netdev - Re: [Patch net v5 5/9] net_sched: Check the return value of qfq_choose_next

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Message-ID: <bzz3jovy7tsxwxa3ek3opnl2xuk7l5fuzqewrxw4syvwfzxkmj@22o6rjgiwnkm>
Date: Tue, 2 Dec 2025 14:18:15 -0700
From: Xiang Mei <xmei5@....edu>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, stephen@...workplumber.org, kuba@...nel.org
Subject: Re: [Patch net v5 5/9] net_sched: Check the return value of
 qfq_choose_next_agg()

On Wed, Nov 26, 2025 at 11:52:40AM -0800, Cong Wang wrote:
> qfq_choose_next_agg() could return NULL so its return value should be
> properly checked unless NULL is acceptable.
> 
> There are two cases we need to deal with:
> 
> 1) q->in_serv_agg, which is okay with NULL since it is either checked or
>    just compared with other pointer without dereferencing. In fact, it
>    is even intentionally set to NULL in one of the cases.
> 
> 2) in_serv_agg, which is a temporary local variable, which is not okay
>    with NULL, since it is dereferenced immediately, hence must be checked.
> 
> This fix corrects one of the 2nd cases, and leaving the 1st case as they are.
> 
> Although this bug is triggered with the netem duplicate change, the root
> cause is still within qfq qdisc.
> 
> Fixes: 462dbc9101ac ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
> Cc: Xiang Mei <xmei5@....edu>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/sched/sch_qfq.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
> index 2255355e51d3..a438ebaf53e0 100644
> --- a/net/sched/sch_qfq.c
> +++ b/net/sched/sch_qfq.c
> @@ -1145,6 +1145,8 @@ static struct sk_buff *qfq_dequeue(struct Qdisc *sch)
>  		 * choose the new aggregate to serve.
>  		 */
>  		in_serv_agg = q->in_serv_agg = qfq_choose_next_agg(q);
> +		if (!in_serv_agg)
> +			return NULL;
>  		skb = qfq_peek_skb(in_serv_agg, &cl, &len);
>  	}
>  	if (!skb)
> -- 
> 2.34.1
>

The explanations make sense to me. The patch also handles the case
correctly.

Reviewed-by: Xiang Mei <xmei5@....edu>