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From: Tycho Andersen <tycho.andersen@canonical.com>
Subject: Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program
Date: Wed, 9 Sep 2015 18:13:15 -0600
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Cc: Kees Cook <keescook@chromium.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Will Drewry <wad@chromium.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Pavel Emelyanov <xemul@parallels.com>,
	"Serge E. Hallyn" <serge.hallyn@ubuntu.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	LKML <linux-kernel@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	Cyrill Gorcunov <gorcunov@parallels.com>
To: Andy Lutomirski <luto@amacapital.net>
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On Wed, Sep 09, 2015 at 04:44:24PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 9, 2015 at 3:34 PM, Tycho Andersen
> <tycho.andersen@canonical.com> wrote:
> >
> > Here's a thought,
> >
> > The set I'm currently proposing effectively separates the ref-counting
> > of the struct seccomp_filter from the struct bpf_prog (by necessity,
> > since we're referring to filters from fds). What if we went a little
> > futher, and made a copy of each seccomp_filter on fork(), keeping it
> > pointed at the same bpf_prog but adding some metadata about how it was
> > inherited (tsk->seccomp.filter->inheritence_count++ perhaps). This
> > would still require this change:
> 
> Won't that break the tsync mechanism?

We'll need the change I posted (is_ancestor comparing the underlying
bpf_prog instead of the seccomp_filter), but then I think it'll work.
I guess we'll need to do some more bookkeeping when we install filters
via TSYNC since each thread would need its own seccomp_filter, and
we'd also have to decide whether a filter installed via TSYNC was
inherited or not.

Am I missing something?

Tycho