Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()

[Miklos Szeredi <miklos@szeredi.hu>]

On Fri, 21 May 2021 at 10:39, Greg Kurz <groug@kaod.org> wrote:
>
> On Fri, 21 May 2021 10:26:27 +0200
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > On Thu, 20 May 2021 at 17:47, Greg Kurz <groug@kaod.org> wrote:
> > >
> > > All submounts share the same virtio-fs device instance as the root
> > > mount. If the same virtiofs filesystem is mounted again, sget_fc()
> > > is likely to pick up any of these submounts and reuse it instead of
> > > the root mount.
> > >
> > > On the server side:
> > >
> > > # mkdir ${some_dir}
> > > # mkdir ${some_dir}/mnt1
> > > # mount -t tmpfs none ${some_dir}/mnt1
> > > # touch ${some_dir}/mnt1/THIS_IS_MNT1
> > > # mkdir ${some_dir}/mnt2
> > > # mount -t tmpfs none ${some_dir}/mnt2
> > > # touch ${some_dir}/mnt2/THIS_IS_MNT2
> > >
> > > On the client side:
> > >
> > > # mkdir /mnt/virtiofs1
> > > # mount -t virtiofs myfs /mnt/virtiofs1
> > > # ls /mnt/virtiofs1
> > > mnt1 mnt2
> > > # grep virtiofs /proc/mounts
> > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > >
> > > And now remount it again:
> > >
> > > # mount -t virtiofs myfs /mnt/virtiofs2
> > > # grep virtiofs /proc/mounts
> > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > > myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
> > > # ls /mnt/virtiofs2
> > > THIS_IS_MNT2
> > >
> > > Submount mnt2 was picked-up instead of the root mount.
> >
>
> > Why is this a problem?
> >
>
> It seems very weird to mount the same filesystem again
> and to end up in one of its submounts. We should have:
>
> # ls /mnt/virtiofs2
> mnt1 mnt2

Okay, sorry, I understand the problem.  The solution is wrong,
however: the position of the submount on that list is no indication
that it's the right one (it's possible that the root sb will go away
and only a sub-sb will remain).

Even just setting a flag in the root, indicating that it's the root
isn't fully going to solve the problem.

Here's issue in full:

case 1:  no connection for "myfs" exists
    - need to create fuse_conn, sb

case 2: connection for "myfs" exists but only sb for submount
    - only create sb for root, reuse fuse_conn

case 3: connection for "myfs" as well as root sb exists
   - reuse sb

I'll think about how to fix this properly, it's probably going to be
rather more involved...

Thanks,
Miklos