netdev - [Patch net 1/2] net_sched: skbprio: Remove overly strict queue assertions

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Message-Id: <20250329222536.696204-2-xiyou.wangcong@gmail.com>
Date: Sat, 29 Mar 2025 15:25:35 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: netdev@...r.kernel.org
Cc: jhs@...atatu.com,
	jiri@...nulli.us,
	Cong Wang <xiyou.wangcong@...il.com>,
	syzbot+a3422a19b05ea96bee18@...kaller.appspotmail.com,
	Nishanth Devarajan <ndev2021@...il.com>
Subject: [Patch net 1/2] net_sched: skbprio: Remove overly strict queue assertions

In the current implementation, skbprio enqueue/dequeue contains an assertion
that fails under certain conditions when SKBPRIO is used as a child qdisc under
TBF with specific parameters. The failure occurs because TBF sometimes peeks at
packets in the child qdisc without actually dequeuing them when tokens are
unavailable.

This peek operation creates a discrepancy between the parent and child qdisc
queue length counters. When TBF later receives a high-priority packet,
SKBPRIO's queue length may show a different value than what's reflected in its
internal priority queue tracking, triggering the assertion.

The fix removes this overly strict assertions in SKBPRIO, they are not
necessary at all.

Reported-by: syzbot+a3422a19b05ea96bee18@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a3422a19b05ea96bee18
Fixes: aea5f654e6b7 ("net/sched: add skbprio scheduler")
Cc: Nishanth Devarajan <ndev2021@...il.com>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
---
 net/sched/sch_skbprio.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/net/sched/sch_skbprio.c b/net/sched/sch_skbprio.c
index 20ff7386b74b..f485f62ab721 100644
--- a/net/sched/sch_skbprio.c
+++ b/net/sched/sch_skbprio.c
@@ -123,8 +123,6 @@ static int skbprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	/* Check to update highest and lowest priorities. */
 	if (skb_queue_empty(lp_qdisc)) {
 		if (q->lowest_prio == q->highest_prio) {
-			/* The incoming packet is the only packet in queue. */
-			BUG_ON(sch->q.qlen != 1);
 			q->lowest_prio = prio;
 			q->highest_prio = prio;
 		} else {
@@ -156,7 +154,6 @@ static struct sk_buff *skbprio_dequeue(struct Qdisc *sch)
 	/* Update highest priority field. */
 	if (skb_queue_empty(hpq)) {
 		if (q->lowest_prio == q->highest_prio) {
-			BUG_ON(sch->q.qlen);
 			q->highest_prio = 0;
 			q->lowest_prio = SKBPRIO_MAX_PRIORITY - 1;
 		} else {
-- 
2.34.1