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

From: Eric Dumazet
Date: Wed Aug 25 2021 - 12:21:17 EST


On Wed, Aug 25, 2021 at 9:04 AM Jakub Kicinski <kuba@xxxxxxxxxx> 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@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?

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@vger, 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.