Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

From: James Bottomley
Date: Fri Feb 17 2017 - 12:21:16 EST

On Fri, 2017-02-17 at 14:57 +1300, Eric W. Biederman wrote:
> I think I am missing something but I completely do not understand
> that subthread that says use file marks and perform the work in the
> vfs. The problem is that fundamentally we need multiple mappings and
> I don't see a mark on a file (even an inherited mark) providing the
> mapping so I don't see the point.

The point of the mark is that it's a statement by the system
administrator that the underlying subtree is safe to be mounted by an
unprivileged container in the containers user view (i.e. with
current_user_ns() == s_user_ns). For the unprivileged container
there's no real arbitrary s_user_ns use case because the unprivileged
container must prove it can set up the mapping, so it would likely
always be mounting from within a user_ns with the mapping it wanted.