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From: ebiederm@xmission.com (Eric W. Biederman)
To: James Bottomley <James.Bottomley@HansenPartnership.com>
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Date: Mon, 20 Feb 2017 17:24:52 +1300
In-Reply-To: <1487351993.4351.17.camel@HansenPartnership.com> (James
        Bottomley's message of "Fri, 17 Feb 2017 09:19:53 -0800")
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Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount
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James Bottomley <James.Bottomley@HansenPartnership.com> writes:

> 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.

As a statement that it is ok for the unprivileged mapping code to
operate that seems reasonable.  I don't currently the need for such an
ok from the system adminstrator, but if you need it a flag that
propagates to children and child directories seems reasonable.

Eric