netdev - Re: [patch 1/1][NETNS][IPV6] protect addrconf from loopback registration

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Date:	Mon, 12 Nov 2007 09:59:52 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Denis V. Lunev" <den@...ru>
Cc:	Daniel Lezcano <dlezcano@...ibm.com>, davem@...emloft.net,
	netdev@...r.kernel.org, xemul@...nvz.org, ebiederm@...ssion.com,
	containers@...ts.osdl.org, yoshfuji@...ux-ipv6.org,
	Benjamin Thery <benjamin.thery@...l.net>
Subject: Re: [patch 1/1][NETNS][IPV6] protect addrconf from loopback registration

"Denis V. Lunev" <den@...ru> writes:

>>> why should we care on down? we are destroying the device. It should
>>> gone. All references to it should also gone. So, we should perform the
>>> cleaning and remove all IPv6 addresses, so notifier should also work.
>> 
>> We need to take care of netdev down, someone can put the loopback down
>> if he wants.
>> 
>>> The code relies on the "persistent" loopback and this is a _bad_ thing.
>>> This is longstanding bug in the code, that the dst_entry should have a
>>> valid reference to a device. This is the only purpose for a loopback
>>> persistence. Though, at the namespace death no such entries must be and
>>> this will be checked during unregister process. This patch definitely
>>> breaks this assumption :(
>>>
>>> Namespaces are good to catch leakage using standard codepaths, so they
>>> should be preserved as much as possible. So, _all_ normal down code
>>> should be called for a loopback device in other than init_net context.
>> 
>> I agree with you, this is a bug in ipv6 and the loopback; when playing
>> with ipv6 we found that the loopback is still referenced 9 times when
>> the system is shutdown.
>
> Pff...  I can't guess right now where the error can be :( We have
> correct behavior of loopback even for IPv6 within OpenVZ. On down the
> count is 0 and device is destroyed correctly without these kludges. So,
> something is definitely wrong.
>
>> The purpose of this patch is to protect the __actual__ code from the new
>> loopback behavior. We are looking at a more generic approach with the
>> namespace for ipv6, as you mentioned, namespaces are good for network
>> leakage detection as we create several instances of the network stack.
>
> The only kludge required is already in place. addrconf_ifdown has a
> protection for init_net.
>         if (dev == init_net.loopback_dev && how == 1)
>                 how = 0;
> Other places should be untouched.
>
> Unregister for a loopback in !init_net is a _valid_ operation and should
> be clean, i.e. without kludges in the path. This is the only way to
> check the ref-counting.

Oh.  Speaking of.  One way to catch this kind of thing during debugging
is to instrument dev_put and dev_hold to add a print statement.  Something
like:

/**
 *	dev_put - release reference to device
 *	@dev: network device
 *
 * Release reference to device to allow it to be freed.
 */
static inline void __dev_put(struct net_device *dev, char *file, char *func, int line)
{
        if (dev->flags & IFF_LOOPBACK)
        	printk("%s: %s.%s.%d\n", __func__, file, file, line);
	atomic_dec(&dev->refcnt);
}

#define dev_put(DEV) __dev_put(DEV, __FILE__, __func__, __LINE__)

/**
 *	dev_hold - get reference to device
 *	@dev: network device
 *
 * Hold reference to device to keep it from being freed.
 */
static inline void __dev_hold(struct net_device *dev, char *file, char *func, int line)
{
        if (dev->flags & IFF_LOOPBACK)
        	printk("%s: %s.%s.%d\n", __func__, file, file, line);
	atomic_inc(&dev->refcnt);
}

#define dev_hold(DEV) __dev_hold(DEV, __FILE__, __func__, __LINE__)

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