netdev - Re: size overflow in function qdisc_tree_decrease_qlen net/sched/sch

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Date:	Tue, 1 Dec 2015 11:17:05 -0800
From:	Cong Wang <cwang@...pensource.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	pageexec@...email.hu, Daniele Fucini <dfucini@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	David Miller <davem@...emloft.net>, spender@...ecurity.net,
	re.emese@...il.com
Subject: Re: size overflow in function qdisc_tree_decrease_qlen net/sched/sch_api.c

On Tue, Dec 1, 2015 at 11:09 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2015-12-01 at 10:43 -0800, Cong Wang wrote:
>
>> This smells hacky... Another way to fix this is to hold the qdisc tree
>> lock in mq_dump(), since it is not a hot path (comparing with
>> enqueue/dequeue)?
>
> Really ? Which qdisc tree lock will protect you exactly ???
>
> Whole point of MQ is that each TX queue has its own lock.
>
> So multiple cpus can call qdisc_tree_decrease_qlen() at the same time,
> holding their own lock.
>
> Clearly modifying mq 'data' is wrong.

Ah, yeah, but mq _seems_ also the only one who modifies sch->q.qlen
in ->dump(), which is the root cause of this bug. I am wondering if it should
just compute the qlen and return it without modifying sch->q.qlen.
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