[PATCH v2] net/sched: Fix divide error in tabledist

[PATCH v2] net/sched: Fix divide error in tabledist

[Manas]

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

v1 -> v2: Fix whitespace
---
 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

Re: [PATCH v2] net/sched: Fix divide error in tabledist

[Cong Wang]

Hi Manas,

On Mon, Dec 22, 2025 at 11:43:06AM +0530, Manas wrote:
> 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

We are reverting the check_netem_in_tree() code due to user-space
breakage. I hope reverting it could also fix the bug you repported here?

See: https://lore.kernel.org/netdev/20251227194135.1111972-3-xiyou.wangcong@gmail.com/

Thanks!
Cong

Re: [PATCH v2] net/sched: Fix divide error in tabledist

[Manas Ghandat]

Hi Cong,

On 12/28/25 01:19, Cong Wang wrote:
> We are reverting the check_netem_in_tree() code due to user-space
> breakage. I hope reverting it could also fix the bug you repported here?
> 
> See: https://lore.kernel.org/netdev/20251227194135.1111972-3-xiyou.wangcong@gmail.com/
> 
> Thanks!
> Cong

Reverting the above patch would fix the bug which I have reported.

Re: [PATCH v2] net/sched: Fix divide error in tabledist

[Manas Ghandat]

Hi,

I just wanted an update since the patch has not been merged yet.

On 12/28/25 20:54, Manas Ghandat wrote:
> 
> Hi Cong,
> 
> On 12/28/25 01:19, Cong Wang wrote:
>> We are reverting the check_netem_in_tree() code due to user-space
>> breakage. I hope reverting it could also fix the bug you repported here?
>>
>> See: https://lore.kernel.org/netdev/20251227194135.1111972-3-xiyou.wangcong@gmail.com/
>>
>> Thanks!
>> Cong
> 
> Reverting the above patch would fix the bug which I have reported.

Re: [PATCH v2] net/sched: Fix divide error in tabledist

[Jamal Hadi Salim]

On Wed, Feb 25, 2026 at 11:35 AM Manas Ghandat <ghandatmanas@gmail.com> wrote:
>
> Hi,
>
> I just wanted an update since the patch has not been merged yet.
>

Patience please...

cheers,
jamal
> On 12/28/25 20:54, Manas Ghandat wrote:
> >
> > Hi Cong,
> >
> > On 12/28/25 01:19, Cong Wang wrote:
> >> We are reverting the check_netem_in_tree() code due to user-space
> >> breakage. I hope reverting it could also fix the bug you repported here?
> >>
> >> See: https://lore.kernel.org/netdev/20251227194135.1111972-3-xiyou.wangcong@gmail.com/
> >>
> >> Thanks!
> >> Cong
> >
> > Reverting the above patch would fix the bug which I have reported.
>