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

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Date:	Mon, 07 Jul 2014 19:35:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	decot@...glers.com
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com, edumazet@...gle.com, amwang@...hat.com,
	antonio@...hcoding.com, jpirko@...hat.com
Subject: Re: [PATCH net-next v1 2/2] netpoll: avoid reference leaks

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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