[PATCH v2] netfilter: Make xt_cgroup independent from net_cls

["Michal Koutný" <mkoutny@suse.com>]

The xt_group matching supports the default hierarchy since commit
c38c4597e4bf3 ("netfilter: implement xt_cgroup cgroup2 path match").
The cgroup v1 matching (based on clsid) and cgroup v2 matching (based on
path) are rather independent. Downgrade the Kconfig dependency to
mere CONFIG_SOCK_GROUP_DATA so that xt_group can be built even without
CONFIG_NET_CLS_CGROUP for path matching.
Also add a message for users when they attempt to specify any
non-trivial clsid.

Link: https://lists.opensuse.org/archives/list/kernel@lists.opensuse.org/thread/S23NOILB7MUIRHSKPBOQKJHVSK26GP6X/
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 net/netfilter/Kconfig     |  2 +-
 net/netfilter/xt_cgroup.c | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

Changes from v1 (https://lore.kernel.org/r/20250228165216.339407-1-mkoutny@suse.com)
- terser guard (Jan)
- verboser message (Florian)
- better select !CGROUP_BPF (kernel test robot)

diff --git a/net/netfilter/Kconfig b/net/netfilter/Kconfig
index df2dc21304efb..346ac2152fa18 100644
--- a/net/netfilter/Kconfig
+++ b/net/netfilter/Kconfig
@@ -1180,7 +1180,7 @@ config NETFILTER_XT_MATCH_CGROUP
 	tristate '"control group" match support'
 	depends on NETFILTER_ADVANCED
 	depends on CGROUPS
-	select CGROUP_NET_CLASSID
+	select SOCK_CGROUP_DATA
 	help
 	Socket/process control group matching allows you to match locally
 	generated packets based on which net_cls control group processes
diff --git a/net/netfilter/xt_cgroup.c b/net/netfilter/xt_cgroup.c
index c0f5e9a4f3c65..c3055e74aa0ea 100644
--- a/net/netfilter/xt_cgroup.c
+++ b/net/netfilter/xt_cgroup.c
@@ -23,6 +23,13 @@ MODULE_DESCRIPTION("Xtables: process control group matching");
 MODULE_ALIAS("ipt_cgroup");
 MODULE_ALIAS("ip6t_cgroup");
 
+#define NET_CLS_CLASSID_INVALID_MSG "xt_cgroup: classid invalid without net_cls cgroups\n"
+
+static bool possible_classid(u32 classid)
+{
+	return IS_ENABLED(CONFIG_CGROUP_NET_CLASSID) || classid == 0;
+}
+
 static int cgroup_mt_check_v0(const struct xt_mtchk_param *par)
 {
 	struct xt_cgroup_info_v0 *info = par->matchinfo;
@@ -30,6 +37,11 @@ static int cgroup_mt_check_v0(const struct xt_mtchk_param *par)
 	if (info->invert & ~1)
 		return -EINVAL;
 
+	if (!possible_classid(info->id)) {
+		pr_info(NET_CLS_CLASSID_INVALID_MSG);
+		return -EINVAL;
+	}
+
 	return 0;
 }
 
@@ -51,6 +63,11 @@ static int cgroup_mt_check_v1(const struct xt_mtchk_param *par)
 		return -EINVAL;
 	}
 
+	if (!possible_classid(info->classid)) {
+		pr_info(NET_CLS_CLASSID_INVALID_MSG);
+		return -EINVAL;
+	}
+
 	info->priv = NULL;
 	if (info->has_path) {
 		cgrp = cgroup_get_from_path(info->path);
@@ -83,6 +100,11 @@ static int cgroup_mt_check_v2(const struct xt_mtchk_param *par)
 		return -EINVAL;
 	}
 
+	if (info->has_classid && !possible_classid(info->classid)) {
+		pr_info(NET_CLS_CLASSID_INVALID_MSG);
+		return -EINVAL;
+	}
+
 	info->priv = NULL;
 	if (info->has_path) {
 		cgrp = cgroup_get_from_path(info->path);

base-commit: dd83757f6e686a2188997cb58b5975f744bb7786
-- 
2.48.1