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Subject: Re: [PATCH bpf-next V2 5/6] bpf: Add MTU check for TC-BPF packets
 after egress hook
To:     Jesper Dangaard Brouer <brouer@redhat.com>,
        John Fastabend <john.fastabend@gmail.com>
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        Jakub Kicinski <kuba@kernel.org>,
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On 10/8/20 10:07 AM, Jesper Dangaard Brouer wrote:
[...]
>>> However, due to both gso and vlan offload, even this is not trivial to do...
>>> The mtu is L3, but drivers/hardware/the wire usually care about L2...
> 
> If net_device->mtu is L3 (1500) and XDP (and TC, right?) operate at L2,
> that likely means that the "strict" bpf_mtu_check (in my BPF-helper) is
> wrong, as XDP (and TC) length at this point include the 14 bytes
> Ethernet header.  I will check and fix.

Yes, both at L2 layer.

> Is this accounted for via net_device->hard_header_len ?

It is, see also ether_setup().