net/sched: ets: Remove drr class from the active list if it changes to strict

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: cd9b50adc6 ("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>

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));