net_sched: unset TCQ_F_CAN_BYPASS when adding filters

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 reproduced easily 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. Note, both fq_codel and sfq clear this flag in their
->bind_tcf() but this is clearly not sufficient when we don't use any
class ID.

Fixes: 23624935e0 ("net_sched: TCQ_F_CAN_BYPASS generalization")
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -2152,6 +2152,7 @@ replay:
 		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:

net/sched/sch_fq_codel.c

@@ -596,8 +596,6 @@ static unsigned long fq_codel_find(struct Qdisc *sch, u32 classid)
 static unsigned long fq_codel_bind(struct Qdisc *sch, unsigned long parent,
 			      u32 classid)
 {
-	/* we cannot bypass queue discipline anymore */
-	sch->flags &= ~TCQ_F_CAN_BYPASS;
 	return 0;
 }
 

net/sched/sch_sfq.c

@@ -824,8 +824,6 @@ static unsigned long sfq_find(struct Qdisc *sch, u32 classid)
 static unsigned long sfq_bind(struct Qdisc *sch, unsigned long parent,
 			      u32 classid)
 {
-	/* we cannot bypass queue discipline anymore */
-	sch->flags &= ~TCQ_F_CAN_BYPASS;
 	return 0;
 }