netdev - Re: [net-next PATCH 3/3] qdisc: catch misconfig of attaching qdisc to tx_queue

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Message-ID: <deacab39-2828-fc5d-7a40-9189b1ceee8b@cogentembedded.com>
Date:   Fri, 4 Nov 2016 15:53:50 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org
Cc:     Phil Sutter <phil@....cc>, Robert Olsson <robert@...julf.se>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [net-next PATCH 3/3] qdisc: catch misconfig of attaching qdisc to
 tx_queue_len zero device

Hello.

On 11/3/2016 4:56 PM, Jesper Dangaard Brouer wrote:

> It is a clear misconfiguration to attach a qdisc to a device with
> tx_queue_len zero, because some qdisc's (namely, pfifo, bfifo, gred,
> htb, plug and sfb) inherit/copy this value as their queue length.
>
> Why should the kernel catch such a misconfiguration?  Because prior to
> introducing the IFF_NO_QUEUE device flag, userspace found a loophole
> in the qdisc config system that allowed them to achieve the equivalent
> of IFF_NO_QUEUE, which is to remove the qdisc code path entirely from
> a device.  The loophole on older kernels is setting tx_queue_len=0,
> *prior* to device qdisc init (the config time is significant, simply
> setting tx_queue_len=0 doesn't trigger the loophole).
>
> This loophole is currently used by Docker[1] to get better performance
> and scalability out of the veth device.  The Docker developers were
> warned[1] that they needed to adjust the tx_queue_len if ever
> attaching a qdisc.  The OpenShift project didn't remember this warning
> and attached a qdisc, this were caught and fixed in[2].
>
> [1] https://github.com/docker/libcontainer/pull/193
> [2] https://github.com/openshift/origin/pull/11126
>
> Instead of fixing every userspace program that used this loophole, and
> forgot to reset the tx_queue_len, prior to attaching a qdisc.  Let's
> catch the misconfiguration on the kernel side.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>  net/sched/sch_api.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> 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

    "Exists" and "compatibility".

> +	 * 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");
> +	}
> +
>  	if (!ops->init || (err = ops->init(sch, tca[TCA_OPTIONS])) == 0) {
>  		if (qdisc_is_percpu_stats(sch)) {
>  			sch->cpu_bstats =

MBR, Sergei