ZFS snapdir readability (Crosspost) OT: mount -t zfs for snapshots

[Jan Behrens]

On Fri, 22 Nov 2019 09:09:52 +0100
Borja Marcos <borjam at sarenet.es> wrote:

> > On 21 Nov 2019, at 17:30, Martin Simmons <martin at lispworks.com> wrote:
> > 
> >> I’ve never seen that and, indeed, on FreeBSD 12.1 it’s not possible. Or I am terribly
> >> sloppy today!
> >> 
> >> Did I miss anything? Is it a new ZFS on Linux feature?
> >> 
> >> root at micro1:~ # mount -t zfs -o ro pool/dataset at snapshot /mnt
> >> mount: unpul/wwwnfsen at antesinst: Device busy
> >> root at micro1:~ # 
> > 
> > It looks like unpul/wwwnfsen at antesinst is already mounted -- probably in .zfs :-)
> 
> (This is a bit of off-topic now, the behavior of mount -t zfs)
> 
> Yep, it’s only uglier!
> 
> Whenever you access any of the directories below .zfs/snapshot the snapshot
> is automagically mounted. It’s also sort of invisible except when you run “mount -v”. In that
> case those snapshot mounts are visible. You can also unmount them.

I also just confirmed that once I manually mount a snapshot, I can't
access it through .zfs/snapshot anymore. Of course, this is doesn't
help with solving the original problem because in order to prohibit
snapshot access for non-privileged users, it would require all
snapshots to be mounted atomically at the same time the associated
filesystem is mounted.

> [...]
> 
> Borja.

Regards,
Jan