On Thu, Apr 08, 2021 at 06:54:52PM +0300, Amir Goldstein wrote:

They are understood to me :) but I didn't want to get into it, because it is
complicated to explain and I wasn't sure if anyone cared...

I started working on open_by_handle_at() in userns for fanotify and fanotify
mostly reports directory fhandle, so no issues with cross-directory renames.
In any case, fanotify never reports "connectable" non-dir file handles.

Because my proposed change ALSO makes it possible to start talking about
userspace nfs server inside userns (in case anyone cares), I wanted to lay
out the path towards a userspace "subtree_check" like solution.

We have to support subdirectory exports and subtree checking because we
already have, but, FWIW, if I were writing a new NFS server from
scratch, I don't think I would.  It's poorly understood, and the effort
would be better spent on more flexible storage management.

--b.

Another thing I am contemplating is, if and when idmapped mount support
is added to overlayfs, we can store an additional "connectable" file handle
in the overlayfs index (whose key is the non-connectable fhandle) and
fix ovl_acceptable() similar to nfsd_acceptable() and then we will be able
to mount an overlayfs inside userns with nfs_export support.

I've included a two liner patch on the fhandle_userns branch to allow
overlayfs inside userns with nfs_export support in the case that
underlying filesystem was mounted inside userns, but that is not such
an interesting use case IMO.

Thanks,
Amir.