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Date:   Tue, 5 Oct 2021 08:41:03 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Cpp Code <cpp.code.lv@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        pshelar@....org, "David S. Miller" <davem@...emloft.net>,
        ovs dev <dev@...nvswitch.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v6] net: openvswitch: IPv6: Add IPv6 extension
 header support

Le 01/10/2021 à 22:42, Cpp Code a écrit :
> On Fri, Oct 1, 2021 at 12:21 AM Nicolas Dichtel
> <nicolas.dichtel@...nd.com> wrote:
>>
>> Le 30/09/2021 à 18:11, Cpp Code a écrit :
>>> On Wed, Sep 29, 2021 at 6:19 AM Jakub Kicinski <kuba@...nel.org> wrote:
>>>>
>>>> On Wed, 29 Sep 2021 08:19:05 +0200 Nicolas Dichtel wrote:
>>>>>> /* Insert a kernel only KEY_ATTR */
>>>>>> #define OVS_KEY_ATTR_TUNNEL_INFO    __OVS_KEY_ATTR_MAX
>>>>>> #undef OVS_KEY_ATTR_MAX
>>>>>> #define OVS_KEY_ATTR_MAX            __OVS_KEY_ATTR_MAX
>>>>> Following the other thread [1], this will break if a new app runs over an old
>>>>> kernel.
>>>>
>>>> Good point.
>>>>
>>>>> Why not simply expose this attribute to userspace and throw an error if a
>>>>> userspace app uses it?
>>>>
>>>> Does it matter if it's exposed or not? Either way the parsing policy
>>>> for attrs coming from user space should have a reject for the value.
>>>> (I say that not having looked at the code, so maybe I shouldn't...)
>>>
>>> To remove some confusion, there are some architectural nuances if we
>>> want to extend code without large refactor.
>>> The ovs_key_attr is defined only in kernel side. Userspace side is
>>> generated from this file. As well the code can be built without kernel
>>> modules.
>>> The code inside OVS repository and net-next is not identical, but I
>>> try to keep some consistency.
>> I didn't get why OVS_KEY_ATTR_TUNNEL_INFO cannot be exposed to userspace.
> 
> OVS_KEY_ATTR_TUNNEL_INFO is compressed version of OVS_KEY_ATTR_TUNNEL
> and for clarity purposes its not exposed to userspace as it will never
> use it.
> I would say it's a coding style as it would not brake anything if exposed.
In fact, it's the best way to keep the compatibility in the long term.
You can define it like this:
OVS_KEY_ATTR_TUNNEL_INFO,  /* struct ip_tunnel_info, reserved for kernel use */

> 
>>
>>>
>>> JFYI This is the file responsible for generating userspace part:
>>> https://github.com/openvswitch/ovs/blob/master/build-aux/extract-odp-netlink-h
>>> This is the how corresponding file for ovs_key_attr looks inside OVS:
>>> https://github.com/openvswitch/ovs/blob/master/datapath/linux/compat/include/linux/openvswitch.h
>>> one can see there are more values than in net-next version.
>> There are still some '#ifdef __KERNEL__'. The standard 'make headers_install'
>> filters them. Why not using this standard mechanism?
> 
> Could you elaborate on this, I don't quite understand the idea!? Which
> ifdef you are referring, the one along OVS_KEY_ATTR_TUNNEL_INFO or
> some other?
My understanding is that this file is used for the userland third party, thus,
theoretically, there should be no '#ifdef __KERNEL__'. uapi headers generated
with 'make headers_install' are filtered to remove them.

> 
>>
>> In this file, there are two attributes (OVS_KEY_ATTR_PACKET_TYPE and
>> OVS_KEY_ATTR_ND_EXTENSIONS) that doesn't exist in the kernel.
>> This will also breaks if an old app runs over a new kernel. I don't see how it
>> is possible to keep the compat between {old|new} {kernel|app}.
> 
> Looks like this most likely is a bug while working on multiple
> versions of code.  Need to do add more padding.
As said above, just define the same uapi for everybody and the problem is gone
forever.


Regards,
Nicolas

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