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

[Chia-Yu Chang (Nokia)]

> -----Original Message-----
> From: Paolo Abeni <pabeni@xxxxxxxxxx> 
> Sent: Thursday, April 16, 2026 3:26 PM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@xxxxxxxxxxxxxxxxxxx>; linux-hardening@xxxxxxxxxxxxxxx; kees@xxxxxxxxxx; gustavoars@xxxxxxxxxx; jhs@xxxxxxxxxxxx; jiri@xxxxxxxxxxx; davem@xxxxxxxxxxxxx; edumazet@xxxxxxxxxx; kuba@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; horms@xxxxxxxxxx; ij@xxxxxxxxxx; ncardwell@xxxxxxxxxx; Koen De Schepper (Nokia) <koen.de_schepper@xxxxxxxxxxxxxxxxxxx>; g.white@xxxxxxxxxxxxx; ingemar.s.johansson@xxxxxxxxxxxx; mirja.kuehlewind@xxxxxxxxxxxx; cheshire@xxxxxxxxx; rs.ietf@xxxxxx; Jason_Livingood@xxxxxxxxxxx; vidhi_goel@xxxxxxxxx
> Subject: Re: [PATCH v1 net 1/1] net/sched: sch_dualpi2: fix limit/memlimit enforcement when dequeueing L-queue
> 
> 
> CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
> 
> 
> 
> On 4/13/26 6:37 PM, chia-yu.chang@xxxxxxxxxxxxxxxxxxx wrote:
> > 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);
> 
> Sashiko says:
> ---
> The drop counter is incremented for the L-queue via rtnl_qdisc_drop(), but it appears the drop counter for the parent qdisc (sch) is not updated.
> Will this cause user-facing statistics for the overall dualpi2 qdisc to underreport drops?
> ---

Hi Paolo,

You are right, this is my miss.
I will add "qdisc_qstats_drop(sch)" for the L-queue dropping case.

Thanks!
Chia-Yu