Re: [VFS-RFC] autofs4 and bind, rbind and move mount requests

[Jeff Moyer <jmoyer@redhat.com>]

==> Regarding [VFS-RFC] autofs4 and bind, rbind and move mount requests; raven@themaw.net adds:

raven> I've been investigating a bug report about bind mounting an autofs
raven> controlled mount point. It is indeed disastrous for autofs. It would
raven> be simple enough it to check and fail silently but that won't give
raven> sensible behavior.

raven> What should the semantics be for these type of mount requests
raven> against autofs?

raven> First, a move mount doesn't make sense as it places the autofs
raven> filesystem in a location that is not specified in the autofs map to
raven> which it belongs. It looks like the user space daemon would loose
raven> contact with the newly mounted filesystem and so it would become
raven> useless and probably not umountable, not to mention how the daemon
raven> would handle it. I believe that this shouldn't be allowed. What do
raven> people think? If we don't treat these as invalid then how should
raven> they behave?

I think it is reasonable to not allow this.

[snip]

raven> Bind mount requests are another question.

raven> In the case of a bind mount we can find ourselves with a dentry in
raven> the bound filesystem that is marked as mounted but can't be followed
raven> because the parent vfsmount is in the source filesystem.

raven> Should the automount functionality go along with the bind mount
raven> filesystem? At this stage there's no straight forward way for autofs
raven> to handle two independent mount trees from the same automount
raven> daemon. It's just not designed to do that.

raven> It's probably possible to make this behave as though the automounted
raven> filesystem is mirrored under the filesystem to which it is
raven> bound. But it's likely problematic. What do people think about this?

raven> I've not really thought enough about recursive bind mounts yet but
raven> on the face of it they look fairly ugly as well.

raven> I know this post is short on detail but hopefully that will come out
raven> if there are people interested in discussing it further.

raven> I look forward to some feedback and hope I can find a realistic
raven> approach to solving this problem.

Yeah, this is really a tricky matter.  On the surface, it really doesn't
make sense to me to do a mount -bind with a source directory of the
automount point.  Given the current semantics of bind mounts, what does
this gain you?  Likely it gains you access to an emtpy directory, or a
directory full of empty directories (in the case of ghosting).

Mount -rbind is equally nonsensical, if you agree with the last paragraph.
You will only get a copy of the vfsmount tree at the time of the mount
-rbind call.  Who knows what was mounted at that time, it really isn't
deterministic.

-Jeff