netdev - Re: [Patch net] net_sched: unset TCQ_F_CAN

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Date:   Sat, 13 Jul 2019 14:54:48 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] net_sched: unset TCQ_F_CAN_BYPASS when adding filters



On 7/12/19 10:17 PM, Cong Wang wrote:
> For qdisc's that support TC filters and set TCQ_F_CAN_BYPASS,
> notably fq_codel, it makes no sense to let packets bypass the TC
> filters we setup in any scenario, otherwise our packets steering
> policy could not be enforced.
> 
> This can be easily reproduced with the following script:
> 
>  ip li add dev dummy0 type dummy
>  ifconfig dummy0 up
>  tc qd add dev dummy0 root fq_codel
>  tc filter add dev dummy0 parent 8001: protocol arp basic action mirred egress redirect dev lo
>  tc filter add dev dummy0 parent 8001: protocol ip basic action mirred egress redirect dev lo
>  ping -I dummy0 192.168.112.1
> 
> Without this patch, packets are sent directly to dummy0 without
> hitting any of the filters. With this patch, packets are redirected
> to loopback as expected.
> 
> This fix is not perfect, it only unsets the flag but does not set it back
> because we have to save the information somewhere in the qdisc if we
> really want that.
> 
> Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM")
> Cc: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/sched/cls_api.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> index 638c1bc1ea1b..5c800b0c810b 100644
> --- a/net/sched/cls_api.c
> +++ b/net/sched/cls_api.c
> @@ -2152,6 +2152,7 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
>  		tfilter_notify(net, skb, n, tp, block, q, parent, fh,
>  			       RTM_NEWTFILTER, false, rtnl_held);
>  		tfilter_put(tp, fh);
> +		q->flags &= ~TCQ_F_CAN_BYPASS;
>  	}
>  
>  errout:
> 

Strange, because sfq and fq_codel are roughly the same for TCQ_F_CAN_BYPASS handling.

Why is fq_codel_bind() not effective ?

If not effective, sfq had the same issue, so the Fixes: tag needs to be refined,
maybe to commit 23624935e0c4 net_sched: TCQ_F_CAN_BYPASS generalization