linux-kernel - Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

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Date:   Thu, 12 Jul 2018 16:33:38 -0700
From:   Jann Horn <jannh@...gle.com>
To:     David Howells <dhowells@...hat.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]

On Thu, Jul 12, 2018 at 4:23 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Thu, Jul 12, 2018 at 3:54 PM David Howells <dhowells@...hat.com> wrote:
> >
> > Theodore Y. Ts'o <tytso@....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...

Er, sorry, of course that's the current behavior, not the behavior of
the suggested API.