netdev - how is the bpfilter sockopt processing supposed to work

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Date:   Fri, 17 Jul 2020 07:52:45 +0200
From:   Christoph Hellwig <hch@....de>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: how is the bpfilter sockopt processing supposed to work

Hi Alexei,

I've just been auditing the sockopt code, and bpfilter looks really
odd.  Both getsockopts and setsockopt eventually end up
in__bpfilter_process_sockopt, which then passes record to the
userspace helper containing the address of the optval buffer.
Which depending on bpf-cgroup might be in user or kernel space.
But even if it is in userspace it would be in a different process
than the bpfiler helper.  What makes all this work?