Re: MRP netlink interface

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 25/05/2020 13:03, Michal Kubecek wrote:
> On Mon, May 25, 2020 at 11:28:27AM +0000, Horatiu Vultur wrote:
> [...]
>> My first approach was to extend the 'struct br_mrp_instance' with a field that
>> contains the priority of the node. But this breaks the backwards compatibility,
>> and then every time when I need to change something, I will break the backwards
>> compatibility. Is this a way to go forward?
> 
> No, I would rather say it's an example showing why passing data
> structures as binary data via netlink is a bad idea. I definitely
> wouldn't advice this approach for any new interface. One of the
> strengths of netlink is the ability to use structured and extensible
> messages.
> 
>> Another approach is to restructure MRP netlink interface. What I was thinking to
>> keep the current attributes (IFLA_BRIDGE_MRP_INSTANCE,
>> IFLA_BRIDGE_MRP_PORT_STATE,...) but they will be nested attributes and each of
>> this attribute to contain the fields of the structures they represents.
>> For example:
>> [IFLA_AF_SPEC] = {
>>     [IFLA_BRIDGE_FLAGS]
>>     [IFLA_BRIDGE_MRP]
>>         [IFLA_BRIDGE_MRP_INSTANCE]
>>             [IFLA_BRIDGE_MRP_INSTANCE_RING_ID]
>>             [IFLA_BRIDGE_MRP_INSTANCE_P_IFINDEX]
>>             [IFLA_BRIDGE_MRP_INSTANCE_S_IFINDEX]
>>         [IFLA_BRIDGE_MRP_RING_ROLE]
>>             [IFLA_BRIDGE_MRP_RING_ROLE_RING_ID]
>>             [IFLA_BRIDGE_MRP_RING_ROLE_ROLE]
>>         ...
>> }
>> And then I can parse each field separately and then fill up the structure
>> (br_mrp_instance, br_mrp_port_role, ...) which will be used forward.
>> Then when this needs to be extended with the priority it would have the
>> following format:
>> [IFLA_AF_SPEC] = {
>>     [IFLA_BRIDGE_FLAGS]
>>     [IFLA_BRIDGE_MRP]
>>         [IFLA_BRIDGE_MRP_INSTANCE]
>>             [IFLA_BRIDGE_MRP_INSTANCE_RING_ID]
>>             [IFLA_BRIDGE_MRP_INSTANCE_P_IFINDEX]
>>             [IFLA_BRIDGE_MRP_INSTANCE_S_IFINDEX]
>>             [IFLA_BRIDGE_MRP_INSTANCE_PRIO]
>>         [IFLA_BRIDGE_MRP_RING_ROLE]
>>             [IFLA_BRIDGE_MRP_RING_ROLE_RING_ID]
>>             [IFLA_BRIDGE_MRP_RING_ROLE_ROLE]
>>         ...
>> }
>> And also the br_mrp_instance will have a field called prio.
>> So now, if the userspace is not updated to have support for setting the prio
>> then the kernel will use a default value. Then if the userspace contains a field
>> that the kernel doesn't know about, then it would just ignore it.
>> So in this way every time when the netlink interface will be extended it would
>> be backwards compatible.
> 
> Silently ignoring unrecognized attributes in userspace requests is what
> most kernel netlink based interfaces have been doing traditionally but
> it's not really a good idea. Essentially it ties your hands so that you
> can only add new attributes which can be silently ignored without doing
> any harm, otherwise you risk that kernel will do something different
> than userspace asked and userspace does not even have a way to find out
> if the feature is supported or not. (IIRC there are even some places
> where ignoring an attribute changes the nature of the request but it is
> still ignored by older kernels.)
> 
> That's why there have been an effort, mostly by Johannes Berg, to
> introduce and promote strict checking for new netlink interfaces and new
> attributes in existing netlink attributes. If you don't have strict
> checking for unknown attributes enabled yet, there isn't much that can
> be done for already released kernels but I would suggest to enable it as
> soon as possible.
> 
> Michal
> 

+1, we don't have strict checking for the bridge main af spec attributes, but
you could add that for new nested interfaces that need to be parsed like the
above