netdev - Re: [PATCH bpf-next V2 5/6] bpf: Add MTU check for TC-BPF packets after egress hook

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Date:   Thu, 8 Oct 2020 10:25:51 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     Maciej Żenczykowski <maze@...gle.com>,
        bpf <bpf@...r.kernel.org>, Linux NetDev <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Shaun Crampton <shaun@...era.io>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Marek Majkowski <marek@...udflare.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Eyal Birger <eyal.birger@...il.com>
Subject: Re: [PATCH bpf-next V2 5/6] bpf: Add MTU check for TC-BPF packets
 after egress hook

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().