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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: <22105.1531436081@warthog.procyon.org.uk>
Date:   Thu, 12 Jul 2018 16:21:35 -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>
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> On Jul 12, 2018, at 3:54 PM, David Howells <dhowells@redhat.com> wrote:
>=20
> Theodore Y. Ts'o <tytso@mit.edu> wrote:
>=20
>> So maybe the answer is that you open /dev/sda1 and /dev/sda2 and then
>> pass the file descriptors to the fsopen object?  We can require that
>> the fd's be opened with O_RDWR and O_EXCL, which has the benefit where
>> if you have multiple block devices, you know *which* block device had
>> a problem with being grabbed for an exclusive open.
>=20
> Would that mean then that doing:
>=20
>    mount /dev/sda3 /a
>    mount /dev/sda3 /b
>=20
> would then fail on the second command because /dev/sda3 is already open
> exclusively?
>=20

I tend to think that this *should* fail using the new API.  The semantics of=
 the second mount request are bizarre at best.=