netdev - Re: [PATCH net-net 0/4] Increase the limit of tuntap queues

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Date:	Mon, 24 Nov 2014 10:02:43 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Miller <davem@...emloft.net>
Cc:	pagupta@...hat.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, jasowang@...hat.com, dgibson@...hat.com,
	vfalico@...il.com, edumazet@...gle.com, vyasevic@...hat.com,
	hkchu@...gle.com, wuzhy@...ux.vnet.ibm.com, xemul@...allels.com,
	therbert@...gle.com, bhutchings@...arflare.com, xii@...gle.com,
	stephen@...workplumber.org, jiri@...nulli.us,
	sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH net-net 0/4] Increase the limit of tuntap queues

On Sun, Nov 23, 2014 at 08:23:21PM -0500, David Miller wrote:
> From: "Michael S. Tsirkin" <mst@...hat.com>
> Date: Sun, 23 Nov 2014 22:30:32 +0200
> 
> > qemu runs in the host, but it's unpriveledged: it gets
> > passed tun FDs by a priveledged daemon, and it only
> > has the rights to some operations,
> > in particular to attach and detach queues.
> > 
> > The assumption always was that this operation is safe
> > and can't make kernel run out of resources.
> 
> This creates a rather rediculous situation in my opinion.
> 
> Configuring a network device is a privileged operation, the daemon
> should be setting this thing up.
> 
> In no other context would we have to worry about something like this.

Right.  Jason corrected me.  I got it wrong:
what qemu does is TUNSETQUEUE and that needs to get a queue
that's already initialized by the daemon.

To create new queues daemon calls TUNSETIFF,
and that already can be used to create new devices,
so it's a priveledged operation.

This means it's safe to just drop the restriction,
exactly as you suggested originally.
-- 
MST
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