[PATCH v3 net 1/1] net/sched: sch_dualpi2: drain both C-queue and L-queue in dualpi2_change()

[chia-yu . chang]

From: Chia-Yu Chang <chia-yu.chang@xxxxxxxxxxxxxxxxxxx>

Fix dualpi2_change() to correctly enforce updated limit and memlimit
values after a configuration change of the dualpi2 qdisc.

Before this patch, dualpi2_change() always attempted to dequeue packets
via the root qdisc (C-queue) when reducing backlog or memory usage, and
unconditionally assumed that a valid skb will be returned. When traffic
classification results in packets being queued in the L-queue while the
C-queue is empty, this leads to a NULL skb dereference during limit or
memlimit enforcement.

This is fixed by first dequeuing from the C-queue path if it is
non-empty. Once the C-queue is empty, packets are dequeued directly from
the L-queue. Return values from qdisc_dequeue_internal() are checked for
both queues. When dequeuing from the L-queue, the parent qdisc qlen and
backlog counters are updated explicitly to keep overall qdisc statistics
consistent.

Fixes: 320d031ad6e4 ("sched: Struct definition and parsing of dualpi2 qdisc")
Reported-by: "Kito Xu (veritas501)" <hxzene@xxxxxxxxx>
Closes: https://lore.kernel.org/netdev/20260413075740.2234828-1-hxzene@xxxxxxxxx/
Signed-off-by: Chia-Yu Chang <chia-yu.chang@xxxxxxxxxxxxxxxxxxx>
---
 net/sched/sch_dualpi2.c | 32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_dualpi2.c b/net/sched/sch_dualpi2.c
index fe6f5e889625..241e6a46bd00 100644
--- a/net/sched/sch_dualpi2.c
+++ b/net/sched/sch_dualpi2.c
@@ -868,11 +868,35 @@ static int dualpi2_change(struct Qdisc *sch, struct nlattr *opt,
 	old_backlog = sch->qstats.backlog;
 	while (qdisc_qlen(sch) > sch->limit ||
 	       q->memory_used > q->memory_limit) {
-		struct sk_buff *skb = qdisc_dequeue_internal(sch, true);
+		struct sk_buff *skb = NULL;
 
-		q->memory_used -= skb->truesize;
-		qdisc_qstats_backlog_dec(sch, skb);
-		rtnl_qdisc_drop(skb, sch);
+		if (qdisc_qlen(sch) > qdisc_qlen(q->l_queue)) {
+			skb = qdisc_dequeue_internal(sch, true);
+			if (unlikely(!skb)) {
+				WARN_ON_ONCE(1);
+				break;
+			}
+			q->memory_used -= skb->truesize;
+			rtnl_qdisc_drop(skb, sch);
+		} else if (qdisc_qlen(q->l_queue)) {
+			skb = qdisc_dequeue_internal(q->l_queue, true);
+			if (unlikely(!skb)) {
+				WARN_ON_ONCE(1);
+				break;
+			}
+			/* L-queue packets are counted in both sch and
+			 * l_queue on enqueue; qdisc_dequeue_internal()
+			 * handled l_queue, so we further account for sch.
+			 */
+			--sch->q.qlen;
+			qdisc_qstats_backlog_dec(sch, skb);
+			q->memory_used -= skb->truesize;
+			rtnl_qdisc_drop(skb, q->l_queue);
+			qdisc_qstats_drop(sch);
+		} else {
+			WARN_ON_ONCE(1);
+			break;
+		}
 	}
 	qdisc_tree_reduce_backlog(sch, old_qlen - qdisc_qlen(sch),
 				  old_backlog - sch->qstats.backlog);
-- 
2.34.1