Re: [Patch v4 net 2/6] net_sched: Check the return value of qfq_choose_next_agg()

[Xiang Mei <xmei5@asu.edu>]

On Sat, Jul 19, 2025 at 03:03:37PM -0700, 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@asu.edu>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.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 f0eb70353744..f328a58c7b98 100644
> --- a/net/sched/sch_qfq.c
> +++ b/net/sched/sch_qfq.c
> @@ -1147,6 +1147,8 @@ 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
>
Reviewed-by: Xiang Mei <xmei5@asu.edu>

Thanks for the explanations and fix. The fix makes sense to me.