Re: [PATCH net] net/sched: ets: use old 'nbands' while purging unused classes

On 8/7/25 12:48, Davide Caratti wrote:

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/ (local)
  [2] https://lore.kernel.org/netdev/d912cbd7-193b-4269-9857-525bee8bbb6a@gmail.com/ (local)

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
Co-developed-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Davide Caratti <redacted>

Can you submit a tdc test case for this bug?

cheers,
Victor