Re: [PATCH] net/sched: Forbid assigning mirred action to a filter attached to the egress

[Jamal Hadi Salim <jhs@mojatatu.com>]

On Sun, Mar 24, 2024 at 11:27 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>
> On Wed, Mar 20, 2024 at 3:34 PM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
> >
> > On Wed, Mar 20, 2024 at 2:26 PM Eric Dumazet <edumazet@google.com> wrote:
> > >
> > > On Wed, Mar 20, 2024 at 7:13 PM Eric Dumazet <edumazet@google.com> wrote:
> > > >
> > > > On Wed, Mar 20, 2024 at 6:50 PM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
> > >
> > > > Nope, you just have to complete the patch, moving around
> > > > dev_xmit_recursion_inc() and dev_xmit_recursion_dec()
> > >
> > > Untested part would be:
> > >
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 303a6ff46e4e16296e94ed6b726621abe093e567..dbeaf67282e8b6ec164d00d796c9fd8e4fd7c332
> > > 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -4259,6 +4259,8 @@ int __dev_queue_xmit(struct sk_buff *skb, struct
> > > net_device *sb_dev)
> > >          */
> > >         rcu_read_lock_bh();
> > >
> > > +       dev_xmit_recursion_inc();
> > > +
> > >         skb_update_prio(skb);
> > >
> > >         qdisc_pkt_len_init(skb);
> > > @@ -4331,9 +4333,7 @@ int __dev_queue_xmit(struct sk_buff *skb, struct
> > > net_device *sb_dev)
> > >                         HARD_TX_LOCK(dev, txq, cpu);
> > >
> > >                         if (!netif_xmit_stopped(txq)) {
> > > -                               dev_xmit_recursion_inc();
> > >                                 skb = dev_hard_start_xmit(skb, dev, txq, &rc);
> > > -                               dev_xmit_recursion_dec();
> > >                                 if (dev_xmit_complete(rc)) {
> > >                                         HARD_TX_UNLOCK(dev, txq);
> > >                                         goto out;
> > > @@ -4353,12 +4353,14 @@ int __dev_queue_xmit(struct sk_buff *skb,
> > > struct net_device *sb_dev)
> > >         }
> > >
> > >         rc = -ENETDOWN;
> > > +       dev_xmit_recursion_dec();
> > >         rcu_read_unlock_bh();
> > >
> > >         dev_core_stats_tx_dropped_inc(dev);
> > >         kfree_skb_list(skb);
> > >         return rc;
> > >  out:
> > > +       dev_xmit_recursion_dec();
> > >         rcu_read_unlock_bh();
> > >         return rc;
> > >  }
> >
> > This removed the deadlock but now every packet being redirected will
> > be dropped. I was wrong earlier on the tc block because that only
> > works on clsact and ingress which are fine not needing this lock.
> > Here's a variation of the earlier patch that may work but comes at the
> > cost of a new per-cpu increment on the qdisc.
> >
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -3789,6 +3789,11 @@ static inline int __dev_xmit_skb(struct sk_buff
> > *skb, struct Qdisc *q,
> >         if (unlikely(contended))
> >                 spin_lock(&q->busylock);
> >
> > +       if (__this_cpu_read(q->recursion_xmit) > 0) {
> > +               //drop
> > +       }
> > +
> > +       __this_cpu_inc(q->recursion_xmit);
> >         spin_lock(root_lock);
> >         if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
> >                 __qdisc_drop(skb, &to_free);
> > @@ -3825,6 +3830,7 @@ static inline int __dev_xmit_skb(struct sk_buff
> > *skb, struct Qdisc *q,
> >                 }
> >         }
> >         spin_unlock(root_lock);
> > +       __this_cpu_dec(q->recursion_xmit);
> >         if (unlikely(to_free))
> >                 kfree_skb_list_reason(to_free,
> >                                       tcf_get_drop_reason(to_free));
> >
>
> And here's a tested version(attached) that fixes both the A->A and A->B->A.

Sent a proper patch as RFC, Eric please take a look.

cheers,
jamal

> cheers,
> jamal