path: root/net/sched/sch_api.c
diff options
authorJesper Dangaard Brouer <brouer@redhat.com>2016-11-03 14:56:11 +0100
committerDavid S. Miller <davem@davemloft.net>2016-11-07 20:15:55 -0500
commit84c46dd865384a1613e797b967552e4a428b5202 (patch)
treefd0201d39d33a237267cf9c0ca62166e3b246469 /net/sched/sch_api.c
parentnet/qdisc: IFF_NO_QUEUE drivers should use consistent TX queue len (diff)
qdisc: catch misconfig of attaching qdisc to tx_queue_len zero device
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@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to '')
1 files changed, 11 insertions, 0 deletions
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");
+ }
if (!ops->init || (err = ops->init(sch, tca[TCA_OPTIONS])) == 0) {
if (qdisc_is_percpu_stats(sch)) {
sch->cpu_bstats =