I also don't quite understand the dislike of variable-sized records.
Don't getdents, inotify, Netlink all use them?  And I think at least for
Netlink, more stuff is added all the time?

Netlink is absolutely atrocious to work with because everything is
variable sized and figuring out the correct allocation size is a
complete nightmare even with the "helpful" macros that are provided.

The bigger problem however is the complete untypedness even of the most
basic things. For example, retrieving the mtu of a network interface
through netlink is a complete nightmare. getdents, inotify, fanotify,
open_by_handle_at()'s struct fiel_handle are all fine. But let's
absolutely not take netlink as a model for anything related to mounts.

And no one is against again variable sized records per se. I think we're
coming to a good compromise here.