Subject: Re: [net-next PATCH 3/3] qdisc: catch misconfig of attaching qdisc to tx_queue_len zero device

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 4 Nov 2016 11:10:42 +0100

On Fri, 4 Nov 2016 10:35:26 +0100 Phil Sutter <[email-masked]> wrote:

> Hi,
> 
> On Thu, Nov 03, 2016 at 02:56:11PM +0100, Jesper Dangaard Brouer wrote:
> [...]
> > diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
> > index 206dc24add3a..f337f1bdd1d4 100644
> > --- a/net/sched/sch_api.c
> > +++ b/net/sched/sch_api.c
> > @@ -960,6 +960,17 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
> >  
> >  	sch->handle = handle;
> >  
> > +	/* This exist to keep backward compatible with a userspace
> > +	 * loophole, what allowed userspace to get IFF_NO_QUEUE
> > +	 * facility on older kernels by setting tx_queue_len=0 (prior
> > +	 * to qdisc init), and then forgot to reinit tx_queue_len
> > +	 * before again attaching a qdisc.
> > +	 */
> > +	if ((dev->priv_flags & IFF_NO_QUEUE) && (dev->tx_queue_len == 0)) {
> > +		dev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
> > +		netdev_info(dev, "Caught tx_queue_len zero misconfig\n");
> > +	}  
> 
> I wonder why this is limited to IFF_NO_QUEUE devices. Do you think there
> is a valid use case for physical ones?

Hmmm, I cannot come up with a useful use-case for physical devices, but
I cannot see why we should save users that had used the loophole on
physical devices, as that is clearly a faulty config to begin with.
See net_crit_ratelimited warning here:
 https://github.com/torvalds/linux/blob/v4.9-rc3/net/core/dev.c#L3403

> Also, if we sanitize here, couldn't we then just get rid of the
> sanitization you're fixing in patch 2?

Without patch 2, then some IFF_NO_QUEUE devices would have a visible
tx_queue_len 0 (e.g. the ones not calling ether_setup()), and that
would be inconsistent (visible from userspace).

> Apart from that, ACK to all the patches. Thanks for cleaning up my mess!
> :)

Thanks for the ACKs

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer