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

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

On Wed, Jul 02, 2025 at 11:04:22AM -0400, Jamal Hadi Salim wrote:
> On Wed, Jul 2, 2025 at 10:12 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
> >
> > On Tue, Jul 1, 2025 at 9:57 PM Cong Wang <xiyou.wangcong@gmail.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

You are exaggerating this. I only added 1 bit to the core data structure,
the code logic remains in the netem, so it is contained within netem.

> > really makes no sense. The ROI is not good.

Speaking of ROI, I think you need to look at the patch stats:

William/Your patch:
 1 file changed, 40 insertions(+)

My patch:
 2 files changed, 4 insertions(+), 4 deletions(-)


> > 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.

Interesting, are you sure this works before my patch?

I don't intend to change any logic except closing the infinite loop. IOW,
if it didn't work before, I don't expect to make it work with this patch,
this patch merely fixes the infinite loop, which is sufficient as a bug fix.
Otherwise it would become a feature improvement. (Don't get me wrong, I
think this feature should be improved rather than simply forbidden, it just
belongs to a different patch.)

Thanks.