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

[Cong Wang <xiyou.wangcong@gmail.com>]

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?

Also, the offending commit is very old and this configuration is not
uncommon at all, how could we even not notice this for such a long time?

Thanks.