net/sched: pass netlink extack to mqprio and taprio offload

With the multiplexed ndo_setup_tc() model which lacks a first-class
struct netlink_ext_ack * argument, the only way to pass the netlink
extended ACK message down to the device driver is to embed it within the
offload structure.

Do this for struct tc_mqprio_qopt_offload and struct tc_taprio_qopt_offload.

Since struct tc_taprio_qopt_offload also contains a tc_mqprio_qopt_offload
structure, and since device drivers might effectively reuse their mqprio
implementation for the mqprio portion of taprio, we make taprio set the
extack in both offload structures to point at the same netlink extack
message.

In fact, the taprio handling is a bit more tricky, for 2 reasons.

First is because the offload structure has a longer lifetime than the
extack structure. The driver is supposed to populate the extack
synchronously from ndo_setup_tc() and leave it alone afterwards.
To not have any use-after-free surprises, we zero out the extack pointer
when we leave taprio_enable_offload().

The second reason is because taprio does overwrite the extack message on
ndo_setup_tc() error. We need to switch to the weak form of setting an
extack message, which preserves a potential message set by the driver.

Signed-off-by: Vladimir Oltean <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Acked-by: Jamal Hadi Salim <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/pkt_sched.h

···
 struct tc_mqprio_qopt_offload {
 	/* struct tc_mqprio_qopt must always be the first element */
 	struct tc_mqprio_qopt qopt;
+	struct netlink_ext_ack *extack;
 	u16 mode;
 	u16 shaper;
 	u32 flags;
···
 
 struct tc_taprio_qopt_offload {
 	struct tc_mqprio_qopt_offload mqprio;
+	struct netlink_ext_ack *extack;
 	u8 enable;
 	ktime_t base_time;
 	u64 cycle_time;

net/sched/sch_mqprio.c

···
 				 const struct tc_mqprio_qopt *qopt,
 				 struct netlink_ext_ack *extack)
 {
-	struct tc_mqprio_qopt_offload mqprio = {.qopt = *qopt};
 	struct mqprio_sched *priv = qdisc_priv(sch);
 	struct net_device *dev = qdisc_dev(sch);
+	struct tc_mqprio_qopt_offload mqprio = {
+		.qopt = *qopt,
+		.extack = extack,
+	};
 	int err, i;
 
 	switch (priv->mode) {

net/sched/sch_taprio.c

···
 		return -ENOMEM;
 	}
 	offload->enable = 1;
+	offload->extack = extack;
 	mqprio_qopt_reconstruct(dev, &offload->mqprio.qopt);
+	offload->mqprio.extack = extack;
 	taprio_sched_to_offload(dev, sched, offload, &caps);
 
 	for (tc = 0; tc < TC_MAX_QUEUE; tc++)
···
 
 	err = ops->ndo_setup_tc(dev, TC_SETUP_QDISC_TAPRIO, offload);
 	if (err < 0) {
-		NL_SET_ERR_MSG(extack,
-			       "Device failed to setup taprio offload");
+		NL_SET_ERR_MSG_WEAK(extack,
+				    "Device failed to setup taprio offload");
 		goto done;
 	}
 
 	q->offloaded = true;
 
 done:
+	/* The offload structure may linger around via a reference taken by the
+	 * device driver, so clear up the netlink extack pointer so that the
+	 * driver isn't tempted to dereference data which stopped being valid
+	 */
+	offload->extack = NULL;
+	offload->mqprio.extack = NULL;
 	taprio_offload_free(offload);
 
 	return err;