netdev - Re: [PATCH net v7 1/2] net/sched: sch_cake: Fix incorrect qlen reduction in cake

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Message-ID: <87wm3bbzwn.fsf@toke.dk>
Date: Thu, 27 Nov 2025 10:14:32 +0100
From: Toke Høiland-Jørgensen <toke@...e.dk>
To: Xiang Mei <xmei5@....edu>, security@...nel.org
Cc: netdev@...r.kernel.org, xiyou.wangcong@...il.com,
 cake@...ts.bufferbloat.net, bestswngs@...il.com, Xiang Mei <xmei5@....edu>
Subject: Re: [PATCH net v7 1/2] net/sched: sch_cake: Fix incorrect qlen
 reduction in cake_drop

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

> In cake_drop(), qdisc_tree_reduce_backlog() is used to update the qlen
> and backlog of the qdisc hierarchy. Its caller, cake_enqueue(), assumes
> that the parent qdisc will enqueue the current packet. However, this
> assumption breaks when cake_enqueue() returns NET_XMIT_CN: the parent
> qdisc stops enqueuing current packet, leaving the tree qlen/backlog
> accounting inconsistent. This mismatch can lead to a NULL dereference
> (e.g., when the parent Qdisc is qfq_qdisc).
>
> This patch computes the qlen/backlog delta in a more robust way by
> observing the difference before and after the series of cake_drop()
> calls, and then compensates the qdisc tree accounting if cake_enqueue()
> returns NET_XMIT_CN.
>
> To ensure correct compensation when ACK thinning is enabled, a new
> variable is introduced to keep qlen unchanged.
>
> Fixes: 15de71d06a40 ("net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit")
> Signed-off-by: Xiang Mei <xmei5@....edu>

Reviewed-by: Toke Høiland-Jørgensen <toke@...e.dk>