linux-kernel - Re: [PATCH net-next v1 2/2] netpoll: avoid reference leaks

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Message-ID: <CAG88wWZ9GiqoOscqG4-ePOWo8bXgSTAQ6=6-tZPahMcxOvQMLg@mail.gmail.com>
Date:	Tue, 8 Jul 2014 12:35:14 -0700
From:	David Decotigny <decot@...glers.com>
To:	David Miller <davem@...emloft.net>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Eric Dumazet <edumazet@...gle.com>, amwang@...hat.com,
	antonio@...hcoding.com, Jiri Pirko <jpirko@...hat.com>
Subject: Re: [PATCH net-next v1 2/2] netpoll: avoid reference leaks

Hi,

Thanks for the feedback. This patch results from manual inspection of
the code. I agree my commit description is abusive: in the case of
bonding, I think everything is fine, there should be no ref leak,
cleanup paths seem clean.

My point was to make things more predictable: ndo_netpoll_cleanup
called anyways to acknowledge actual loss of a ref to npinfo,
irrespective of whether it's the last ref or not. Without this patch,
calling ndo_netpoll_cleanup would depend on some timing behavior, hard
to predict, and users of the API have better be careful to reclaim the
refs manually anyways: as a consequence, not sure this callback is
actually required in its current inception.

On Mon, Jul 7, 2014 at 7:35 PM, David Miller <davem@...emloft.net> wrote:
> From: David Decotigny <decot@...glers.com>
> Date: Mon, 30 Jun 2014 16:50:10 -0700
>
>> This ensures that the ndo_netpoll_cleanup callback is called for every
>> device that provides one. Otherwise there is a risk of reference leak
>> with bonding for example, which depends on this callback to cleanup
>> the slaves' references to netpoll info.
>>
>> Tested:
>>   see patch "netpoll: fix use after free"
>>
>> Signed-off-by: David Decotigny <decot@...glers.com>
>
> I definitely don't understand this.
>
> Why would we call the cleanup function of an object before it's
> reference count hits zero?  It is exactly the act of reaching a
> zero refcount which should trigger invoking the cleanup callback.
>
> If a refcount is being released in another location without checking
> if it hits zero and invoking the cleanup if so, _THAT_ is the bug.
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