linux-kernel - net/sched: Fix divide error in tabledist

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Message-ID: <f69b2c8f-8325-4c2e-a011-6dbc089f30e4@gmail.com>
Date: Thu, 11 Dec 2025 23:36:32 +0530
From: Manas Ghandat <ghandatmanas@...il.com>
To: stephen@...workplumber.org
Cc: xiyou.wangcong@...il.com, Jiri Pirko <jiri@...nulli.us>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: net/sched: Fix divide error in tabledist

Previously, a duplication check was added to ensure that a
duplicating netem cannot exist in a tree with other netems. When
check_netem_in_tree() fails after parameter updates, the qdisc
structure is left in an inconsistent state with some new values
applied but duplicate not updated. Move the tree validation check
before modifying any qdisc parameters
Fixes: ec8e0e3d7ade ("net/sched: Restrict conditions for adding 
duplicating netems to qdisc tree")
Reported-by: Manas Ghandat <ghandatmanas@...il.com>
Signed-off-by: Manas Ghandat <ghandatmanas@...il.com>
---
net/sched/sch_netem.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 32a5f3304046..1a2b498ada83 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -1055,6 +1055,11 @@ static int netem_change(struct Qdisc *sch, struct 
nlattr *opt,
q->loss_model = CLG_RANDOM;
}
+ ret = check_netem_in_tree(sch, qopt->duplicate, extack);
+ if (ret)
+ goto unlock;
+ q->duplicate = qopt->duplicate;
+
if (delay_dist)
swap(q->delay_dist, delay_dist);
if (slot_dist)
@@ -1068,12 +1073,6 @@ static int netem_change(struct Qdisc *sch, struct 
nlattr *opt,
q->counter = 0;
q->loss = qopt->loss;
- ret = check_netem_in_tree(sch, qopt->duplicate, extack);
- if (ret)
- goto unlock;
-
- q->duplicate = qopt->duplicate;
-
/* for compatibility with earlier versions.
* if gap is set, need to assume 100% probability
*/
-- 
2.43.0