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

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Message-ID: <CAM0EoM=1DyZsgYnuTjXB88L=41g00pjat+Jq4jThpciXzcEKJQ@mail.gmail.com>
Date: Wed, 20 Mar 2024 15:34:32 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: renmingshuai <renmingshuai@...wei.com>, xiyou.wangcong@...il.com, jiri@...nulli.us, 
	davem@...emloft.net, vladbu@...dia.com, netdev@...r.kernel.org, 
	yanan@...wei.com, liaichun@...wei.com, caowangbao@...wei.com, 
	Eric Dumazet <eric.dumazet@...il.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Victor Nogueira <victor@...atatu.com>, 
	Pedro Tammela <pctammela@...atatu.com>, Davide Caratti <dcaratti@...hat.com>
Subject: Re: [PATCH] net/sched: Forbid assigning mirred action to a filter
 attached to the egress

On Wed, Mar 20, 2024 at 2:26 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Mar 20, 2024 at 7:13 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Wed, Mar 20, 2024 at 6:50 PM Jamal Hadi Salim <jhs@...atatu.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));


cheers,
jamal