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From:   Jann Horn <jannh@google.com>
Date:   Thu, 12 Jul 2018 16:23:54 -0700
Message-ID: <CAG48ez1OF0OfdmnRsV5Ne2mOt-j3Pgf7QRsn3q3Hzjpus9LHGA@mail.gmail.com>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]
To:     David Howells <dhowells@redhat.com>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>
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On Thu, Jul 12, 2018 at 3:54 PM David Howells <dhowells@redhat.com> wrote:
>
> Theodore Y. Ts'o <tytso@mit.edu> wrote:
>
> > 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.
>
> Would that mean then that doing:
>
>         mount /dev/sda3 /a
>         mount /dev/sda3 /b
>
> would then fail on the second command because /dev/sda3 is already open
> exclusively?

Not exactly. mount_bdev() uses FMODE_EXCL, which locks out parallel
usage *with a different filesystem type*. This is the effect:

# strace -e trace=mount mount -t vfat /dev/loop0 mount
mount("/dev/loop0", "/home/jannh/tmp/x/mount", "vfat", MS_MGC_VAL, NULL) = 0
+++ exited with 0 +++
# strace -e trace=mount mount -t ext4 /dev/loop0 mount
mount("/dev/loop0", "/home/jannh/tmp/x/mount", "ext4", MS_MGC_VAL,
NULL) = -1 EBUSY (Device or resource busy)
mount: /home/jannh/tmp/x/mount: /dev/loop0 already mounted on
/home/jannh/tmp/x/mount.
+++ exited with 32 +++

I don't really understand why it's not more strict though...