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

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 16 Jun 2022 16:43:36 -0700
Peilin Ye <yepeilin.cs@gmail.com> wrote:

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