netdev - [PATCH net] sch_cake: Return __NET_XMIT_STOLEN when consuming enqueued skb

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Message-Id: <20220831092103.442868-1-toke@toke.dk>
Date:   Wed, 31 Aug 2022 11:21:03 +0200
From:   Toke Høiland-Jørgensen <toke@...e.dk>
To:     Toke Høiland-Jørgensen <toke@...e.dk>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, cake@...ts.bufferbloat.net,
        netdev@...r.kernel.org
Subject: [PATCH net] sch_cake: Return __NET_XMIT_STOLEN when consuming enqueued skb

When the GSO splitting feature of sch_cake is enabled, GSO superpackets
will be broken up and the resulting segments enqueued in place of the
original skb. In this case, CAKE calls consume_skb() on the original skb,
but still returns NET_XMIT_SUCCESS. This can confuse parent qdiscs into
assuming the original skb still exists, when it really has been freed. Fix
this by adding the __NET_XMIT_STOLEN flag to the return value in this case.

Fixes: 0c850344d388 ("sch_cake: Conditionally split GSO segments")
Signed-off-by: Toke Høiland-Jørgensen <toke@...e.dk>
---
 net/sched/sch_cake.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index a43a58a73d09..a04928082e4a 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1713,6 +1713,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	}
 	idx--;
 	flow = &b->flows[idx];
+	ret = NET_XMIT_SUCCESS;
 
 	/* ensure shaper state isn't stale */
 	if (!b->tin_backlog) {
@@ -1771,6 +1772,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 
 		qdisc_tree_reduce_backlog(sch, 1-numsegs, len-slen);
 		consume_skb(skb);
+		ret |= __NET_XMIT_STOLEN;
 	} else {
 		/* not splitting */
 		cobalt_set_enqueue_time(skb, now);
@@ -1904,7 +1906,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		}
 		b->drop_overlimit += dropped;
 	}
-	return NET_XMIT_SUCCESS;
+	return ret;
 }
 
 static struct sk_buff *cake_dequeue_one(struct Qdisc *sch)
-- 
2.37.2