Re: tun: add IFF_TUN_EXCL flag to avoid opening a persistent device.

[David Miller <davem@davemloft.net>]

From: David Woodhouse <dwmw2@infradead.org>
Date: Thu, 23 Apr 2009 18:04:18 +0100

> When creating a certain types of VPN, NetworkManager will first attempt
> to find an available tun device by iterating through 'vpn%d' until it
> finds one that isn't already busy. Then it'll set that to be persistent
> and owned by the otherwise unprivileged user that the VPN dæmon itself
> runs as.
> 
> There's a race condition here -- during the period where the vpn%d
> device is created and we're waiting for the VPN dæmon to actually
> connect and use it, if we try to create _another_ device we could end up
> re-using the same one -- because trying to open it again doesn't get
> -EBUSY as it would while it's _actually_ busy.
> 
> So solve this, we add an IFF_TUN_EXCL flag which causes tun_set_iff() to
> fail if it would be opening an existing persistent tundevice -- so that
> we can make sure we're getting an entirely _new_ device.
> 
> Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

Applied to net-next-2.6, thanks David.