Re: [PATCH net] net/sched: sch_netem: Fix arithmetic in netem_dump() for 32-bit platforms
From: Stephen Hemminger
Date: Thu Jun 16 2022 - 23:08:42 EST
On Thu, 16 Jun 2022 16:43:36 -0700
Peilin Ye <yepeilin.cs@xxxxxxxxx> wrote:
> From: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> Fixes: 112f9cb65643 ("netem: convert to qdisc_watchdog_schedule_ns")
> Reviewed-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> Signed-off-by: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>
> ---
> net/sched/sch_netem.c | 4 ++--
Thanks for fixing.
Guess it is time to run netem on one of the Pi's.
Acked-by: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>