netdev - Re: [PATCH net] ip[6]: don't register inet[6]dev when dev is down

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Date:   Sat, 8 Jul 2017 11:56:59 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Hongjun Li <hongjun.li@...nd.com>
Subject: Re: [PATCH net] ip[6]: don't register inet[6]dev when dev is down

On Fri, Jul 7, 2017 at 5:39 AM, Nicolas Dichtel
<nicolas.dichtel@...nd.com> wrote:
> Le 06/07/2017 à 20:16, Cong Wang a écrit :
>> On Thu, Jul 6, 2017 at 5:08 AM, Nicolas Dichtel
>> <nicolas.dichtel@...nd.com> wrote:
>>> Le 06/07/2017 à 00:43, Cong Wang a écrit :
>>>> On Wed, Jul 5, 2017 at 8:57 AM, Nicolas Dichtel
>>>> <nicolas.dichtel@...nd.com> wrote:
>>>>> When a device changes from one netns to another, it's first unregistered,
>>>>> then the netns reference is updated and the dev is registered in the new
>>>>> netns. Thus, when a slave moves to another netns, it is first
>>>>> unregistered. This triggers a NETDEV_UNREGISTER event which is caught by
>>>>> the bonding driver. The driver calls bond_release(), which calls
>>>>> dev_set_mtu() and thus triggers NETDEV_CHANGEMTU (the device is still in
>>>>> the old netns).
>>>>
>>>> I think in this special case it is meaningless to send
>>>> NETDEV_CHANGEMTU, because the device is dying within
>>>> its old netns, who still cares about its mtu change?
>>>>
>>>> Something like the attached patch...
>>> Yes, your patch seems good and I hesitated with something like this.
>>> But I don't see a valid case where the inet[6]dev must be created on a down
>>> interface. I think the patch is valid, even with your patch.
>>
>> Your patch is more risky because it affects normal CHANGEMTU path,
>> I am not sure if it is correct to not to add idev when it is down either.
> Why would it be needed to add this idev on a down interface?
> If idev wasn't there I don't see why changing the mtu would justify to create
> this idev.
>


There must be a reason to check idev->if_flags & IF_READY instead
of IFF_UP.


>>
>> This is a very unusual path, we don't have to take the risk.
> I still think that this approach is better for two reasons:
>  - we don't know if another path like this exists (need an audit) and it would
> be easy to add one again by side effect in the future;

Perhaps we need to add a warning for these events triggered after
UNREGISTER, except UNREGISTER_FINAL, in case of trouble.
But again, the CHANGEMTU case is so special because of the
idev.


>  - the patch is easy to backport in older kernel.
>

Easy to backport doesn't mean easy to verify. ;) As David said, this
code is mess, especially for the keep_addr_on_down logic.