From: Chia-Yu Chang Date: Fri, 17 Apr 2026 15:25:51 +0000 (+0200) Subject: net/sched: sch_dualpi2: drain both C-queue and L-queue in dualpi2_change() X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=478ed6b7d2577439c610f91fa8759a4c878a4264;p=thirdparty%2Fkernel%2Fstable.git net/sched: sch_dualpi2: drain both C-queue and L-queue in dualpi2_change() 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)" Closes: https://lore.kernel.org/netdev/20260413075740.2234828-1-hxzene@gmail.com/ Signed-off-by: Chia-Yu Chang Link: https://patch.msgid.link/20260417152551.71648-1-chia-yu.chang@nokia-bell-labs.com Signed-off-by: Paolo Abeni --- 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);