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Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <26064.1531440190@warthog.procyon.org.uk>
Date:   Thu, 12 Jul 2018 17:24:36 -0700
Cc:     "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>
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To:     David Howells <dhowells@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jul 12, 2018, at 5:03 PM, David Howells <dhowells@redhat.com> wrote:
>=20
> Andy Lutomirski <luto@amacapital.net> wrote:
>=20
>>>> I tend to think that this *should* fail using the new API.  The semanti=
cs
>>>> of the second mount request are bizarre at best.
>>>=20
>>> You still have to support existing behaviour lest you break userspace.
>>>=20
>>=20
>> I assume the existing behavior is that a bind mount is created?  If so, t=
he
>> new mount(8) tool could do it in user code.
>=20
> You have a race there.
>=20
> Also you can't currently directly create a bind mount from userspace as yo=
u
> can only bind from another path point - which you may not be able to acces=
s
> (either by permission failure or because it's not in your mount namespace)=
.
>=20

Are you trying to preserve the magic bind semantics with the new API?  If yo=
u are, I think it should be by explicit opt in only. Otherwise you risk havi=
ng your shiny new way to specify fs options get ignored when the magic bind m=
ount happens.=20=