netfilter: nf_tables: meter: pick a set backend that supports updates

in nftables, 'meter' can be used to instantiate a hash-table at run
time:

rule add filter forward iif "internal" meter hostacct { ip saddr counter}
nft list meter ip filter hostacct
table ip filter {
  meter hostacct {
    type ipv4_addr
    elements = { 192.168.0.1 : counter packets 8 bytes 2672, ..

because elemets get added on the fly, the kernel must chose a set
backend type that implements the ->update() function, otherwise
rule insertion fails with EOPNOTSUPP.

Therefore, skip set types that lack ->update, and also
make sure we do not discard a (bad) candidate when we did yet
find any candidate at all.  This could happen when userspace prefers
low memory footprint -- the set implementation currently checked might
not be a fit at all.  Make sure we pick it anyway (!bops).  In
case next candidate is a better fix, it will be chosen instead.

But in case nothing else is found we at least have a non-ideal
match rather than no match at all.

Fixes: 6c03ae21 ("netfilter: nft_set_hash: add non-resizable hashtable implementation")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -2446,6 +2446,9 @@ EXPORT_SYMBOL_GPL(nft_unregister_set);
 
 static bool nft_set_ops_candidate(const struct nft_set_ops *ops, u32 flags)
 {
+	if ((flags & NFT_SET_EVAL) && !ops->update)
+		return false;
+
 	return (flags & ops->features) == (flags & NFT_SET_FEATURES);
 }
 
@@ -2510,7 +2513,7 @@ nft_select_set_ops(const struct nft_ctx *ctx,
 				if (est.space == best.space &&
 				    est.lookup < best.lookup)
 					break;
-			} else if (est.size < best.size) {
+			} else if (est.size < best.size || !bops) {
 				break;
 			}
 			continue;

net/netfilter/nft_set_hash.c

@@ -674,7 +674,7 @@ static const struct nft_set_ops *
 nft_hash_select_ops(const struct nft_ctx *ctx, const struct nft_set_desc *desc,
 		    u32 flags)
 {
-	if (desc->size && !(flags & NFT_SET_TIMEOUT)) {
+	if (desc->size && !(flags & (NFT_SET_EVAL | NFT_SET_TIMEOUT))) {
 		switch (desc->klen) {
 		case 4:
 			return &nft_hash_fast_ops;