# Re: \[PATCH net\] net: vlan: set header\_ops to match hard\_header\_len when hw offload is toggled

- From: Chen Zhen \<hidden\>
- Date: 2026-01-06 02:04:58 
- List: netdev
- Page: https://ratatoskr.run/netdev/2026/01/3353907
- Thread: https://ratatoskr.run/netdev/2025/12/3349438/t.md

```text
On 26/1/6 8:22, Jakub Kicinski wrote:
> On Wed, 31 Dec 2025 11:54:19 +0800 Chen Zhen wrote:
>> skbuff: skb_under_panic: text:ffffffff95b33e66 len:90 put:14 head:ffff915ac1967440 data:ffff915ac196743e tail:0x58 end:0x180 dev:br0.10
>> ------------[ cut here ]------------
>> kernel BUG at net/core/skbuff.c:197!
>> Call Trace:
>>  <TASK>
>>  skb_push+0x39/0x40
>>  eth_header+0x26/0xb0
>>  vlan_dev_hard_header+0x58/0x130 [8021q]
>>  neigh_connected_output+0xae/0x100
>>  ip6_finish_output2+0x2cc/0x650
>>  ? nf_hook_slow+0x41/0xc0
>>  ip6_finish_output+0x27/0xd0
>>  ndisc_send_skb+0x1d0/0x370
>>  ? __pfx_dst_output+0x10/0x10
>>  ndisc_send_ns+0x5a/0xb0
>>  addrconf_dad_work+0x2b5/0x380
>>  process_one_work+0x17f/0x320
> 
> Please run this stack trace thru script/decode_stacktrace
> and you can cut off here, no need to include functions
> below process_one_work, they are irrelevant.
> 
>>  worker_thread+0x26d/0x2f0
>>  ? __pfx_worker_thread+0x10/0x10
>>  kthread+0xcc/0x100
>>  ? __pfx_kthread+0x10/0x10
>>  ret_from_fork+0x30/0x50
>>  ? __pfx_kthread+0x10/0x10
>>  ret_from_fork_asm+0x1b/0x30
>>  </TASK>
>>
>> This bug can be easily reproduced by these steps:
>>
>>  ip link add veth0 type veth peer name veth1
>>  ip link set veth0 up
>>  ip link set veth1 up
>>  ethtool -K veth0 tx-vlan-hw-insert off
>>  # vlandev.header_ops = vlan_header_ops, hard_header_len = 18(hard_header_len + VLAN_HLEN)
>>  ip link add link veth0 name veth0.10 type vlan id 10 reorder_hdr off
>>  ip addr add 192.168.10.1/24 dev veth0.10
>>  ip link set veth0.10 up
>>  # vlandev.hard_header_len = 14(hard_header_len)
>>  ethtool -K veth0 tx-vlan-hw-insert on
>>  # Panic!
> 
> Instead of putting this in the commit message please add a selftest
> which will automatically catch re-occurrence of the issue.
> 
>> The reason is that when NETIF_F_HW_VLAN_CTAG_TX is off, vlandev.hard_header_len will be set to
>> dev->hard_header_len since commit 029f5fc31cdb ("8021q: set hard_header_len when VLAN offload features
>> are toggled"), but the header_ops remains unchanged. Then neigh_connected_output() will call
>> vlan_dev_hard_header() and panic in skb_push() because reorder_hdr is off.
> 
> Please wrap commit messages at 70 columns.

Thanks for your patient review and I will send patch v2 soon.

Best regards,
Chen Zhen
```