netdev - Re: [PATCH net v3] net/sched: sch_cake: Fix incorrect qlen reduction in cake

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Message-ID: <875xbejcel.fsf@toke.dk>
Date: Thu, 13 Nov 2025 14:21:54 +0100
From: Toke Høiland-Jørgensen <toke@...e.dk>
To: Xiang Mei <xmei5@....edu>, security@...nel.org
Cc: netdev@...r.kernel.org, cake@...ts.bufferbloat.net, bestswngs@...il.com,
 Xiang Mei <xmei5@....edu>
Subject: Re: [PATCH net v3] net/sched: sch_cake: Fix incorrect qlen
 reduction in cake_drop

Xiang Mei <xmei5@....edu> writes:

> In cake_drop(), qdisc_tree_reduce_backlog() is called to decrement
> the qlen of the qdisc hierarchy. However, this can incorrectly reduce
> qlen when the dropped packet was never enqueued, leading to a possible
> NULL dereference (e.g., when QFQ is the parent qdisc).
>
> This happens when cake_enqueue() returns NET_XMIT_CN: the parent
> qdisc does not enqueue the skb, but cake_drop() still reduces backlog.
>
> This patch avoids the extra reduction by checking whether the packet
> was actually enqueued. It also moves qdisc_tree_reduce_backlog()
> out of cake_drop() to keep backlog accounting consistent.
>
> Fixes: 15de71d06a40 ("net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit")
> Signed-off-by: Xiang Mei <xmei5@....edu>
> ---
> v2: add missing cc
> v3: move qdisc_tree_reduce_backlog out of cake_drop
>
>  net/sched/sch_cake.c | 40 ++++++++++++++++++++++++----------------
>  1 file changed, 24 insertions(+), 16 deletions(-)
>
> diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
> index 32bacfc314c2..179cafe05085 100644
> --- a/net/sched/sch_cake.c
> +++ b/net/sched/sch_cake.c
> @@ -1597,7 +1597,6 @@ static unsigned int cake_drop(struct Qdisc *sch, struct sk_buff **to_free)
>  
>  	qdisc_drop_reason(skb, sch, to_free, SKB_DROP_REASON_QDISC_OVERLIMIT);
>  	sch->q.qlen--;
> -	qdisc_tree_reduce_backlog(sch, 1, len);
>  
>  	cake_heapify(q, 0);
>  
> @@ -1750,7 +1749,9 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>  	ktime_t now = ktime_get();
>  	struct cake_tin_data *b;
>  	struct cake_flow *flow;
> -	u32 idx, tin;
> +	u32 dropped = 0;
> +	u32 idx, tin, prev_qlen, prev_backlog, drop_id;
> +	bool same_flow = false;
>  
>  	/* choose flow to insert into */
>  	idx = cake_classify(sch, &b, skb, q->flow_mode, &ret);
> @@ -1927,24 +1928,31 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>  	if (q->buffer_used > q->buffer_max_used)
>  		q->buffer_max_used = q->buffer_used;
>  
> -	if (q->buffer_used > q->buffer_limit) {
> -		bool same_flow = false;
> -		u32 dropped = 0;
> -		u32 drop_id;
> +	if (q->buffer_used <= q->buffer_limit)
> +		return NET_XMIT_SUCCESS;

While this does reduce indentation, I don't think it's an overall
improvement; the overflow condition is the exceptional case, so it's
clearer to keep it inside the if statement (which also keeps the
variable scope smaller).

> -		while (q->buffer_used > q->buffer_limit) {
> -			dropped++;
> -			drop_id = cake_drop(sch, to_free);
> +	prev_qlen = sch->q.qlen;
> +	prev_backlog = sch->qstats.backlog;
>  
> -			if ((drop_id >> 16) == tin &&
> -			    (drop_id & 0xFFFF) == idx)
> -				same_flow = true;
> -		}
> -		b->drop_overlimit += dropped;
> +	while (q->buffer_used > q->buffer_limit) {
> +		dropped++;
> +		drop_id = cake_drop(sch, to_free);
> +		if ((drop_id >> 16) == tin &&
> +		    (drop_id & 0xFFFF) == idx)
> +			same_flow = true;
> +	}
> +	b->drop_overlimit += dropped;

The 'dropped' variable is wholly redundant after this change, so let's
just get rid of it and use the prev_qlen for this statistic instead.

-Toke