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

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Date:   Wed, 25 Aug 2021 23:19:42 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Zhongya Yan <yan2228598786@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.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>
Subject: Re: [PATCH] net: tcp_drop adds `reason` parameter for tracing v2

On Wed, 25 Aug 2021 08:47:46 -0700
Eric Dumazet <edumazet@...gle.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
> 
> 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.
> 
> If you really want to include __FILE__ and __LINE__, these both can be
> stringified and included in the report, with the help of macros.

I agree the __LINE__ is pointless, but if this has a tracepoint
involved, then you can simply enable the stacktrace trigger to it and
it will save a stack trace in the ring buffer for you.

   echo stacktrace > /sys/kernel/tracing/events/tcp/tcp_drop/trigger

And when the event triggers it will record a stack trace. You can also
even add a filter to do it only for specific reasons.

   echo 'stacktrace if reason == 1' > /sys/kernel/tracing/events/tcp/tcp_drop/trigger

And it even works for flags:

   echo 'stacktrace if reason & 0xa' > /sys/kernel/tracing/events/tcp/tcp_drop/trigger

Which gives another reason to use an enum over a string.

-- Steve