netdev - Re: [PATCH net-next v3 3/6] tcp: add dropreasons in tcp_rcv_state

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Date: Tue, 13 Feb 2024 14:36:52 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com, 
	kernelxing@...cent.com, kuba@...nel.org, netdev@...r.kernel.org, 
	pabeni@...hat.com
Subject: Re: [PATCH net-next v3 3/6] tcp: add dropreasons in tcp_rcv_state_process()

On Tue, Feb 13, 2024 at 12:07 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From: Jason Xing <kerneljasonxing@...il.com>
> Date: Tue, 13 Feb 2024 09:48:04 +0800
> > On Mon, Feb 12, 2024 at 11:33 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Mon, Feb 12, 2024 at 10:29 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> > > >
> > > > From: Jason Xing <kernelxing@...cent.com>
> > >
> > >
> > > >                         if (!acceptable)
> > > > -                               return 1;
> > > > +                               /* This reason isn't clear. We can refine it in the future */
> > > > +                               return SKB_DROP_REASON_TCP_CONNREQNOTACCEPTABLE;
> > >
> > > tcp_conn_request() might return 0 when a syncookie has been generated.
> > >
> > > Technically speaking, the incoming SYN was not dropped :)
> >
> > Hi Eric, Kuniyuki
> >
> > Sorry, I should have checked tcp_conn_request() carefully last night.
> > Today, I checked tcp_conn_request() over and over again.
> >
> > I didn't find there is any chance to return a negative/positive value,
> > only 0. It means @acceptable is always true and it should never return
> > TCP_CONNREQNOTACCEPTABLE for TCP ipv4/6 protocol and never trigger a
> > reset in this way.
>
> Ah right, I remember I digged the same thing before and even in the
> initial commit, conn_request() always returned 0 and tcp_rcv_state_process()
> tested it with if (retval < 0).

Good. Thanks for your double check :)

>
> I think we can clean up the leftover with some comments above
> ->conn_request() definition so that we can convert it to void
> when we deprecate DCCP in the near future.

In the next version, I will remove the new
SKB_DROP_REASON_TCP_CONNREQNOTACCEPTABLE and the comment line which I
added and keep it as the old way, namely, returning 1.

>
>
> >
> > For DCCP, there are chances to return -1 in dccp_v4_conn_request().
> > But I don't think we've already added drop reasons in DCCP before.
> >
> > If I understand correctly, there is no need to do any refinement or
> > even introduce TCP_CONNREQNOTACCEPTABLE new dropreason about the
> > .conn_request() for TCP.
> >
> > Should I add a NEW kfree_skb_reason() in tcp_conn_request() for those
> > labels, like drop_and_release, drop_and_free, drop, and not return a
> > drop reason to its caller tcp_rcv_state_process()?
>
> Most interested reasons will be covered by
>
>   - reqsk q : net_info_ratelimited() in tcp_syn_flood_action() or
>               net_dbg_ratelimited() in pr_drop_req() or
>               __NET_INC_STATS(net, LINUX_MIB_LISTENDROPS) in tcp_listendrop()
>   - accept q: NET_INC_STATS(net, LINUX_MIB_LISTENOVERFLOWS) or
>               __NET_INC_STATS(net, LINUX_MIB_LISTENDROPS) in tcp_listendrop()
>
> and could be refined by drop reason, but I'm not sure if drop reason
> is used under such a pressured situation.

Interesting. Let us wait for Eric's response.

Thanks,
Jason

>
> Also, these failures are now treated with consume_skb().
>
> Whichever is fine to me, no strong preference.


>
>
> >
> > Please correct me if I'm wrong...
> >
> > Thanks,
> > Jason
> >
> > >
> > > I think you need to have a patch to change tcp_conn_request() and its
> > > friends to return a 'refined' drop_reason
> > > to avoid future questions / patches.