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    Subject[PATCH 5.18 065/181] net/sched: sch_netem: Fix arithmetic in netem_dump() for 32-bit platforms
    Date
    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 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;
    --
    2.35.1


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