Date: Tue, 20 Dec 2016 10:11:50 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: David Ahern <dsahern@gmail.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Andy Lutomirski <luto@kernel.org>, Daniel Mack <daniel@zonque.org>,
        =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= <mic@digikod.net>,
        Kees Cook <keescook@chromium.org>, Jann Horn <jann@thejh.net>,
        Tejun Heo <tj@kernel.org>, "David S. Miller" <davem@davemloft.net>,
        Thomas Graf <tgraf@suug.ch>, Michael Kerrisk <mtk.manpages@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>
Subject: Re: Potential issues (security and otherwise) with the current
 cgroup-bpf API
Message-ID: <20161220091150.GJ3124@twins.programming.kicks-ass.net>
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On Mon, Dec 19, 2016 at 05:56:24PM -0800, Andy Lutomirski wrote:
> >> Huh?  My example in the original email attaches a program in a
> >> sub-hierarchy.  Are you saying that 4.11 could make that example stop
> >> working?
> >
> > Are you suggesting sub-cgroups should not be allowed to override the filter of a parent cgroup?
> 
> Yes, exactly.  I think there are two sensible behaviors:
> 
> a) sub-cgroups cannot have a filter at all of the parent has a filter.
> (This is the "punt" approach -- it lets different semantics be
> assigned later without breaking userspace.)
> 
> b) sub-cgroups can have a filter if a parent does, too.  The semantics
> are that the sub-cgroup filter runs first and all side-effects occur.
> If that filter says "reject" then ancestor filters are skipped.  If
> that filter says "accept", then the ancestor filter is run and its
> side-effects happen as well.  (And so on, all the way up to the root.)

So from what I understand the proposed cgroup is not in fact
hierarchical at all.

@TJ, I thought you were enforcing all new cgroups to be properly
hierarchical, that would very much include this one.