netfilter: nf_tables: export set count and backend name to userspace

JIRA: https://issues.redhat.com/browse/RHEL-115582
Upstream Status: commit 0014af8

commit 0014af8
Author: Florian Westphal <fw@strlen.de>
Date:   Tue Apr 8 15:55:53 2025 +0200

    netfilter: nf_tables: export set count and backend name to userspace

    nf_tables picks a suitable set backend implementation (bitmap, hash,
    rbtree..) based on the userspace requirements.

    Figuring out the chosen backend requires information about the set flags
    and the kernel version.  Export this to userspace so nft can include this
    information in '--debug=netlink' output.

    Signed-off-by: Florian Westphal <fw@strlen.de>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Signed-off-by: Florian Westphal <fwestpha@redhat.com>

Author: fw-strlen

include/uapi/linux/netfilter/nf_tables.h

@@ -394,6 +394,8 @@ enum nft_set_field_attributes {
  * @NFTA_SET_HANDLE: set handle (NLA_U64)
  * @NFTA_SET_EXPR: set expression (NLA_NESTED: nft_expr_attributes)
  * @NFTA_SET_EXPRESSIONS: list of expressions (NLA_NESTED: nft_list_attributes)
+ * @NFTA_SET_TYPE: set backend type (NLA_STRING)
+ * @NFTA_SET_COUNT: number of set elements (NLA_U32)
  */
 enum nft_set_attributes {
 	NFTA_SET_UNSPEC,
@@ -415,6 +417,8 @@ enum nft_set_attributes {
 	NFTA_SET_HANDLE,
 	NFTA_SET_EXPR,
 	NFTA_SET_EXPRESSIONS,
+	NFTA_SET_TYPE,
+	NFTA_SET_COUNT,
 	__NFTA_SET_MAX
 };
 #define NFTA_SET_MAX		(__NFTA_SET_MAX - 1)

net/netfilter/nf_tables_api.c

@@ -4564,6 +4564,8 @@ static const struct nla_policy nft_set_policy[NFTA_SET_MAX + 1] = {
 	[NFTA_SET_HANDLE]		= { .type = NLA_U64 },
 	[NFTA_SET_EXPR]			= { .type = NLA_NESTED },
 	[NFTA_SET_EXPRESSIONS]		= NLA_POLICY_NESTED_ARRAY(nft_expr_policy),
+	[NFTA_SET_TYPE]			= { .type = NLA_REJECT },
+	[NFTA_SET_COUNT]		= { .type = NLA_REJECT },
 };
 
 static const struct nla_policy nft_concat_policy[NFTA_SET_FIELD_MAX + 1] = {
@@ -4758,6 +4760,27 @@ static u32 nft_set_userspace_size(const struct nft_set_ops *ops, u32 size)
 	return size;
 }
 
+static noinline_for_stack int
+nf_tables_fill_set_info(struct sk_buff *skb, const struct nft_set *set)
+{
+	unsigned int nelems;
+	char str[40];
+	int ret;
+
+	ret = snprintf(str, sizeof(str), "%ps", set->ops);
+
+	/* Not expected to happen and harmless: NFTA_SET_TYPE is dumped
+	 * to userspace purely for informational/debug purposes.
+	 */
+	DEBUG_NET_WARN_ON_ONCE(ret >= sizeof(str));
+
+	if (nla_put_string(skb, NFTA_SET_TYPE, str))
+		return -EMSGSIZE;
+
+	nelems = nft_set_userspace_size(set->ops, atomic_read(&set->nelems));
+	return nla_put_be32(skb, NFTA_SET_COUNT, htonl(nelems));
+}
+
 static int nf_tables_fill_set(struct sk_buff *skb, const struct nft_ctx *ctx,
 			      const struct nft_set *set, u16 event, u16 flags)
 {
@@ -4838,6 +4861,9 @@ static int nf_tables_fill_set(struct sk_buff *skb, const struct nft_ctx *ctx,
 
 	nla_nest_end(skb, nest);
 
+	if (nf_tables_fill_set_info(skb, set))
+		goto nla_put_failure;
+
 	if (set->num_exprs == 1) {
 		nest = nla_nest_start_noflag(skb, NFTA_SET_EXPR);
 		if (nf_tables_fill_expr_info(skb, set->exprs[0], false) < 0)