[PATCH net] net/sched: sch_dualpi2: avoid double memory accounting on peek

[shaos <shawnq1@163.com>]

qdisc_dequeue_internal() removes skbs from gso_skb without invoking
the qdisc dequeue callback. Their truesize was already subtracted
when they were first dequeued for peek or requeued after a transmit
failure. dualpi2_change() currently subtracts it again while trimming
the queue, wrapping the u32 memory_used counter.

Skip the second decrement when the skb comes from gso_skb. Packets
dequeued directly from the C-queue still need the accounting here;
the L-queue path is unchanged.

The issue was identified by LLM-assisted source analysis and
reproduced with a purpose-built rtnetlink/AF_PACKET test in QEMU.
The test uses CAP_NET_ADMIN in a user-created network namespace and
CAP_NET_RAW for its AF_PACKET sender. It does not modify the kernel
or use error injection.

On Linux 7.4-rc2, the unpatched reproducer wrapped memory_used to
4294964992 and subsequent packets remained rejected after restoring
the memory limit. With this patch, the counter remains valid and a
packet is accepted after restoring the limit.

The modified sch_dualpi2.o builds with W=1 under allmodconfig and
allyesconfig. Full-tree W=1 builds stop on unrelated warnings in
lib/tests/stackinit_kunit.c, arch/x86/kernel/callthunks.c, and
net/8021q/vlan.c. The QEMU regression passes; tc-testing was not run.

Fixes: 8f9516daedd6 ("sched: Add enqueue/dequeue of dualpi2 qdisc")
Link: https://lkml.iu.edu/hypermail/linux/kernel/2610.2/16461.html
Signed-off-by: shaos <shawnq1@163.com>
---
 net/sched/sch_dualpi2.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_dualpi2.c b/net/sched/sch_dualpi2.c
index 4947def7c49ef..ccf6359eeebc3 100644
--- a/net/sched/sch_dualpi2.c
+++ b/net/sched/sch_dualpi2.c
@@ -909,14 +909,20 @@ static int dualpi2_change(struct Qdisc *sch, struct nlattr *opt,
 	while (qdisc_qlen(sch) > sch->limit ||
 	       q->memory_used > q->memory_limit) {
 		struct sk_buff *skb = NULL;
+		bool already_dequeued;
 
 		if (qdisc_qlen(sch) > qdisc_qlen(q->l_queue)) {
+			already_dequeued = skb_peek(&sch->gso_skb);
+
 			skb = qdisc_dequeue_internal(sch, true);
 			if (unlikely(!skb)) {
 				WARN_ON_ONCE(1);
 				break;
 			}
-			WRITE_ONCE(q->memory_used, q->memory_used - skb->truesize);
+			/* A peeked or requeued skb was already accounted. */
+			if (!already_dequeued)
+				WRITE_ONCE(q->memory_used,
+					   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);