linux-kernel - [PATCH v2 3/8] sysctl: add helper to register a sysctl mount point

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Date:   Wed, 24 Nov 2021 15:14:30 -0800
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     akpm@...ux-foundation.org, keescook@...omium.org,
        yzaikin@...gle.com, nixiaoming@...wei.com, ebiederm@...ssion.com,
        steve@....org, gregkh@...uxfoundation.org, rafael@...nel.org,
        tytso@....edu, viro@...iv.linux.org.uk, pmladek@...e.com,
        senozhatsky@...omium.org, rostedt@...dmis.org,
        john.ogness@...utronix.de, dgilbert@...erlog.com,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        mcgrof@...badil.infradead.org, mcgrof@...nel.org,
        linux-scsi@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 3/8] 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.

Suggested-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
---
 fs/proc/proc_sysctl.c  | 13 +++++++++++++
 include/linux/sysctl.h |  7 +++++++
 2 files changed, 20 insertions(+)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 6d462644bb00..aa743bbb8400 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -35,6 +35,19 @@ struct ctl_table sysctl_mount_point[] = {
 	{ }
 };
 
+/**
+ * register_sysctl_mount_point() - registers a sysctl mount point
+ * @path: path for the mount point
+ *
+ * Used to create a permanently empty directory to serve as mount point.
+ * There are some subtle but important permission checks this allows in the
+ * case of unprivileged mounts.
+ */
+struct ctl_table_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 e4da44567f18..7946b532e964 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -208,6 +208,8 @@ extern int sysctl_init(void);
 extern void __register_sysctl_init(const char *path, struct ctl_table *table,
 				 const char *table_name);
 #define register_sysctl_init(path, table) __register_sysctl_init(path, table, #table)
+extern struct ctl_table_header *register_sysctl_mount_point(const char *path);
+
 void do_sysctl_args(void);
 
 extern int pwrsw_enabled;
@@ -223,6 +225,11 @@ static inline struct ctl_table_header *register_sysctl_table(struct ctl_table *
 	return NULL;
 }
 
+static inline 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)
 {
-- 
2.33.0