[PATCH net v2 0/2] ets: use old 'nbands' while purging unused classes

- patch 1/2 fixes a NULL dereference in the control path of sch_ets qdisc
- patch 2/2 extends kselftests to verify effectiveness of the above fix

Changes since v1:
 - added a kselftest (thanks Victor) 

Davide Caratti (2):
  net/sched: ets: use old 'nbands' while purging unused classes
  selftests: net/forwarding: test purge of active DWRR classes

 net/sched/sch_ets.c                                   | 11 ++++++-----
 tools/testing/selftests/net/forwarding/sch_ets.sh     |  1 +
 .../testing/selftests/net/forwarding/sch_ets_tests.sh |  8 ++++++++
 3 files changed, 15 insertions(+), 5 deletions(-)

-- 
2.47.0

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/d912cbd7-193b-4269-9857-525bee8bbb6a@gmail.com/

Cc: stable@vger.kernel.org
Fixes: 103406b38c60 ("net/sched: Always pass notifications when child class becomes empty")
Fixes: c062f2a0b04d ("net/sched: sch_ets: don't remove idle classes from the round-robin list")
Fixes: dcc68b4d8084 ("net: sch_ets: Add a new Qdisc")
Reported-by: Li Shuang <redacted>
Closes: https://issues.redhat.com/browse/RHEL-108026
Reviewed-by: Petr Machata <petrm@nvidia.com>
Co-developed-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Davide Caratti <redacted>
---
 net/sched/sch_ets.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/net/sched/sch_ets.c b/net/sched/sch_ets.c
index 037f764822b9..82635dd2cfa5 100644
--- a/net/sched/sch_ets.c
+++ b/net/sched/sch_ets.c

@@ -651,6 +651,12 @@ 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) {

@@ -658,11 +664,6 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 			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));
 

-- 
2.47.0

Extend sch_ets.sh to add a reproducer for problematic list deletions when
active DWRR class are purged by ets_qdisc_change() [1] [2].

[1] https://lore.kernel.org/netdev/e08c7f4a6882f260011909a868311c6e9b54f3e4.1639153474.git.dcaratti@redhat.com/
[2] https://lore.kernel.org/netdev/f3b9bacc73145f265c19ab80785933da5b7cbdec.1754581577.git.dcaratti@redhat.com/

Suggested-by: Victor Nogueira <redacted>
Signed-off-by: Davide Caratti <redacted>
---
 tools/testing/selftests/net/forwarding/sch_ets.sh       | 1 +
 tools/testing/selftests/net/forwarding/sch_ets_tests.sh | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/tools/testing/selftests/net/forwarding/sch_ets.sh b/tools/testing/selftests/net/forwarding/sch_ets.sh
index 1f6f53e284b5..6269d5e23487 100755
--- a/tools/testing/selftests/net/forwarding/sch_ets.sh
+++ b/tools/testing/selftests/net/forwarding/sch_ets.sh

@@ -11,6 +11,7 @@ ALL_TESTS="
 	ets_test_strict
 	ets_test_mixed
 	ets_test_dwrr
+	ets_test_plug
 	classifier_mode
 	ets_test_strict
 	ets_test_mixed

diff --git a/tools/testing/selftests/net/forwarding/sch_ets_tests.sh b/tools/testing/selftests/net/forwarding/sch_ets_tests.sh
index 08240d3e3c87..79d837a2868a 100644
--- a/tools/testing/selftests/net/forwarding/sch_ets_tests.sh
+++ b/tools/testing/selftests/net/forwarding/sch_ets_tests.sh

@@ -224,3 +224,11 @@ ets_test_dwrr()
 	ets_set_dwrr_two_bands
 	xfail_on_slow ets_dwrr_test_01
 }
+
+ets_test_plug()
+{
+	ets_change_qdisc $put 2 "3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3" "1514 1514"
+	tc qdisc add dev $put handle 20: parent 10:4 plug
+	start_traffic_pktsize 100 $h1.10 192.0.2.1 192.0.2.2 00:c1:a0:c1:a0:00 "-c 1"
+	ets_qdisc_setup $put 2
+}

-- 
2.47.0

On 8/12/25 13:40, Davide Caratti wrote:

Extend sch_ets.sh to add a reproducer for problematic list deletions when
active DWRR class are purged by ets_qdisc_change() [1] [2].

[1] https://lore.kernel.org/netdev/e08c7f4a6882f260011909a868311c6e9b54f3e4.1639153474.git.dcaratti@redhat.com/
[2] https://lore.kernel.org/netdev/f3b9bacc73145f265c19ab80785933da5b7cbdec.1754581577.git.dcaratti@redhat.com/

Suggested-by: Victor Nogueira <redacted>
Signed-off-by: Davide Caratti <redacted>

LGTM, thanks!

Acked-by: Victor Nogueira <redacted>

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski [off-list ref]:

On Tue, 12 Aug 2025 18:40:28 +0200 you wrote:

- patch 1/2 fixes a NULL dereference in the control path of sch_ets qdisc
- patch 2/2 extends kselftests to verify effectiveness of the above fix

Changes since v1:
 - added a kselftest (thanks Victor)

Davide Caratti (2):
  net/sched: ets: use old 'nbands' while purging unused classes
  selftests: net/forwarding: test purge of active DWRR classes

[...]

Here is the summary with links:
  - [net,v2,1/2] net/sched: ets: use old 'nbands' while purging unused classes
    https://git.kernel.org/netdev/net/c/87c6efc5ce9c
  - [net,v2,2/2] selftests: net/forwarding: test purge of active DWRR classes
    https://git.kernel.org/netdev/net/c/774a2ae6617b

You are awesome, thank you!
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