netdev - Re: [PATCH net-next 0/6] net: reset MAC header consistently across L3 virtual devices

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Date:   Mon, 28 Jun 2021 12:46:24 -0600
From:   David Ahern <dsahern@...il.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Simon Horman <simon.horman@...ronome.com>,
        Martin Varghese <martin.varghese@...ia.com>,
        Eli Cohen <elic@...dia.com>, Jiri Benc <jbenc@...hat.com>,
        Tom Herbert <tom@...bertland.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Harald Welte <laforge@...monks.org>,
        Andreas Schultz <aschultz@...p.net>,
        Jonas Bonn <jonas@...rbonn.se>
Subject: Re: [PATCH net-next 0/6] net: reset MAC header consistently across L3
 virtual devices

On 6/28/21 9:04 AM, Guillaume Nault wrote:
> On Sun, Jun 27, 2021 at 09:56:53AM -0600, David Ahern wrote:
>> On 6/26/21 2:53 PM, Guillaume Nault wrote:
>>> On Sat, Jun 26, 2021 at 11:50:19AM -0600, David Ahern wrote:
>>>> On 6/25/21 7:32 AM, Guillaume Nault wrote:
>>>>> Some virtual L3 devices, like vxlan-gpe and gre (in collect_md mode),
>>>>> reset the MAC header pointer after they parsed the outer headers. This
>>>>> accurately reflects the fact that the decapsulated packet is pure L3
>>>>> packet, as that makes the MAC header 0 bytes long (the MAC and network
>>>>> header pointers are equal).
>>>>>
>>>>> However, many L3 devices only adjust the network header after
>>>>> decapsulation and leave the MAC header pointer to its original value.
>>>>> This can confuse other parts of the networking stack, like TC, which
>>>>> then considers the outer headers as one big MAC header.
>>>>>
>>>>> This patch series makes the following L3 tunnels behave like VXLAN-GPE:
>>>>> bareudp, ipip, sit, gre, ip6gre, ip6tnl, gtp.
>>>>>
>>>>> The case of gre is a bit special. It already resets the MAC header
>>>>> pointer in collect_md mode, so only the classical mode needs to be
>>>>> adjusted. However, gre also has a special case that expects the MAC
>>>>> header pointer to keep pointing to the outer header even after
>>>>> decapsulation. Therefore, patch 4 keeps an exception for this case.
>>>>>
>>>>> Ideally, we'd centralise the call to skb_reset_mac_header() in
>>>>> ip_tunnel_rcv(), to avoid manual calls in ipip (patch 2),
>>>>> sit (patch 3) and gre (patch 4). That's unfortunately not feasible
>>>>> currently, because of the gre special case discussed above that
>>>>> precludes us from resetting the MAC header unconditionally.
>>>>
>>>> What about adding a flag to ip_tunnel indicating if it can be done (or
>>>> should not be done since doing it is the most common)?
>>>
>>> That's feasible. I didn't do it here because I wanted to keep the
>>> patch series focused on L3 tunnels. Modifying ip_tunnel_rcv()'s
>>> prototype would also require updating erspan_rcv(), which isn't L3
>>> (erspan carries Ethernet frames). I was feeling such consolidation
>>> would be best done in a follow up patch series.
>>
>> I was thinking a flag in 'struct ip_tunnel'. It's the private data for
>> those netdevices, so a per-instance setting. I haven't walked through
>> the details to know if it would work.
> 
> I didn't think about that. Good idea, that looks perfectly doable. But
> I'd still prefer to centralise the skb_reset_mac_header() call in a
> dedicated patch set. I we did it here, we'd have to not reset the mac
> header by default, to guarantee that unrelated tunnels wouldn't be
> affected.
> However, I think that the default behaviour should be to reset the mac
> header and that only special cases, like the one in ip_gre, should
> explicitely turn that off. Therefore, we'd need a follow up patch
> anyway, to change the way this "reset_mac" flag would be set.
> 
> IMHO, the current approach has the advantage of clearly separating the
> new feature from the refactoring. But if you feel strongly about using
> a flag in struct ip_tunnel, I can rework this series.
> 

I am accustomed to doing refactoring first to add some new feature in
the simplest and clearest way.