sysctl: allow registration of const struct ctl_table

Putting structure, especially those containing function pointers,
into read-only memory makes the safer and easier to reason about.
Change the sysctl registration APIs to allow registration of
"const struct ctl_table".

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org> # security/*
Signed-off-by: Joel Granados <joel.granados@kernel.org>

Author: t-8ch

fs/proc/proc_sysctl.c

@@ -1359,7 +1359,7 @@ static struct ctl_dir *sysctl_mkdir_p(struct ctl_dir *dir, const char *path)
  */
 struct ctl_table_header *__register_sysctl_table(
 	struct ctl_table_set *set,
-	const char *path, struct ctl_table *table, size_t table_size)
+	const char *path, const struct ctl_table *table, size_t table_size)
 {
 	struct ctl_table_root *root = set->dir.header.root;
 	struct ctl_table_header *header;
@@ -1420,7 +1420,7 @@ struct ctl_table_header *__register_sysctl_table(
  *
  * See __register_sysctl_table for more details.
  */
-struct ctl_table_header *register_sysctl_sz(const char *path, struct ctl_table *table,
+struct ctl_table_header *register_sysctl_sz(const char *path, const struct ctl_table *table,
 					    size_t table_size)
 {
 	return __register_sysctl_table(&sysctl_table_root.default_set,
@@ -1449,7 +1449,7 @@ EXPORT_SYMBOL(register_sysctl_sz);
  *
  * Context: if your base directory does not exist it will be created for you.
  */
-void __init __register_sysctl_init(const char *path, struct ctl_table *table,
+void __init __register_sysctl_init(const char *path, const struct ctl_table *table,
 				 const char *table_name, size_t table_size)
 {
 	struct ctl_table_header *hdr = register_sysctl_sz(path, table, table_size);

include/linux/sysctl.h

@@ -223,13 +223,13 @@ extern void retire_sysctl_set(struct ctl_table_set *set);
 
 struct ctl_table_header *__register_sysctl_table(
 	struct ctl_table_set *set,
-	const char *path, struct ctl_table *table, size_t table_size);
-struct ctl_table_header *register_sysctl_sz(const char *path, struct ctl_table *table,
+	const char *path, const struct ctl_table *table, size_t table_size);
+struct ctl_table_header *register_sysctl_sz(const char *path, const struct ctl_table *table,
 					    size_t table_size);
 void unregister_sysctl_table(struct ctl_table_header * table);
 
 extern int sysctl_init_bases(void);
-extern void __register_sysctl_init(const char *path, struct ctl_table *table,
+extern void __register_sysctl_init(const char *path, const struct ctl_table *table,
 				 const char *table_name, size_t table_size);
 #define register_sysctl_init(path, table)	\
 	__register_sysctl_init(path, table, #table, ARRAY_SIZE(table))
@@ -251,7 +251,7 @@ extern int no_unaligned_warning;
 
 #else /* CONFIG_SYSCTL */
 
-static inline void register_sysctl_init(const char *path, struct ctl_table *table)
+static inline void register_sysctl_init(const char *path, const struct ctl_table *table)
 {
 }
 
@@ -261,7 +261,7 @@ static inline struct ctl_table_header *register_sysctl_mount_point(const char *p
 }
 
 static inline struct ctl_table_header *register_sysctl_sz(const char *path,
-							  struct ctl_table *table,
+							  const struct ctl_table *table,
 							  size_t table_size)
 {
 	return NULL;