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

[Jakub Kicinski]

On Wed, 25 Aug 2021 08:47:46 -0700 Eric Dumazet wrote:
> On Wed, Aug 25, 2021 at 8:41 AM Zhongya Yan <yan2228598786@xxxxxxxxx> 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?

> If you really want to include __FILE__ and __LINE__, these both can be
> stringified and included in the report, with the help of macros.