[PATCH v1 net 1/1] net/sched: sch_dualpi2: fix limit/memlimit enforcement when dequeueing L-queue

[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")
Signed-off-by: Chia-Yu Chang <chia-yu.chang@xxxxxxxxxxxxxxxxxxx>
---
 net/sched/sch_dualpi2.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/net/sched/sch_dualpi2.c b/net/sched/sch_dualpi2.c
index 6d7e6389758d..56d4422970b6 100644
--- a/net/sched/sch_dualpi2.c
+++ b/net/sched/sch_dualpi2.c
@@ -872,11 +872,25 @@ 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);
-
-		q->memory_used -= skb->truesize;
-		qdisc_qstats_backlog_dec(sch, skb);
-		rtnl_qdisc_drop(skb, sch);
+		int c_len = qdisc_qlen(sch) - qdisc_qlen(q->l_queue);
+		struct sk_buff *skb = NULL;
+
+		if (c_len) {
+			skb = qdisc_dequeue_internal(sch, true);
+			if (!skb)
+				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 (!skb)
+				break;
+			q->memory_used -= skb->truesize;
+			rtnl_qdisc_drop(skb, q->l_queue);
+			/* Keep the overall qdisc stats consistent */
+			--sch->q.qlen;
+			qdisc_qstats_backlog_dec(sch, skb);
+		}
 	}
 	qdisc_tree_reduce_backlog(sch, old_qlen - qdisc_qlen(sch),
 				  old_backlog - sch->qstats.backlog);
-- 
2.34.1