[PATCH 5.4 20/60] net/sched: sch_netem: Fix arithmetic in netem_dump() for 32-bit platforms

Greg Kroah-Hartman
2022-06-27T11:31:26Z

From: Peilin Ye <peilin.ye@bytedance.com>

[ Upstream commit a2b1a5d40bd12b44322c2ccd40bb0ec1699708b6 ]

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] https://github.com/raspberrypi/linux/issues/3512

Reported-by: Yuming Chen <chenyuming.junnan@bytedance.com>
Fixes: 112f9cb65643 ("netem: convert to qdisc_watchdog_schedule_ns")
Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Link: https://lore.kernel.org/r/20220616234336.2443-1-yepeilin.cs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/sched/sch_netem.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index f4101a920d1f..1802f134aa40 100644
--- a/net/sched/sch_netem.c
+++ b/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;
--
2.35.1