net/sched: fq_pie: re-factor fix for fq_pie endless loop

the patch that fixed an endless loop in_fq_pie_init() was not considering
that 65535 is a valid class id. The correct bugfix for this infinite loop
is to change 'idx' to become an u32, like Colin proposed in the past [1].

Fix this as follows:
 - restore 65536 as maximum possible values of 'flows_cnt'
 - use u32 'idx' when iterating on 'q->flows'
 - fix the TDC selftest

This reverts commit bb2f930.

[1] https://lore.kernel.org/netdev/20210407163808.499027-1-colin.king@canonical.com/

CC: Colin Ian King <colin.king@canonical.com>
CC: stable@vger.kernel.org
Fixes: bb2f930 ("net/sched: fix infinite loop in sch_fq_pie")
Fixes: ec97ecf ("net: sched: add Flow Queue PIE packet scheduler")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>

net/sched/sch_fq_pie.c

@@ -297,9 +297,9 @@ static int fq_pie_change(struct Qdisc *sch, struct nlattr *opt,
 			goto flow_error;
 		}
 		q->flows_cnt = nla_get_u32(tb[TCA_FQ_PIE_FLOWS]);
-		if (!q->flows_cnt || q->flows_cnt >= 65536) {
+		if (!q->flows_cnt || q->flows_cnt > 65536) {
 			NL_SET_ERR_MSG_MOD(extack,
-					   "Number of flows must range in [1..65535]");
+					   "Number of flows must range in [1..65536]");
 			goto flow_error;
 		}
 	}
@@ -367,7 +367,7 @@ static void fq_pie_timer(struct timer_list *t)
 	struct fq_pie_sched_data *q = from_timer(q, t, adapt_timer);
 	struct Qdisc *sch = q->sch;
 	spinlock_t *root_lock; /* to lock qdisc for probability calculations */
-	u16 idx;
+	u32 idx;
 
 	root_lock = qdisc_lock(qdisc_root_sleeping(sch));
 	spin_lock(root_lock);
@@ -388,7 +388,7 @@ static int fq_pie_init(struct Qdisc *sch, struct nlattr *opt,
 {
 	struct fq_pie_sched_data *q = qdisc_priv(sch);
 	int err;
-	u16 idx;
+	u32 idx;
 
 	pie_params_init(&q->p_params);
 	sch->limit = 10 * 1024;
@@ -500,7 +500,7 @@ static int fq_pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
 static void fq_pie_reset(struct Qdisc *sch)
 {
 	struct fq_pie_sched_data *q = qdisc_priv(sch);
-	u16 idx;
+	u32 idx;
 
 	INIT_LIST_HEAD(&q->new_flows);
 	INIT_LIST_HEAD(&q->old_flows);

tools/testing/selftests/tc-testing/tc-tests/qdiscs/fq_pie.json

@@ -9,11 +9,11 @@
         "setup": [
             "$IP link add dev $DUMMY type dummy || /bin/true"
         ],
-        "cmdUnderTest": "$TC qdisc add dev $DUMMY root fq_pie flows 65536",
-        "expExitCode": "2",
+        "cmdUnderTest": "$TC qdisc add dev $DUMMY handle 1: root fq_pie flows 65536",
+        "expExitCode": "0",
         "verifyCmd": "$TC qdisc show dev $DUMMY",
-        "matchPattern": "qdisc",
-        "matchCount": "0",
+        "matchPattern": "qdisc fq_pie 1: root refcnt 2 limit 10240p flows 65536",
+        "matchCount": "1",
         "teardown": [
             "$IP link del dev $DUMMY"
         ]