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

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Message-ID: <20141025013352.GE11289@acer.localdomain>
Date:	Sat, 25 Oct 2014 02:33:52 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Cong Wang <cwang@...pensource.com>
Cc:	John Fastabend <john.fastabend@...il.com>,
	Wang Bo <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 05:57:59PM -0700, Cong Wang wrote:
> On Fri, Oct 24, 2014 at 5:33 PM, Patrick McHardy <kaber@...sh.net> wrote:
> >
> > Its about having a sane API.
> 
> I don't see why calling ->destroy() on failure is not sane in qdisc case.
> I never want to argue general case.

Because it makes things more complicated. You need to keep track of what
was actually initialized since you can't assume a consistent state in
->destroy() anymore. If ->init() fails, it knows where it failed,
->destroy() can't know that.

Look at htb_destroy() for an example. It starts with

	cancel_work_sync(&q->work);

Was that actually initialized and can be cancled? You don't know.
Next comes

	qdisc_watchdog_cancel(&q->watchdog);

Same here, if the error happened before it was initialized, crash.

These are just the first two lines. You get the problem.
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