netdev - Re: [PATCH net v2] net: Reset MAC header for direct packet transmission

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Date:   Mon, 29 Mar 2021 14:41:59 +0200
From:   Julian Wiedmann <jwi@...ux.ibm.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Kurt Kanzenbach <kurt@...utronix.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Antoine Tenart <atenart@...nel.org>,
        Wei Wang <weiwan@...gle.com>,
        Cong Wang <cong.wang@...edance.com>,
        Taehee Yoo <ap420073@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net v2] net: Reset MAC header for direct packet
 transmission

On 29.03.21 14:13, Eric Dumazet wrote:
> On Mon, Mar 29, 2021 at 12:30 PM Kurt Kanzenbach <kurt@...utronix.de> wrote:
>>
>> On Mon Mar 29 2021, Eric Dumazet wrote:
>>> Note that last year, I addressed the issue differently in commit
>>> 96cc4b69581db68efc9749ef32e9cf8e0160c509
>>> ("macvlan: do not assume mac_header is set in macvlan_broadcast()")
>>> (amended with commit 1712b2fff8c682d145c7889d2290696647d82dab
>>> "macvlan: use skb_reset_mac_header() in macvlan_queue_xmit()")
>>>
>>> My reasoning was that in TX path, when ndo_start_xmit() is called, MAC
>>> header is essentially skb->data,
>>> so I was hoping to _remove_ skb_reset_mac_header(skb) eventually from
>>> the fast path (aka __dev_queue_xmit),
>>> because most drivers do not care about MAC header, they just use skb->data.
>>>
>>> I understand it is more difficult to review drivers instead of just
>>> adding more code in  __dev_direct_xmit()
>>>
>>> In hsr case, I do not really see why the existing check can not be
>>> simply reworked ?
>>
>> It can be reworked, no problem. I just thought it might be better to add
>> it to the generic code just in case there are more drivers suffering
>> from the issue.
> 
> Note that I have a similar issue pending in ipvlan.
> 
> Still, I think I prefer the non easy way to not add more stuff in fast path.
> 

Can we apply this fix (and propagate it to stable), and then remove the
skb_reset_mac_header() from _both_ xmit paths through net-next?