Also note that only setting MS_SHARED makes sense on a new mount
returned by fsmount(2) because

 - MS_PRIVATE is a no op, due to mount already being private

 - same for MS_SLAVE, since it's only different from MS_PRIVATE  on
mounts receiving propagation, which a new mount by definition isn't

 - MS_UNBINDABLE just prevents move_mount(2) from working so that's
not really useful, though at least it does something

A more interesting issue is whether we'd want to control the
propagation of the target when moving into a shared tree.  I.e. should
there be a MOVE_MOUNT_DONTPROPAGATE flag for move_mount(20 that
prevents the new mount from being propagated...

Thanks,
Miklos