netdev - Re: [PATCH net-next 09/10] netfilter: get ipv6 pktlen properly in length

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Message-ID: <65c32f21-ab74-5863-4d65-b87543f8aa89@gmail.com>
Date:   Thu, 19 Jan 2023 08:41:04 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>, Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, davem@...emloft.net,
        kuba@...nel.org, Paolo Abeni <pabeni@...hat.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Pravin B Shelar <pshelar@....org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Ilya Maximets <i.maximets@....org>,
        Aaron Conole <aconole@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Paul Moore <paul@...l-moore.com>,
        Guillaume Nault <gnault@...hat.com>
Subject: Re: [PATCH net-next 09/10] netfilter: get ipv6 pktlen properly in
 length_mt6

On 1/18/23 8:13 PM, Eric Dumazet wrote:
> On Thu, Jan 19, 2023 at 2:19 AM Xin Long <lucien.xin@...il.com> wrote:
> 
>> I think that IPv6 BIG TCP has a similar problem, below is the tcpdump in
>> my env (RHEL-8), and it breaks too:
>>
>> 19:43:59.964272 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
>> 19:43:59.964282 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
>> 19:43:59.964292 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
>> 19:43:59.964300 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
>> 19:43:59.964308 IP6 2001:db8:1::1 > 2001:db8:2::1: [|HBH]
>>
> 
> Please make sure to use a not too old tcpdump.
> 
>> it doesn't show what we want from the TCP header either.
>>
>> For the latest tcpdump on upstream, it can display headers well for
>> IPv6 BIG TCP. But we can't expect all systems to use the tcpdump
>> that supports HBH parsing.
> 
> User error. If an admin wants to diagnose TCP potential issues, it should use
> a correct version.

Both of those just fall under "if you want a new feature, update your
tools."


> 
>>
>> For IPv4 BIG TCP, it's just a CFLAGS change to support it in "tcpdump,"
>> and 'tshark' even supports it by default.
> 
> Not with privacy _requirements_, where only the headers are captured.
> 
> I am keeping a NACK, until you make sure you do not break this
> important feature.

I think the request here is to keep the snaplen in place (e.g., to make
only headers visible to userspace) while also returning the >64kB packet
length as meta data.

My last pass on the packet socket code suggests this is possible;
someone (Xin) needs to work through the details.