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

Commit Message

Peilin Ye June 16, 2022, 11:43 p.m. UTC

From: Peilin Ye <[e-mail]>

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 <[e-mail]>
Fixes: 112f9cb65643 ("netem: convert to qdisc_watchdog_schedule_ns")
Reviewed-by: Cong Wang <[e-mail]>
Signed-off-by: Peilin Ye <[e-mail]>
---
 net/sched/sch_netem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Stephen Hemminger June 17, 2022, 3:08 a.m. UTC | #1

On Thu, 16 Jun 2022 16:43:36 -0700
Peilin Ye <[e-mail]> wrote:

> From: Peilin Ye <[e-mail]>
> 
> 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 <[e-mail]>
> Fixes: 112f9cb65643 ("netem: convert to qdisc_watchdog_schedule_ns")
> Reviewed-by: Cong Wang <[e-mail]>
> Signed-off-by: Peilin Ye <[e-mail]>
> ---
>  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 <[e-mail]>

[e-mail] June 18, 2022, 3:40 a.m. UTC | #2

Hello:

This patch was applied to netdev/net.git (master)
by Jakub Kicinski <[e-mail]>:

On Thu, 16 Jun 2022 16:43:36 -0700 you wrote:
> From: Peilin Ye <[e-mail]>
> 
> 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
>                                                ^^^^^^^^^^^^^^^^
> 
> [...]

Here is the summary with links:
  - [net] net/sched: sch_netem: Fix arithmetic in netem_dump() for 32-bit platforms
    https://git.kernel.org/netdev/net/c/a2b1a5d40bd1

You are awesome, thank you!

Patch

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index ed4ccef5d6a8..5449ed114e40 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;