netdev - Re: [PATCH][NETNS] Make ifindex generation per-namespace

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Message-ID: <m1przoxk1v.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 09 Oct 2007 11:41:16 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Pavel Emelyanov <xemul@...nvz.org>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>, devel@...nvz.org
Subject: Re: [PATCH][NETNS] Make ifindex generation per-namespace

Pavel Emelyanov <xemul@...nvz.org> writes:

> Currently indexes for netdevices come sequentially one by
> one, and the same stays true even for devices that are 
> created for namespaces.
>
> Side effects of this are:
>  * lo device has not 1 index in a namespace. This may break
>    some userspace that relies on it (and AFAIR something
>    really broke in OpenVZ VEs without this);

As it happens lo hasn't been registered first for some time
so it hasn't had ifindex of 1 in the normal kernel.

>  * after some time namespaces will have devices with indexes
>    like 1000000 os similar. This might be confusing for a
>    human (tools will not mind).

Only if we wind up creating that many devices.

> So move the (currently "global" and static) ifindex variable
> on the struct net, making the indexes allocation look more
> like on a standalone machine.
>
> Moreover - when we have indexes intersect between namespaces,
> we may catch more BUGs in the future related to "wrong device 
> was found for a given index".

Not yet.

I know there are several data structures internal to the kernel that
are indexed by ifindex, and not struct net_device *.  There is the
iflink field in struct net_device.  We need a way to refer to network
devices in other namespaces in rtnetlink in an unambiguous way.   I
don't see any real problems with a global ifindex assignment until
we start migrating applications.

So please hold off on this until the kernel has been audited and
we have removed all of the uses of ifindex that assume ifindex is
global, that we can find.

Right now a namespace local ifindex seems to be just asking for
trouble.

Eric

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