netdev - Re: [PATCH net-next 1/2] net/sched: act

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Message-ID: <CAM0EoMmzt1tDpoqK=mMZoj1=6UU2Ytim2aqJWOBAZmPfNyZSfQ@mail.gmail.com>
Date: Thu, 27 Nov 2025 09:45:06 -0500
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net, pabeni@...hat.com, 
	jiri@...nulli.us, xiyou.wangcong@...il.com, netdev@...r.kernel.org, 
	dcaratti@...hat.com
Subject: Re: [PATCH net-next 1/2] net/sched: act_mirred: Fix infinite loop

On Wed, Nov 26, 2025 at 3:30 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Nov 26, 2025 at 12:20 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> >
> > On Wed, Nov 26, 2025 at 1:20 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Wed, Nov 26, 2025 at 10:14 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> > >
> > > > It's the multiport redirection, particularly to ingress. When it get
> > > > redirected to ingress it will get queued and then transitioned back.
> > > > xmit struct wont catch this as a recursion, so MIRRED_NEST_LIMIT will
> > > > not help you.
> > > > Example (see the first accompanying tdc test):
> > > > packet showing up on port0:ingress mirred redirect --> port1:egress
> > > > packet showing up on port1:egress mirred redirect --> port0:ingress
> > >
> > > Have you tried recording both devices ?
> > >
> > > diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
> > > index f27b583def78e4afecc7112854b93d59c2520201..711fc2e31cb0451c07a39f9c94226357d5faec09
> > > 100644
> > > --- a/net/sched/act_mirred.c
> > > +++ b/net/sched/act_mirred.c
> > > @@ -445,15 +445,17 @@ TC_INDIRECT_SCOPE int tcf_mirred_act(struct sk_buff *skb,
> > >                 return retval;
> > >         }
> > >         for (i = 0; i < xmit->sched_mirred_nest; i++) {
> > > -               if (xmit->sched_mirred_dev[i] != dev)
> > > +               if (xmit->sched_mirred_dev[i] != dev &&
> > > +                   xmit->sched_mirred_dev[i] != skb->dev)
> > >                         continue;
> > > -               pr_notice_once("tc mirred: loop on device %s\n",
> > > -                              netdev_name(dev));
> > > +               pr_notice_once("tc mirred: loop on device %s/%s\n",
> > > +                              netdev_name(dev), netdev_name(skb->dev));
> > >                 tcf_action_inc_overlimit_qstats(&m->common);
> > >                 return retval;
> > >         }
> > >
> > >         xmit->sched_mirred_dev[xmit->sched_mirred_nest++] = dev;
> > > +       xmit->sched_mirred_dev[xmit->sched_mirred_nest++] = skb->dev;
> > >
> > >         m_mac_header_xmit = READ_ONCE(m->tcfm_mac_header_xmit);
> > >         m_eaction = READ_ONCE(m->tcfm_eaction);
> >
> > Did you mean not to decrement sched_mirred_nest twice?
>
> No, sorry, we should decrement twice of course.
>

Ok, I tested.
While it "fixes" it - it's not really a fix. It works by ignoring direction.
Example, this is not a loop but currently would be claimed to be a
loop because port0 appears twice:
port0 ingress --> port1 ingress --> port1 egress
Note: port0 ingress and port0 egress cannot create a loop.

cheers,
jamal