netdev - Question about tcp_filter() in tcp_v6_do

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Message-ID: <c49c55de-5356-0804-9baf-f0ed12829055@gmail.com>
Date:   Wed, 7 Jun 2017 15:35:49 -0700
From:   Chenbo Feng <chenbofeng.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     Lorenzo Colitti <lorenzo@...gle.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Chenbo Feng <fengc@...gle.com>
Subject: Question about tcp_filter() in tcp_v6_do_rcv()

Hello everybody,

I am testing eBPF programs on ipv6 and I just find out the tcp_filter() 
function (previously named sk_filter()) is called both in tcp_v6_rcv() 
and tcp_v6_do_rcv(). In contrast, it is only called by tcp_v4_rcv() in 
ipv4 layer. I guess this implementation is used to capture some corner 
cases in ipv6 ingress route but I cannot find why. Could I know why we 
need this in two similar places in ipv6 transportation layer?

I have tried to dig into the commit history and the related code path 
but I did not see any obvious reason for doing so. And my problem with 
it is when a eBPF program is attached to a socket or a cgroup in ingress 
side, the filter program will be applied on some packets twice. And it 
affect the accuracy when using eBPF program for traffic accounting.

Thanks!

Chenbo Feng