netdev - Re: Question re. skb

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Date:   Thu, 2 May 2019 13:50:00 -0400
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Florian Westphal <fw@...len.de>, Lorenz Bauer <lmb@...udflare.com>
Cc:     herbert@...dor.apana.org.au, netdev@...r.kernel.org
Subject: Re: Question re. skb_orphan for TPROXY

On 4/16/19 8:00 AM, Florian Westphal 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.
> 

Florian, it is the responsibility of the loopback code to perform the skb_orphan()

I am confident we can revert 71f9dacd2e4d23 and fix the
paths that eventually miss the skb_orphan() call.

loopback_xmit() properly calls skb_orphan(), we also need to make sure that any kind 
of loopback (veth and others) do the same.

This is a prereq so that XDP or tc code can implement early demux earlier.

As a bonus we remove one skb_orphan() in rx fast path ;)

Note that skb_scrub_packet() used to call skb_orphan(), we need to a bit smarter
and insert it only in __dev_forward_skb()