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Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>, Amir Goldstein <amir73il@gmail.com>,
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Date: Fri, 17 Feb 2017 09:19:53 -0800
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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.

James