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From:   David Howells <dhowells@redhat.com>
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To:     Andy Lutomirski <luto@amacapital.net>
Cc:     dhowells@redhat.com, "Theodore Y. Ts'o" <tytso@mit.edu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Lutomirski <luto@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]
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Date:   Fri, 13 Jul 2018 01:03:10 +0100
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Andy Lutomirski <luto@amacapital.net> wrote:

> >> I tend to think that this *should* fail using the new API.  The semantics
> >> of the second mount request are bizarre at best.
> > 
> > You still have to support existing behaviour lest you break userspace.
> > 
> 
> I assume the existing behavior is that a bind mount is created?  If so, the
> new mount(8) tool could do it in user code.

You have a race there.

Also you can't currently directly create a bind mount from userspace as you
can only bind from another path point - which you may not be able to access
(either by permission failure or because it's not in your mount namespace).

David