[PATCH 6.12 058/567] net/sched: ets: Remove drr class from the active list if it changes to strict

Greg Kroah-Hartman
2026-01-06T17:17:28Z

From: Victor Nogueira <victor@mojatatu.com>

[ Upstream commit b1e125ae425aba9b45252e933ca8df52a843ec70 ]

Whenever a user issues an ets qdisc change command, transforming a
drr class into a strict one, the ets code isn't checking whether that
class was in the active list and removing it. This means that, if a
user changes a strict class (which was in the active list) back to a drr
one, that class will be added twice to the active list [1].

Doing so with the following commands:

tc qdisc add dev lo root handle 1: ets bands 2 strict 1
tc qdisc add dev lo parent 1:2 handle 20: \
tbf rate 8bit burst 100b latency 1s
tc filter add dev lo parent 1: basic classid 1:2
ping -c1 -W0.01 -s 56 127.0.0.1
tc qdisc change dev lo root handle 1: ets bands 2 strict 2
tc qdisc change dev lo root handle 1: ets bands 2 strict 1
ping -c1 -W0.01 -s 56 127.0.0.1

Will trigger the following splat with list debug turned on:

[ 59.279014][ T365] ------------[ cut here ]------------
[ 59.279452][ T365] list_add double add: new=ffff88801d60e350, prev=ffff88801d60e350, next=ffff88801d60e2c0.
[ 59.280153][ T365] WARNING: CPU: 3 PID: 365 at lib/list_debug.c:35 __list_add_valid_or_report+0x17f/0x220
[ 59.280860][ T365] Modules linked in:
[ 59.281165][ T365] CPU: 3 UID: 0 PID: 365 Comm: tc Not tainted 6.18.0-rc7-00105-g7e9f13163c13-dirty #239 PREEMPT(voluntary)
[ 59.281977][ T365] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 59.282391][ T365] RIP: 0010:__list_add_valid_or_report+0x17f/0x220
[ 59.282842][ T365] Code: 89 c6 e8 d4 b7 0d ff 90 0f 0b 90 90 31 c0 e9 31 ff ff ff 90 48 c7 c7 e0 a0 22 9f 48 89 f2 48 89 c1 4c 89 c6 e8 b2 b7 0d ff 90 <0f> 0b 90 90 31 c0 e9 0f ff ff ff 48 89 f7 48 89 44 24 10 4c 89 44
...
[ 59.288812][ T365] Call Trace:
[ 59.289056][ T365] <TASK>
[ 59.289224][ T365] ? srso_alias_return_thunk+0x5/0xfbef5
[ 59.289546][ T365] ets_qdisc_change+0xd2b/0x1e80
[ 59.289891][ T365] ? __lock_acquire+0x7e7/0x1be0
[ 59.290223][ T365] ? __pfx_ets_qdisc_change+0x10/0x10
[ 59.290546][ T365] ? srso_alias_return_thunk+0x5/0xfbef5
[ 59.290898][ T365] ? __mutex_trylock_common+0xda/0x240
[ 59.291228][ T365] ? __pfx___mutex_trylock_common+0x10/0x10
[ 59.291655][ T365] ? srso_alias_return_thunk+0x5/0xfbef5
[ 59.291993][ T365] ? srso_alias_return_thunk+0x5/0xfbef5
[ 59.292313][ T365] ? trace_contention_end+0xc8/0x110
[ 59.292656][ T365] ? srso_alias_return_thunk+0x5/0xfbef5
[ 59.293022][ T365] ? srso_alias_return_thunk+0x5/0xfbef5
[ 59.293351][ T365] tc_modify_qdisc+0x63a/0x1cf0

Fix this by always checking and removing an ets class from the active list
when changing it to strict.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/tree/net/sched/sch_ets.c?id=ce052b9402e461a9aded599f5b47e76bc727f7de#n663

Fixes: cd9b50adc6bb9 ("net/sched: ets: fix crash when flipping from 'strict' to 'quantum'")
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20251208190125.1868423-1-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/sched/sch_ets.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/net/sched/sch_ets.c b/net/sched/sch_ets.c
index ae46643e596d3..306e046276d46 100644
--- a/net/sched/sch_ets.c
+++ b/net/sched/sch_ets.c
@@ -664,6 +664,10 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
q->classes[i].deficit = quanta[i];
}
}
+ for (i = q->nstrict; i < nstrict; i++) {
+ if (cl_is_active(&q->classes[i]))
+ list_del_init(&q->classes[i].alist);
+ }
WRITE_ONCE(q->nstrict, nstrict);
memcpy(q->prio2band, priomap, sizeof(priomap));

--
2.51.0