Commit 986afaf8 authored Apr 17, 2026 by Stephen Hemminger Committed by Jakub Kicinski Apr 27, 2026

net/sched: netem: only reseed PRNG when seed is explicitly provided



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: 4072d97d ("netem: add prng attribute to netem_sched_data")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260418032027.900913-4-stephen@networkplumber.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent 4185701f

net/sched/sch_netem.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -1112,11 +1112,10 @@ 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);
		}

		unlock:
		sch_tree_unlock(sch);
		@@ -1139,6 +1138,9 @@ 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");