netdev - Re: [PATCH net v2 1/2] net: Fix return value of qdisc ingress handling on success

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Date:   Sat, 1 Oct 2022 13:19:50 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Paul Blakey <paulb@...dia.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Vlad Buslov <vladbu@...dia.com>, Oz Shlomo <ozsh@...dia.com>,
        Roi Dayan <roid@...dia.com>, netdev@...r.kernel.org,
        Saeed Mahameed <saeedm@...dia.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net v2 1/2] net: Fix return value of qdisc ingress
 handling on success

On Wed, Sep 28, 2022 at 10:55:49AM +0300, Paul Blakey wrote:
> 
> 
> On 25/09/2022 21:00, Cong Wang wrote:
> > On Sun, Sep 25, 2022 at 11:14:21AM +0300, Paul Blakey wrote:
> > > Currently qdisc ingress handling (sch_handle_ingress()) doesn't
> > > set a return value and it is left to the old return value of
> > > the caller (__netif_receive_skb_core()) which is RX drop, so if
> > > the packet is consumed, caller will stop and return this value
> > > as if the packet was dropped.
> > > 
> > > This causes a problem in the kernel tcp stack when having a
> > > egress tc rule forwarding to a ingress tc rule.
> > > The tcp stack sending packets on the device having the egress rule
> > > will see the packets as not successfully transmitted (although they
> > > actually were), will not advance it's internal state of sent data,
> > > and packets returning on such tcp stream will be dropped by the tcp
> > > stack with reason ack-of-unsent-data. See reproduction in [0] below.
> > > 
> > 
> > Hm, but how is this return value propagated to egress? I checked
> > tcf_mirred_act() code, but don't see how it is even used there.
> > 
> > 318         err = tcf_mirred_forward(want_ingress, skb2);
> > 319         if (err) {
> > 320 out:
> > 321                 tcf_action_inc_overlimit_qstats(&m->common);
> > 322                 if (tcf_mirred_is_act_redirect(m_eaction))
> > 323                         retval = TC_ACT_SHOT;
> > 324         }
> > 325         __this_cpu_dec(mirred_rec_level);
> > 326
> > 327         return retval;
> > 
> > 
> > What am I missing?
> 
> for the ingress acting act_mirred it will return TC_ACT_CONSUMED above
> the code you mentioned (since redirect=1, use_reinsert=1. Although
> TC_ACT_STOLEN which is the retval set for this action, will also act the
> same)
> 
> 
> It is propagated as such (TX stack starting from tcp):
> 

Sorry for my misunderstanding.

I meant to say those TC_ACT_* return value, not NET_RX_*, but I worried
too much here, as mirred lets user specify the return value.

BTW, it seems you at least miss the drop case, which is NET_RX_DROP for
TC_ACT_SHOT at least? Possibly other code paths in sch_handle_ingress()
too.

Thanks.