Re: [PATCH net-next] net: xdp: don't assume an Ethernet header in generic XDP

[Alexander Lobakin]

From: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
Date: Thu, 13 Aug 2026 19:32:24 +0800

> Kenneth Lee reported a bug [1]:
> 
>   skb_vlan_push got skb with skb->data not at mac header (offset 14)
>   WARNING: net/core/skbuff.c:6474 at skb_vlan_push+0x5ec/0x8a0
>    ____bpf_skb_vlan_push net/core/filter.c:3239 [inline]
>    bpf_skb_vlan_push+0x216/0x8b0 net/core/filter.c:3229
>    bpf_prog_run_data_pointers+0x17c/0x240 include/linux/filter.h:917
>    tcf_bpf_act+0x31c/0x940 net/sched/act_bpf.c:50
>    tcf_action_exec+0x2c1/0x720 net/sched/act_api.c:1147
>    tcf_classify+0x6e4/0x1080 net/sched/cls_api.c:1860
>    tc_run+0x31c/0x5d0 net/core/dev.c:4411
>    sch_handle_ingress net/core/dev.c:4486 [inline]
>    __netif_receive_skb_core+0x141b/0x2ec0 net/core/dev.c:6054
> 
> bpf_prog_run_generic_xdp() just assumes xdp->data is an Ethernet header:
Most XDP programs expect Ethernet header at the beginning of a frame.
Unless you write a custom one which doesn't.
But then you may face that XDP_TX, XDP_REDIRECT won't work properly --
cpumap Rx, each .ndo_xdp_xmit() implementation -- all expect Ethernet
header at the beginning.

Thanks,
Olek