netdev - Re: How to display IPv4 array?

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Date: Tue, 12 Mar 2024 16:04:56 +0000
From: Donald Hunter <donald.hunter@...il.com>
To: Jiri Pirko <jiri@...nulli.us>
Cc: Hangbin Liu <liuhangbin@...il.com>, Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: How to display IPv4 array?

On Tue, 12 Mar 2024 at 12:12, Jiri Pirko <jiri@...nulli.us> wrote:
>
> Tue, Mar 12, 2024 at 11:08:33AM CET, liuhangbin@...il.com wrote:
> >Hi Jakub,
> >
> >I plan to add bond support for Documentation/netlink/specs/rt_link.yaml. While
> >dealing with the attrs. I got a problem about how to show the bonding arp/ns
> >targets. Because the arp/ns targets are filled as an array[1]. I tried
> >something like:
> >
> >  -
> >    name: linkinfo-bond-attrs
> >    name-prefix: ifla-bond-
> >    attributes:
> >      -
> >        name: arp-ip-target
> >        type: nest
> >        nested-attributes: ipv4-addr
> >  -
> >    name: ipv4-addr
> >    attributes:
> >      -
> >        name: addr
> >        type: binary
> >        display-hint: ipv4
> >
> >But this failed with error: Exception: Space 'ipv4-addr' has no attribute with value '0'
> >Do you have any suggestion?
> >
> >[1] https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/tree/drivers/net/bonding/bond_netlink.c#n670
>
> Yeah, that's odd use of attr type, here it is an array index. I'm pretty
> sure I saw this in the past on different netlink places.
> I believe that is not supported with the existing ynl code.
>
> Perhaps something like the following might work:
>       -
>         name: arp-ip-target
>         type: binary
>         display-hint: ipv4
>         nested-array: true
>
> "nested-array" would tell the parser to expect a nest that has attr
> type of value of array index, "type" is the same for all array members.
> The output will be the same as in case of "multi-attr", array index
> ignored (I don't see what it would be good for to the user).

I'd say that this construct looks more like nest-type-value, except
that it contains a value rather than a set of nested attributes. It is
documented here:

https://docs.kernel.org/userspace-api/netlink/genetlink-legacy.html#type-value

You can see an example of nest-type-value in the nlctrl.yaml in net-next:

      -
        name: policy
        type: nest-type-value
        type-value: [ policy-id, attr-id ]
        nested-attributes: policy-attrs

It has one or more indices that are stored as nest keys, followed by a
set of nested attributes. Perhaps this could be extended to support an
attribute instead of the nested-attributes ?

> Other existing attrs considered:
>
> "nested-attributes" does not make much sense for this usecase IMO as the
> attr type is array index, the mapping fails.
>
> "multi-attr" also counts with valid attr type and no nest.
>
> Makes sense?
>