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 15:17:41 -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 2:45 PM, Patrick McHardy <kaber@...sh.net> wrote:
> On Fri, Oct 24, 2014 at 01:52:00PM -0700, Cong Wang wrote:
>> 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.
>
> In which cases does it actually fail after that? Usually this is
> called once initialization is complete.

How about tbf_change() in tbf_init()? If tbf_change() fails,
watchdog is still there if we don't call ->destroy(). Yes,
I know the timer is started, the point is we do miss something
clean up, even trivial.

tbf is not the only one who calls xxx_change() in xxx_init().

>
>> ->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().
>
> No, why do you think that? ->init() is supposed to clean up after
> itself if it fails, both qdisc_destroy() and ->destroy are only
> supposed to be called after ->init() succeeded.


Why? This would end up with calling xxx_destroy() in every
failure path in ->init(). Moving it up to caller makes the code cleaner
and smaller.

>
> Its simply symetrical, as everywhere else in the kernel. If a sub-init
> funtion fails, it should clean up and return an error. We don't
> destroy things we've never successfully initialized, they're supposed
> to clean up after themselves.

Most (if not all) ->destroy() are able to clean partially initialized qdisc,
I don't see why it  could be a problem here.

We don't have to keep with other kernel subsystem as long as it makes
sense, net_sched subsystem is pretty much self-contained.
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