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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Yousuk Seung , Neal Cardwell , linux-kernel@vger.kernel.org (open list) Subject: [PATCH net v4 8/8] net/sched: netem: fix slot delay calculation overflow Date: Mon, 6 Apr 2026 10:25:16 -0700 Message-ID: <20260406172627.210894-9-stephen@networkplumber.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260406172627.210894-1-stephen@networkplumber.org> References: <20260406172627.210894-1-stephen@networkplumber.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit get_slot_next() computes a random delay between min_delay and max_delay using: get_random_u32() * (max_delay - min_delay) >> 32 This overflows signed 64-bit arithmetic when the delay range exceeds approximately 2.1 seconds (2^31 nanoseconds), producing a negative result that effectively disables slot-based pacing. This is a realistic configuration for WAN emulation (e.g., slot 1s 5s). Use mul_u64_u32_shr() which handles the widening multiply without overflow. Fixes: 0a9fe5c375b5 ("netem: slotting with non-uniform distribution") Signed-off-by: Stephen Hemminger Reviewed-by: Simon Horman --- net/sched/sch_netem.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c index c82f76af41aa..d47e1b0d7942 100644 --- a/net/sched/sch_netem.c +++ b/net/sched/sch_netem.c @@ -660,9 +660,8 @@ static void get_slot_next(struct netem_sched_data *q, u64 now) if (!q->slot_dist) next_delay = q->slot_config.min_delay + - (get_random_u32() * - (q->slot_config.max_delay - - q->slot_config.min_delay) >> 32); + mul_u64_u32_shr(q->slot_config.max_delay - q->slot_config.min_delay, + get_random_u32(), 32); else next_delay = tabledist(q->slot_config.dist_delay, (s32)(q->slot_config.dist_jitter), -- 2.53.0