net/sched: ets: use old 'nbands' while purging unused classes

JIRA: https://issues.redhat.com/browse/RHEL-107519

commit 87c6efc
Author: Davide Caratti <[email protected]>
Date:   Tue Aug 12 18:40:29 2025 +0200

    net/sched: ets: use old 'nbands' while purging unused classes

    Shuang reported sch_ets test-case [1] crashing in ets_class_qlen_notify()
    after recent changes from Lion [2]. The problem is: in ets_qdisc_change()
    we purge unused DWRR queues; the value of 'q->nbands' is the new one, and
    the cleanup should be done with the old one. The problem is here since my
    first attempts to fix ets_qdisc_change(), but it surfaced again after the
    recent qdisc len accounting fixes. Fix it purging idle DWRR queues before
    assigning a new value of 'q->nbands', so that all purge operations find a
    consistent configuration:

     - old 'q->nbands' because it's needed by ets_class_find()
     - old 'q->nstrict' because it's needed by ets_class_is_strict()

     BUG: kernel NULL pointer dereference, address: 0000000000000000
     #PF: supervisor read access in kernel mode
     #PF: error_code(0x0000) - not-present page
     PGD 0 P4D 0
     Oops: Oops: 0000 [#1] SMP NOPTI
     CPU: 62 UID: 0 PID: 39457 Comm: tc Kdump: loaded Not tainted 6.12.0-116.el10.x86_64 #1 PREEMPT(voluntary)
     Hardware name: Dell Inc. PowerEdge R640/06DKY5, BIOS 2.12.2 07/09/2021
     RIP: 0010:__list_del_entry_valid_or_report+0x4/0x80
     Code: ff 4c 39 c7 0f 84 39 19 8e ff b8 01 00 00 00 c3 cc cc cc cc 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa <48> 8b 17 48 8b 4f 08 48 85 d2 0f 84 56 19 8e ff 48 85 c9 0f 84 ab
     RSP: 0018:ffffba186009f400 EFLAGS: 00010202
     RAX: 00000000000000d6 RBX: 0000000000000000 RCX: 0000000000000004
     RDX: ffff9f0fa29b69c0 RSI: 0000000000000000 RDI: 0000000000000000
     RBP: ffffffffc12c2400 R08: 0000000000000008 R09: 0000000000000004
     R10: ffffffffffffffff R11: 0000000000000004 R12: 0000000000000000
     R13: ffff9f0f8cfe0000 R14: 0000000000100005 R15: 0000000000000000
     FS:  00007f2154f37480(0000) GS:ffff9f269c1c0000(0000) knlGS:0000000000000000
     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
     CR2: 0000000000000000 CR3: 00000001530be001 CR4: 00000000007726f0
     DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
     DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
     PKRU: 55555554
     Call Trace:
      <TASK>
      ets_class_qlen_notify+0x65/0x90 [sch_ets]
      qdisc_tree_reduce_backlog+0x74/0x110
      ets_qdisc_change+0x630/0xa40 [sch_ets]
      __tc_modify_qdisc.constprop.0+0x216/0x7f0
      tc_modify_qdisc+0x7c/0x120
      rtnetlink_rcv_msg+0x145/0x3f0
      netlink_rcv_skb+0x53/0x100
      netlink_unicast+0x245/0x390
      netlink_sendmsg+0x21b/0x470
      ____sys_sendmsg+0x39d/0x3d0
      ___sys_sendmsg+0x9a/0xe0
      __sys_sendmsg+0x7a/0xd0
      do_syscall_64+0x7d/0x160
      entry_SYSCALL_64_after_hwframe+0x76/0x7e
     RIP: 0033:0x7f2155114084
     Code: 89 02 b8 ff ff ff ff eb bb 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa 80 3d 25 f0 0c 00 00 74 13 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 48 83 ec 28 89 54 24 1c 48 89
     RSP: 002b:00007fff1fd7a988 EFLAGS: 00000202 ORIG_RAX: 000000000000002e
     RAX: ffffffffffffffda RBX: 0000560ec063e5e0 RCX: 00007f2155114084
     RDX: 0000000000000000 RSI: 00007fff1fd7a9f0 RDI: 0000000000000003
     RBP: 00007fff1fd7aa60 R08: 0000000000000010 R09: 000000000000003f
     R10: 0000560ee9b3a010 R11: 0000000000000202 R12: 00007fff1fd7aae0
     R13: 000000006891ccde R14: 0000560ec063e5e0 R15: 00007fff1fd7aad0
      </TASK>

     [1] https://lore.kernel.org/netdev/e08c7f4a6882f260011909a868311c6e9b54f3e4.1639153474.git.dcaratti@redhat.com/
     [2] https://lore.kernel.org/netdev/[email protected]/

    Cc: [email protected]
    Fixes: 103406b ("net/sched: Always pass notifications when child class becomes empty")
    Fixes: c062f2a ("net/sched: sch_ets: don't remove idle classes from the round-robin list")
    Fixes: dcc68b4 ("net: sch_ets: Add a new Qdisc")
    Reported-by: Li Shuang <[email protected]>
    Closes: https://issues.redhat.com/browse/RHEL-108026
    Reviewed-by: Petr Machata <[email protected]>
    Co-developed-by: Ivan Vecera <[email protected]>
    Signed-off-by: Ivan Vecera <[email protected]>
    Signed-off-by: Davide Caratti <[email protected]>
    Link: https://patch.msgid.link/7928ff6d17db47a2ae7cc205c44777b1f1950545.1755016081.git.dcaratti@redhat.com
    Signed-off-by: Jakub Kicinski <[email protected]>

Signed-off-by: Ivan Vecera <[email protected]>

Author: ivecera

net/sched/sch_ets.c

@@ -651,18 +651,19 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 
 	sch_tree_lock(sch);
 
+	for (i = nbands; i < oldbands; i++) {
+		if (i >= q->nstrict && q->classes[i].qdisc->q.qlen)
+			list_del_init(&q->classes[i].alist);
+		qdisc_purge_queue(q->classes[i].qdisc);
+	}
+
 	WRITE_ONCE(q->nbands, nbands);
 	for (i = nstrict; i < q->nstrict; i++) {
 		if (q->classes[i].qdisc->q.qlen) {
 			list_add_tail(&q->classes[i].alist, &q->active);
 			q->classes[i].deficit = quanta[i];
 		}
 	}
-	for (i = q->nbands; i < oldbands; i++) {
-		if (i >= q->nstrict && q->classes[i].qdisc->q.qlen)
-			list_del_init(&q->classes[i].alist);
-		qdisc_purge_queue(q->classes[i].qdisc);
-	}
 	WRITE_ONCE(q->nstrict, nstrict);
 	memcpy(q->prio2band, priomap, sizeof(priomap));