netfilter: nf_tables: allow lookups in dynamic sets

This un-breaks lookups in sets that have the 'dynamic' flag set.
Given this active example configuration:

table filter {
  set set1 {
    type ipv4_addr
    size 64
    flags dynamic,timeout
    timeout 1m
  }

  chain input {
     type filter hook input priority 0; policy accept;
  }
}

... this works:
nft add rule ip filter input add @set1 { ip saddr }

-> whenever rule is triggered, the source ip address is inserted
into the set (if it did not exist).

This won't work:
nft add rule ip filter input ip saddr @set1 counter
Error: Could not process rule: Operation not supported

In other words, we can add entries to the set, but then can't make
matching decision based on that set.

That is just wrong -- all set backends support lookups (else they would
not be very useful).
The failure comes from an explicit rejection in nft_lookup.c.

Looking at the history, it seems like NFT_SET_EVAL used to mean
'set contains expressions' (aka. "is a meter"), for instance something like

 nft add rule ip filter input meter example { ip saddr limit rate 10/second }
 or
 nft add rule ip filter input meter example { ip saddr counter }

The actual meaning of NFT_SET_EVAL however, is
'set can be updated from the packet path'.

'meters' and packet-path insertions into sets, such as
'add @set { ip saddr }' use exactly the same kernel code (nft_dynset.c)
and thus require a set backend that provides the ->update() function.

The only set that provides this also is the only one that has the
NFT_SET_EVAL feature flag.

Removing the wrong check makes the above example work.
While at it, also fix the flag check during set instantiation to
allow supported combinations only.

Fixes: 8aeff920 ("netfilter: nf_tables: add stateful object reference to set elements")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -3562,8 +3562,11 @@ static int nf_tables_newset(struct net *net, struct sock *nlsk,
 			      NFT_SET_OBJECT))
 			return -EINVAL;
 		/* Only one of these operations is supported */
-		if ((flags & (NFT_SET_MAP | NFT_SET_EVAL | NFT_SET_OBJECT)) ==
-			     (NFT_SET_MAP | NFT_SET_EVAL | NFT_SET_OBJECT))
+		if ((flags & (NFT_SET_MAP | NFT_SET_OBJECT)) ==
+			     (NFT_SET_MAP | NFT_SET_OBJECT))
+			return -EOPNOTSUPP;
+		if ((flags & (NFT_SET_EVAL | NFT_SET_OBJECT)) ==
+			     (NFT_SET_EVAL | NFT_SET_OBJECT))
 			return -EOPNOTSUPP;
 	}
 

net/netfilter/nft_lookup.c

@@ -73,9 +73,6 @@ static int nft_lookup_init(const struct nft_ctx *ctx,
 	if (IS_ERR(set))
 		return PTR_ERR(set);
 
-	if (set->flags & NFT_SET_EVAL)
-		return -EOPNOTSUPP;
-
 	priv->sreg = nft_parse_register(tb[NFTA_LOOKUP_SREG]);
 	err = nft_validate_register_load(priv->sreg, set->klen);
 	if (err < 0)