[PATCH net v2 09/10] net/sched: netem: only reseed PRNG when seed is explicitly provided

[Stephen Hemminger <stephen@networkplumber.org>]

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 <stephen@networkplumber.org>
---
 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 3bed25f494d3..46f57828ee1c 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -1123,11 +1123,10 @@ 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);
@@ -1150,6 +1149,9 @@ 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");
-- 
2.51.0