netns: Introduce sysctl root for read-only net sysctls.

This one stores all ctl-heads in one list and restricts the
permissions not give write access to non-init net namespaces.
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -201,8 +201,11 @@ extern void unregister_pernet_gen_device(int id, struct pernet_operations *);
 struct ctl_path;
 struct ctl_table;
 struct ctl_table_header;
+
 extern struct ctl_table_header *register_net_sysctl_table(struct net *net,
 	const struct ctl_path *path, struct ctl_table *table);
+extern struct ctl_table_header *register_net_sysctl_rotable(
+	const struct ctl_path *path, struct ctl_table *table);
 extern void unregister_net_sysctl_table(struct ctl_table_header *header);
 
 #endif /* __NET_NET_NAMESPACE_H */

net/sysctl_net.c

@@ -40,6 +40,27 @@ static struct ctl_table_root net_sysctl_root = {
 	.lookup = net_ctl_header_lookup,
 };
 
+static LIST_HEAD(net_sysctl_ro_tables);
+static struct list_head *net_ctl_ro_header_lookup(struct ctl_table_root *root,
+		struct nsproxy *namespaces)
+{
+	return &net_sysctl_ro_tables;
+}
+
+static int net_ctl_ro_header_perms(struct ctl_table_root *root,
+		struct nsproxy *namespaces, struct ctl_table *table)
+{
+	if (namespaces->net_ns == &init_net)
+		return table->mode;
+	else
+		return table->mode & ~0222;
+}
+
+static struct ctl_table_root net_sysctl_ro_root = {
+	.lookup = net_ctl_ro_header_lookup,
+	.permissions = net_ctl_ro_header_perms,
+};
+
 static int sysctl_net_init(struct net *net)
 {
 	INIT_LIST_HEAD(&net->sysctl_table_headers);
@@ -64,6 +85,7 @@ static __init int sysctl_init(void)
 	if (ret)
 		goto out;
 	register_sysctl_root(&net_sysctl_root);
+	register_sysctl_root(&net_sysctl_ro_root);
 out:
 	return ret;
 }
@@ -80,6 +102,14 @@ struct ctl_table_header *register_net_sysctl_table(struct net *net,
 }
 EXPORT_SYMBOL_GPL(register_net_sysctl_table);
 
+struct ctl_table_header *register_net_sysctl_rotable(const
+		struct ctl_path *path, struct ctl_table *table)
+{
+	return __register_sysctl_paths(&net_sysctl_ro_root,
+			&init_nsproxy, path, table);
+}
+EXPORT_SYMBOL_GPL(register_net_sysctl_rotable);
+
 void unregister_net_sysctl_table(struct ctl_table_header *header)
 {
 	unregister_sysctl_table(header);