netdev - Re: [PATCH net] net/sched: Fix use of wild pointer in mq_destroy() when qdisc

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Date:	Fri, 24 Oct 2014 13:52:00 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	John Fastabend <john.fastabend@...il.com>, wang.bo116@....com.cn,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, cui.yunfeng@....com.cn
Subject: Re: [PATCH net] net/sched: Fix use of wild pointer in mq_destroy()
 when qdisc_alloc fail

On Fri, Oct 24, 2014 at 12:14 PM, Patrick McHardy <kaber@...sh.net> wrote:
> On Fri, Oct 24, 2014 at 11:13:56AM -0700, Cong Wang wrote:
>> On Fri, Oct 24, 2014 at 10:49 AM, John Fastabend
>> <john.fastabend@...il.com> wrote:
>> >
>> > Patch looks fine, another way to fix this would be drop the
>> > mq_destroy() call in the error path. I'm not convinced one
>> > is any better than the other but maybe some other folks have
>> > opinions, it seems a bit wrong to call mq_destroy twice so in
>> > that sense it may be a bit nicer to drop the mq_destroy().
>>
>> Dropping mq_destroy() in error path is indeed better,
>> because upper layer does cleanup intentionally.
>> Look at what other qdisc's do. :)
>
> I would argue that the qdisc_destroy() call in qdisc_create_dflt()
> is wrong, it should instead free the qdisc and release the module
> reference manually as done in qdisc_create().
>
> qdisc_destroy() should only be called for fully initialized qdiscs.

Probably, but at least ->destroy() should be called, looking at
those calling qdisc_watchdog_init(), they are supposed to call
qdisc_watchdog_cancel() when >init() fails after that.

->destroy() is supposed to be able to clean up even partially
initialized qdisc's. So, for qdisc_create_dflt() we should probably
just call ->destroy().

Reading the code again, seems it is inconsistent with qdisc_create(),
where ->destroy() is skipped when ->init() fails. Hmm, we have
a bigger problem here.

I am working on a patch now. Thanks for pointing this out.
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