linux-kernel - Re: Detecting if you are running in a container

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Date:	Wed, 12 Oct 2011 13:57:02 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Theodore Tso <tytso@....EDU>, Matt Helsley <matthltc@...ibm.com>,
	Lennart Poettering <mzxreary@...inter.de>,
	Kay Sievers <kay.sievers@...y.org>,
	linux-kernel@...r.kernel.org, harald@...hat.com, david@...ar.dk,
	greg@...ah.com, Linux Containers <containers@...ts.osdl.org>,
	Linux Containers <lxc-devel@...ts.sourceforge.net>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: Detecting if you are running in a container

On Tue, Oct 11, 2011 at 02:16:24PM -0700, Eric W. Biederman wrote:
> It actually isn't much complexity and for the most part the code that
> I care about in that area is already merged.  In principle all I care
> about are having the identiy checks go from:
> (uid1 == uid2) to ((user_ns1 == user_ns2) && (uid1 == uid2))
> 
> There are some per subsystem sysctls that do make sense to make per
> subsystem and that work is mostly done.  I expect there are a few
> more in the networking stack that interesting to make per network
> namespace.
> 
> The only real world issue right now that I am aware of is the user
> namespace aren't quite ready for prime-time and so people run into
> issues where something like sysctl -a during bootup sets a bunch of
> sysctls and they change sysctls they didn't mean to.  Once the
> user namespaces are in place accessing a truly global sysctl will
> result in EPERM when you are in a container and everyone will be
> happy. ;)
> 
> 
> Where all of this winds up interesting in the field of oncoming kernel
> work is that uids are persistent and are stored in file systems.  So
> once we have all of the permission checks in the kernel tweaked to care
> about user namespaces we next look at the filesystems.   The easy
> initial implementation is going to be just associating a user namespace
> with a super block.  But farther out being able to store uids from
> different user namespaces on the same filesystem becomes an interesting
> problem.

Yipes.  Why would anyone want to do that?

--b.

> We already have things like user mapping in 9p and nfsv4 so it isn't
> wholly uncharted territory.  But it could get interesting.   Just
> a heads up.
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