On Thu,  7 Sep 2023 11:23:58 +0200 Maxime Chevallier wrote:

 - the netlink API would need polishing, I struggle a bit with finding
   the correct netlink design pattern to return variale-length list of u32.

Think of them as a list, not an array.

Dump them one by one, don't try to wrap them in any way:
https://docs.kernel.org/next/userspace-api/netlink/specs.html#multi-attr-arrays
People have tried other things in the past:
https://docs.kernel.org/next/userspace-api/netlink/genetlink-legacy.html#attribute-type-nests
but in the end they add constraints and pain for little benefit.