netdev - Re: [PATCH] ip6_gre: don't allow to remove the fb_tunnel

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Date:	Tue, 15 Apr 2014 09:57:28 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org, xeb@...l.ru
Subject: Re: [PATCH] ip6_gre: don't allow to remove the fb_tunnel_dev

Le 15/04/2014 06:04, David Miller a écrit :
> From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
> Date: Mon, 14 Apr 2014 17:11:38 +0200
>
>> It's possible to remove the FB tunnel with the command 'ip link del ip6gre0' but
>> this is unsafe, the module always supposes that this device exists. For example,
>> ip6gre_tunnel_lookup() may use it unconditionally.
>>
>> Let's add a rtnl handler for dellink, which will never remove the FB tunnel (we
>> let ip6gre_destroy_tunnels() do the job).
>>
>> Introduced by commit c12b395a4664 ("gre: Support GRE over IPv6").
>>
>> CC: Dmitry Kozlov <xeb@...l.ru>
>> Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
>
> I don't see how we ever get rid of fb_tunnel_dev and can therefore
> remove the module successfully.
>
> It is created by the per-netns initialization, but since it isn't
> added to the hashes I don't see how the per-netns exit code can
> end up unregistering and freeing it up.
>
> How is this supposed to work?
It is added to the hashes in ip6gre_init_net() in bucket [0][0]:
#define tunnels_wc      tunnels[0]
[snip]
         rcu_assign_pointer(ign->tunnels_wc[0],
                            netdev_priv(ign->fb_tunnel_dev));

Thus the tunnel is deleted by the loop in ip6gre_destroy_tunnels().
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