net_sched: sch_sfq: use a temporary work area for validating configuration

Many configuration parameters have influence on others (e.g. divisor
-> flows -> limit, depth -> limit) and so it is difficult to correctly
do all of the validation before applying the configuration. And if a
validation error is detected late it is difficult to roll back a
partially applied configuration.

To avoid these issues use a temporary work area to update and validate
the configuration and only then apply the configuration to the
internal state.

Signed-off-by: Octavian Purdila <tavip@google.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: tavip

net/sched/sch_sfq.c

@@ -631,6 +631,15 @@ static int sfq_change(struct Qdisc *sch, struct nlattr *opt,
 	struct red_parms *p = NULL;
 	struct sk_buff *to_free = NULL;
 	struct sk_buff *tail = NULL;
+	unsigned int maxflows;
+	unsigned int quantum;
+	unsigned int divisor;
+	int perturb_period;
+	u8 headdrop;
+	u8 maxdepth;
+	int limit;
+	u8 flags;
+
 
 	if (opt->nla_len < nla_attr_size(sizeof(*ctl)))
 		return -EINVAL;
@@ -656,36 +665,59 @@ static int sfq_change(struct Qdisc *sch, struct nlattr *opt,
 		NL_SET_ERR_MSG_MOD(extack, "invalid limit");
 		return -EINVAL;
 	}
+
 	sch_tree_lock(sch);
+
+	limit = q->limit;
+	divisor = q->divisor;
+	headdrop = q->headdrop;
+	maxdepth = q->maxdepth;
+	maxflows = q->maxflows;
+	perturb_period = q->perturb_period;
+	quantum = q->quantum;
+	flags = q->flags;
+
+	/* update and validate configuration */
 	if (ctl->quantum)
-		q->quantum = ctl->quantum;
-	WRITE_ONCE(q->perturb_period, ctl->perturb_period * HZ);
+		quantum = ctl->quantum;
+	perturb_period = ctl->perturb_period * HZ;
 	if (ctl->flows)
-		q->maxflows = min_t(u32, ctl->flows, SFQ_MAX_FLOWS);
+		maxflows = min_t(u32, ctl->flows, SFQ_MAX_FLOWS);
 	if (ctl->divisor) {
-		q->divisor = ctl->divisor;
-		q->maxflows = min_t(u32, q->maxflows, q->divisor);
+		divisor = ctl->divisor;
+		maxflows = min_t(u32, maxflows, divisor);
 	}
 	if (ctl_v1) {
 		if (ctl_v1->depth)
-			q->maxdepth = min_t(u32, ctl_v1->depth, SFQ_MAX_DEPTH);
+			maxdepth = min_t(u32, ctl_v1->depth, SFQ_MAX_DEPTH);
 		if (p) {
-			swap(q->red_parms, p);
-			red_set_parms(q->red_parms,
+			red_set_parms(p,
 				      ctl_v1->qth_min, ctl_v1->qth_max,
 				      ctl_v1->Wlog,
 				      ctl_v1->Plog, ctl_v1->Scell_log,
 				      NULL,
 				      ctl_v1->max_P);
 		}
-		q->flags = ctl_v1->flags;
-		q->headdrop = ctl_v1->headdrop;
+		flags = ctl_v1->flags;
+		headdrop = ctl_v1->headdrop;
 	}
 	if (ctl->limit) {
-		q->limit = min_t(u32, ctl->limit, q->maxdepth * q->maxflows);
-		q->maxflows = min_t(u32, q->maxflows, q->limit);
+		limit = min_t(u32, ctl->limit, maxdepth * maxflows);
+		maxflows = min_t(u32, maxflows, limit);
 	}
 
+	/* commit configuration */
+	q->limit = limit;
+	q->divisor = divisor;
+	q->headdrop = headdrop;
+	q->maxdepth = maxdepth;
+	q->maxflows = maxflows;
+	WRITE_ONCE(q->perturb_period, perturb_period);
+	q->quantum = quantum;
+	q->flags = flags;
+	if (p)
+		swap(q->red_parms, p);
+
 	qlen = sch->q.qlen;
 	while (sch->q.qlen > q->limit) {
 		dropped += sfq_drop(sch, &to_free);