Re: [PATCH net-next 02/11] tools: ynl-gen: generate nested array policies

["Asbjørn Sloth Tønnesen" <ast@fiberby.net>]

On 9/8/25 7:54 AM, Johannes Berg wrote:
> On Sat, 2025-09-06 at 14:13 +0000, Asbjørn Sloth Tønnesen wrote:
>> Johannes introduced NLA_NESTED_ARRAY and the NLA_POLICY_NESTED_ARRAY()
>> macro in commit 1501d13596b9 for use in nl80211, and it's therefore
>> used in net/wireless/nl80211.c, but outside of that the macro is
>> only sparsely adopted (only by mac80211_hwsim.c and nf_tables_api.c).
>>
>> Wireguard adopts the macro in this RFC patch:
>> https://lore.kernel.org/netdev/20250904220255.1006675-2-ast@fiberby.net/
> 
 > I think the general consensus now is that preference should be towards
 > arrays being expressed by giving the attribute holding the array
 > multiple times, i.e. each occurrence of an attribute holds a single
 > entry of the array:
 >
 > [header][type1:a1][type2:b][type1:a2][type1:a3]
 >
 > resulting in an array
 >
 > [a1, a2, a3] and a separate value "b",
 >
 > rather than a nested array:
 >
 > [header][type1:[1:a1][2:a2][3:a3]][type2:b]
 >
 >
 > Of course if each entry has multiple values, then you'd still need
 > nesting:
 >
 > [header][type1:[subtype1:x1][subtype2:x2]][type1:[subtype1:y1][subtype2:y2]]
 >
 > would be an array
 >
 > [[x1, x2], [y1, y2]].

Thank you for the consensus write up. Should we prohibit indexed-array with sub-type
nest for families with a genetlink protocol?

It is currently only used in families with a netlink-raw or genetlink-legacy protocol.

> I can't get rid of the nested array types in nl80211 though, of course.

Wireguard is already in the same boat. It is not using the term, nor the policy,
but it is doing the validation in the handler through, so it can adopt a
NLA_POLICY_NESTED_ARRAY() policy, instead of a plain '{ .type = NLA_NESTED }'.

Comments on the protocol in include/uapi/linux/wireguard.h:
 > WGDEVICE_A_PEERS: NLA_NESTED
 >   0: NLA_NESTED
 >     WGPEER_A_PUBLIC_KEY: NLA_EXACT_LEN, len WG_KEY_LEN
 >     [..]
 >   0: NLA_NESTED
 >     ...
 >   ...

Given that, as Jacob pointed out, there are more families with nested arrays in
their YNL spec, than those using NLA_NESTED_ARRAY, then it appears that there
are more families already in the boat.