net/sched: tbf: correct backlog statistic for GSO packets

[ Upstream commit 1596a135e3180c92e42dd1fbcad321f4fb3e3b17 ]

When the length of a GSO packet in the tbf qdisc is larger than the burst
size configured the packet will be segmented by the tbf_segment function.
Whenever this function is used to enqueue SKBs, the backlog statistic of
the tbf is not increased correctly. This can lead to underflows of the
'backlog' byte-statistic value when these packets are dequeued from tbf.

Reproduce the bug:
Ensure that the sender machine has GSO enabled. Configured the tbf on
the outgoing interface of the machine as follows (burstsize = 1 MTU):
$ tc qdisc add dev <oif> root handle 1: tbf rate 50Mbit burst 1514 latency 50ms

Send bulk TCP traffic out via this interface, e.g., by running an iPerf3
client on this machine. Check the qdisc statistics:
$ tc -s qdisc show dev <oif>

The 'backlog' byte-statistic has incorrect values while traffic is
transferred, e.g., high values due to u32 underflows. When the transfer
is stopped, the value is != 0, which should never happen.

This patch fixes this bug by updating the statistics correctly, even if
single SKBs of a GSO SKB cannot be enqueued.

Fixes: e43ac79a ("sch_tbf: segment too big GSO packets")
Signed-off-by: Martin Ottens <martin.ottens@fau.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241125174608.1484356-1-martin.ottens@fau.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/sched/sch_tbf.c

@@ -207,7 +207,7 @@ static int tbf_segment(struct sk_buff *skb, struct Qdisc *sch,
 	struct tbf_sched_data *q = qdisc_priv(sch);
 	struct sk_buff *segs, *nskb;
 	netdev_features_t features = netif_skb_features(skb);
-	unsigned int len = 0, prev_len = qdisc_pkt_len(skb);
+	unsigned int len = 0, prev_len = qdisc_pkt_len(skb), seg_len;
 	int ret, nb;
 
 	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
@@ -218,21 +218,27 @@ static int tbf_segment(struct sk_buff *skb, struct Qdisc *sch,
 	nb = 0;
 	skb_list_walk_safe(segs, segs, nskb) {
 		skb_mark_not_on_list(segs);
-		qdisc_skb_cb(segs)->pkt_len = segs->len;
-		len += segs->len;
+		seg_len = segs->len;
+		qdisc_skb_cb(segs)->pkt_len = seg_len;
 		ret = qdisc_enqueue(segs, q->qdisc, to_free);
 		if (ret != NET_XMIT_SUCCESS) {
 			if (net_xmit_drop_count(ret))
 				qdisc_qstats_drop(sch);
 		} else {
 			nb++;
+			len += seg_len;
 		}
 	}
 	sch->q.qlen += nb;
-	if (nb > 1)
+	sch->qstats.backlog += len;
+	if (nb > 0) {
 		qdisc_tree_reduce_backlog(sch, 1 - nb, prev_len - len);
-	consume_skb(skb);
-	return nb > 0 ? NET_XMIT_SUCCESS : NET_XMIT_DROP;
+		consume_skb(skb);
+		return NET_XMIT_SUCCESS;
+	}
+
+	kfree_skb(skb);
+	return NET_XMIT_DROP;
 }
 
 static int tbf_enqueue(struct sk_buff *skb, struct Qdisc *sch,