linux-kernel - Re: [PATCH] net: tcp_drop adds `reason` parameter for tracing v2

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Date:   Wed, 25 Aug 2021 09:20:37 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Zhongya Yan <yan2228598786@...il.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, hengqi.chen@...il.com,
        Yonghong Song <yhs@...com>, Neil Spring <ntspring@...com>
Subject: Re: [PATCH] net: tcp_drop adds `reason` parameter for tracing v2

On Wed, Aug 25, 2021 at 9:04 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 25 Aug 2021 08:47:46 -0700 Eric Dumazet wrote:
> > On Wed, Aug 25, 2021 at 8:41 AM Zhongya Yan <yan2228598786@...il.com> wrote:
> > > @@ -5703,15 +5700,15 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
> > >                         TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
> > >                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
> > >                 tcp_send_challenge_ack(sk, skb);
> > > -               goto discard;
> > > +               tcp_drop(sk, skb, TCP_DROP_MASK(__LINE__, TCP_VALIDATE_INCOMING));
> >
> > I'd rather use a string. So that we can more easily identify _why_ the
> > packet was drop, without looking at the source code
> > of the exact kernel version to locate line number 1057
>
> Yeah, the line number seems like a particularly bad idea. Hopefully
> strings won't be problematic, given we can expect most serious users
> to feed the tracepoints via BPF. enum would be more convenient there,
> I'd think.
>
> > You can be sure that we will get reports in the future from users of
> > heavily modified kernels.
> > Having to download a git tree, or apply semi-private patches is a no go.
>
> I'm slightly surprised by this angle. Are there downstream kernels with
> heavily modified TCP other than Google's?

Not sure why Google is mentioned here ?
Have you ever received a public report about TCP behavior in a Google kernel ?

Over the years, we received hundreds of TCP bug reports on
netdev@...r, where users claim to use  kernel version 4.19 (or other),
when in fact they use 4.19.xxx
It takes in general multiple emails exchange before we get a more
realistic version number.
Not to mention distro kernels, or even worse private kernels, which
are not exactly easy to track for us upstream developers.