[PATCH] net/sched: fq_pie: enforce memory_limit without wrapped accounting

[Samuel Moelius]

fq_pie checks memory usage by adding the next packet size to the current
usage before comparing against memory_limit.  That addition can wrap and
make an over-limit enqueue look acceptable.

The scheduler can also fail to enforce memory_limit consistently across
enqueue paths.

Use overflow-safe accounting and reject packets that would exceed
memory_limit.

Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@xxxxxxxxxxxxxxx>
---
 net/sched/sch_fq_pie.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_fq_pie.c b/net/sched/sch_fq_pie.c
index 7becbf5362b3..752c1c32913a 100644
--- a/net/sched/sch_fq_pie.c
+++ b/net/sched/sch_fq_pie.c
@@ -152,7 +152,8 @@ static int fq_pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	sel_flow = &q->flows[idx];
 	/* Checks whether adding a new packet would exceed memory limit */
 	get_pie_cb(skb)->mem_usage = skb->truesize;
-	memory_limited = q->memory_usage > q->memory_limit + skb->truesize;
+	memory_limited = (u64)q->memory_usage + skb->truesize >
+			 q->memory_limit;
 
 	/* Checks if the qdisc is full */
 	if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
@@ -160,6 +161,7 @@ static int fq_pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		goto out;
 	} else if (unlikely(memory_limited)) {
 		q->overmemory++;
+		goto out;
 	}
 
 	reason = QDISC_DROP_CONGESTED;
-- 
2.43.0