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Date:   Tue, 16 Apr 2019 17:00:02 +0200
From:   Florian Westphal <fw@strlen.de>
To:     Lorenz Bauer <lmb@cloudflare.com>
Cc:     herbert@gondor.apana.org.au, netdev@vger.kernel.org
Subject: Re: Question re. skb_orphan for TPROXY
Message-ID: <20190416150002.cbkih4lfrna4ywdu@breakpoint.cc>
References: <CACAyw9-pYyvkUBOzdD+XQBEKdGGB9foJ5ph5sdjiuE4_uyEoJg@mail.gmail.com>
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Lorenz Bauer <lmb@cloudflare.com> wrote:
> Apologies for contacting you out of the blue. I'm currently trying to
> understand how TPROXY works under the hood. As part of this endeavour,
> I've stumbled upon the commit attached to this email.
> 
> From the commit message I infer that somewhere, TPROXY relies on a
> check of skb->sk == NULL to function. However, I can't figure out
> where! I've traced TPROXY from NF_HOOK(NF_INET_PRE_ROUTING) just after
> the call to skb_orphan to __inet_lookup_skb / skb_steal_sock called
> from the TCP and UDP receive functions, and as far as I can tell there
> is no such check. Can you maybe shed some light on this?

Without the skb_orphan udp/tcp might steal tunnel/ppp etc. socket
instead of tproxy assigned tcp/udp socket.