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

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Date:   Thu, 19 Jan 2023 19:10:27 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     David Ahern <dsahern@...il.com>,
        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 Thu, Jan 19, 2023 at 5:51 PM Xin Long <lucien.xin@...il.com> wrote:
>
> On Thu, Jan 19, 2023 at 10:41 AM David Ahern <dsahern@...il.com> wrote:
> >
> > 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.
> >
> To be honest, I don't really like such a change in a packet socket,
> I tried, and the code doesn't look nice.
>
> I'm thinking since skb->len is trustable, why don't we use
> IP_MAX_MTU(0xFFFF) as iph->tot_len for IPv4 BIG TCP?
> namely, only change these 2 helpers to:
>
> static inline unsigned int iph_totlen(const struct sk_buff *skb, const
> struct iphdr *iph)
> {
>         u16 len = ntohs(iph->tot_len);
>
>         return (len < IP_MAX_MTU || !skb_is_gso_tcp(skb)) ? len :
>                 skb->len - skb_network_offset(skb);
> }
>
> static inline void iph_set_totlen(struct iphdr *iph, unsigned int len)
> {
>         iph->tot_len = len < IP_MAX_MTU ? htons(len) : htons(IP_MAX_MTU);
> }
>
> What do you think?

I think this is a no go for me.

I think I stated clearly what was the problem.
If you care about TCP diagnostics, you want the truth, not truncated
sequence ranges,
making it impossible to know if a packet was sent.

Without headers describing precisely payload length (solution taken in
IPv6 BI TCP),
you have to augment AF_PACKET to provide this information in
additional meta-data.