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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , =?UTF-8?q?Fran=C3=A7ois=20Michel?= , linux-kernel@vger.kernel.org (open list) Subject: [PATCH net 3/5] net/sched: netem: only reseed PRNG when seed is explicitly provided Date: Sat, 28 Mar 2026 11:21:47 -0700 Message-ID: <20260328182336.392817-4-stephen@networkplumber.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260328182336.392817-1-stephen@networkplumber.org> References: <20260328182336.392817-1-stephen@networkplumber.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit netem_change() unconditionally reseeds the PRNG on every tc change command. If TCA_NETEM_PRNG_SEED is not specified, a new random seed is generated, destroying reproducibility for users who set a deterministic seed on a previous change. Move the initial random seed generation to netem_init() and only reseed in netem_change() when TCA_NETEM_PRNG_SEED is explicitly provided by the user. Fixes: 4072d97ddc44 ("netem: add prng attribute to netem_sched_data") Signed-off-by: Stephen Hemminger --- net/sched/sch_netem.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c index 6cc48b698e48..448097fc88a4 100644 --- a/net/sched/sch_netem.c +++ b/net/sched/sch_netem.c @@ -1111,11 +1111,10 @@ static int netem_change(struct Qdisc *sch, struct nlattr *opt, /* capping jitter to the range acceptable by tabledist() */ q->jitter = min_t(s64, abs(q->jitter), INT_MAX); - if (tb[TCA_NETEM_PRNG_SEED]) + if (tb[TCA_NETEM_PRNG_SEED]) { q->prng.seed = nla_get_u64(tb[TCA_NETEM_PRNG_SEED]); - else - q->prng.seed = get_random_u64(); - prandom_seed_state(&q->prng.prng_state, q->prng.seed); + prandom_seed_state(&q->prng.prng_state, q->prng.seed); + } unlock: sch_tree_unlock(sch); @@ -1138,6 +1137,9 @@ static int netem_init(struct Qdisc *sch, struct nlattr *opt, return -EINVAL; q->loss_model = CLG_RANDOM; + q->prng.seed = get_random_u64(); + prandom_seed_state(&q->prng.prng_state, q->prng.seed); + ret = netem_change(sch, opt, extack); if (ret) pr_info("netem: change failed\n"); -- 2.53.0