netdev - Re: [Patch net 1/2] netem: Fix skb duplication logic to prevent infinite loops

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Message-ID: <CAM0EoMmBdZBzfUAms5-0hH5qF5ODvxWfgqrbHaGT6p3-uOD6vg@mail.gmail.com>
Date: Wed, 2 Jul 2025 11:04:22 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, will@...lsroot.io, stephen@...workplumber.org, 
	Savino Dicanosa <savy@...t3mfailure.io>
Subject: Re: [Patch net 1/2] netem: Fix skb duplication logic to prevent
 infinite loops

On Wed, Jul 2, 2025 at 10:12 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On Tue, Jul 1, 2025 at 9:57 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > On Tue, Jul 01, 2025 at 04:13:05PM -0700, Cong Wang wrote:
> > > diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> > > index fdd79d3ccd8c..33de9c3e4d1b 100644
> > > --- a/net/sched/sch_netem.c
> > > +++ b/net/sched/sch_netem.c
> > > @@ -460,7 +460,8 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
> > >       skb->prev = NULL;
> > >
> > >       /* Random duplication */
> > > -     if (q->duplicate && q->duplicate >= get_crandom(&q->dup_cor, &q->prng))
> > > +     if (tc_skb_cb(skb)->duplicate &&
> >
> > Oops, this is clearly should be !duplicate... It was lost during my
> > stupid copy-n-paste... Sorry for this mistake.
> >
>
> I understood you earlier, Cong. My view still stands:
> You are adding logic to a common data structure for a use case that
> really makes no sense. The ROI is not good.
> BTW: I am almost certain you will hit other issues when this goes out
> or when you actually start to test and then you will have to fix more
> spots.
>
Here's an example that breaks it:

sudo tc qdisc add dev lo root handle 1: prio bands 3 priomap 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
sudo tc filter add dev lo parent 1:0 protocol ip bpf obj
netem_bug_test.o sec classifier/pass classid 1:1
sudo tc qdisc add dev lo parent 1:1 handle 10: netem limit 4 duplicate 100%
sudo tc qdisc add dev lo parent 10: handle 30: netem gap 1 limit 4
duplicate 100% delay 1us reorder 100%

And the ping 127.0.0.1 -c 1
I had to fix your patch for correctness (attached)


the ebpf prog is trivial - make it just return the classid or even zero.

William, as a middle ground can you take a crack at using cb_ext -
take a look for example at struct tc_skb_ext_alloc in cls_api.c (that
one is safe to extend).


cheers
jamal

> cheers,
> jamal

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