net/sched: sch_netem: Fix arithmetic in netem_dump() for 32-bit platforms

As reported by Yuming, currently tc always show a latency of UINT_MAX
for netem Qdisc's on 32-bit platforms:

    $ tc qdisc add dev dummy0 root netem latency 100ms
    $ tc qdisc show dev dummy0
    qdisc netem 8001: root refcnt 2 limit 1000 delay 275s  275s
                                               ^^^^^^^^^^^^^^^^

Let us take a closer look at netem_dump():

        qopt.latency = min_t(psched_tdiff_t, PSCHED_NS2TICKS(q->latency,
                             UINT_MAX);

qopt.latency is __u32, psched_tdiff_t is signed long,
(psched_tdiff_t)(UINT_MAX) is negative for 32-bit platforms, so
qopt.latency is always UINT_MAX.

Fix it by using psched_time_t (u64) instead.

Note: confusingly, users have two ways to specify 'latency':

  1. normally, via '__u32 latency' in struct tc_netem_qopt;
  2. via the TCA_NETEM_LATENCY64 attribute, which is s64.

For the second case, theoretically 'latency' could be negative.  This
patch ignores that corner case, since it is broken (i.e. assigning a
negative s64 to __u32) anyways, and should be handled separately.

Thanks Ted Lin for the analysis [1] .

[1] raspberrypi/linux#3512

Reported-by: Yuming Chen <[email protected]>
Fixes: 112f9cb ("netem: convert to qdisc_watchdog_schedule_ns")
Reviewed-by: Cong Wang <[email protected]>
Signed-off-by: Peilin Ye <[email protected]>
Acked-by: Stephen Hemminger <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: peilin-ye

net/sched/sch_netem.c

@@ -1146,9 +1146,9 @@ static int netem_dump(struct Qdisc *sch, struct sk_buff *skb)
 	struct tc_netem_rate rate;
 	struct tc_netem_slot slot;
 
-	qopt.latency = min_t(psched_tdiff_t, PSCHED_NS2TICKS(q->latency),
+	qopt.latency = min_t(psched_time_t, PSCHED_NS2TICKS(q->latency),
 			     UINT_MAX);
-	qopt.jitter = min_t(psched_tdiff_t, PSCHED_NS2TICKS(q->jitter),
+	qopt.jitter = min_t(psched_time_t, PSCHED_NS2TICKS(q->jitter),
 			    UINT_MAX);
 	qopt.limit = q->limit;
 	qopt.loss = q->loss;