nft -f fails to restore ruleset listing with cetain dynamic set types

[Florian Westphal <fw@strlen.de>]

I was given this reproducer:

----------------
#!/bin/bash -x
nft flush ruleset

nft add table ip filter
nft add chain ip filter output { type filter hook output priority 0 \; }
nft add set ip filter black-list '{type ipv4_addr; flags dynamic; }'
nft add rule ip filter output oifname lo meta l4proto udp update @black-list { ip daddr } counter
nft list ruleset |tee ruleset

nft -f ruleset # success
nft flush ruleset
nft list ruleset
sleep 1
nft -f ruleset # failed

-----------

The ruleset generated by 'nft list ruleset'
cannot be restored.

Ruleset looks like this:

table ip filter {
	set black-list {
		type ipv4_addr
		size 65535
	}

	chain output {
		type filter hook output priority filter; policy accept;
		oifname "lo" meta l4proto udp update @black-list { ip daddr } counter packets 0 bytes 0
	}
}

Reason for the failure on restore is that the set is created with 'size
65536' but no 'dynamic' flag.

This causes a failure because the kernel picks the 'hash' set that
lacks the update() function.

The initial commands work because lack of 'size' picks the rhash
backend, so the ->update works since the check for the timeout flag was
removed in commit "netfilter: nft_dynset: relax superfluous check on set
updates".

First restore works because the set is not created anew, so this is
still 'rhash' backend.

Second restore fails because 'hash' is chosen.

Workaround is to edit the ruleset file to add 'flags dynamic' or 'flags
timeout', or both.

Any suggestions on how to fix this?

I am tempted to axe all hash backends except rbtree, rhash and pipapo,
this internal autopick logic is super confusing to end users and even
as developer I can't even see what the kernel did without perf probes or printk
mangling :/