netdev - Re: Question re. skb

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Message-ID: <CACAyw988GP=gwREKA=SxmfTq+eDP=W537Mz0wyAVLVb2YWjFmw@mail.gmail.com>
Date:   Thu, 18 Apr 2019 14:01:49 +0200
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Florian Westphal <fw@...len.de>
Cc:     herbert@...dor.apana.org.au, netdev@...r.kernel.org
Subject: Re: Question re. skb_orphan for TPROXY

Hello Florian,

Thank you, that makes sense. I guess that technically early demux also
relies on the skb_orphan call to function? I think that's what
confused me.

Best
Lorenz

On Tue, 16 Apr 2019 at 17:00, Florian Westphal <fw@...len.de> wrote:
>
> Lorenz Bauer <lmb@...udflare.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.

-- 
Lorenz Bauer  |  Systems Engineer
25 Lavington St., London SE1 0NZ

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