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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 08:30:24 +0100
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Andy Lutomirski <luto@amacapital.net> wrote:

> > 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).
> > 
> 
> Are you trying to preserve the magic bind semantics with the new API?

No, I'm pointing out that you can't emulate this by doing a bind mount from
userspace if you can't access the thing you're binding from.

Now, we could create a syscall that just picks up an extant superblock using a
device and attaches it to a mount for you, but that would have to be at least
partially parameterised - which would be very fs-dependent - so that it can
know whether or not you're allowed to create another mount to that sb.

What you're talking about is emulating sget() in userspace - when we have to
do it in the kernel anyway if we still offer mount(2).

David