sysctl: add helper to register a sysctl mount point

The way to create a subdirectory on top of sysctl_mount_point is
a bit obscure, and *why* we do that even so more. Provide a helper
which makes it clear why we do this.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

2 files changed, 17 insertions, 0 deletions

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 69cbd70bac1c6..3b13aa724bff1 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -34,6 +34,16 @@ struct ctl_table sysctl_mount_point[] = {
 	{ }
 };
 
+/*
+ * Used to create permanently empty directories to serve as mount points.
+ * There are some subtle but important permission checks this allows in the
+ * case of unprivileged mounts.
+ */
+struct sysctl_header *register_sysctl_mount_point(const char *path)
+{
+	return register_sysctl(path, sysctl_mount_point);
+}
+
 static bool is_empty_dir(struct ctl_table_header *head)
 {
 	return head->ctl_table[0].child == sysctl_mount_point;
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 9a855a5cefa08..c384aa53b9002 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -207,6 +207,8 @@ void unregister_sysctl_table(struct ctl_table_header * table);
 extern int sysctl_init(void);
 extern void register_sysctl_init(const char *path, struct ctl_table *table,
 				 const char *table_name);
+extern struct sysctl_header *register_sysctl_mount_point(const char *path);
+
 void do_sysctl_args(void);
 
 extern int pwrsw_enabled;
@@ -222,6 +224,11 @@ static inline struct ctl_table_header *register_sysctl_table(struct ctl_table *
 	return NULL;
 }
 
+static struct sysctl_header *register_sysctl_mount_point(const char *path)
+{
+	return NULL
+}
+
 static inline struct ctl_table_header *register_sysctl_paths(
 			const struct ctl_path *path, struct ctl_table *table)
 {