landlock: Define access_mask_t to enforce a consistent access mask size

Create and use the access_mask_t typedef to enforce a consistent access
mask size and uniformly use a 16-bits type. This will helps transition
to a 32-bits value one day.

Add a build check to make sure all (filesystem) access rights fit in.
This will be extended with a following commit.

Reviewed-by: Paul Moore <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: [e-mail]
Signed-off-by: Mickaël Salaün <[e-mail]>

security/landlock/fs.c

···
  * @path: Should have been checked by get_path_from_fd().
  */
 int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
-			    const struct path *const path, u32 access_rights)
+			    const struct path *const path,
+			    access_mask_t access_rights)
 {
 	int err;
 	struct landlock_object *object;
···
 
 static inline u64 unmask_layers(const struct landlock_ruleset *const domain,
 				const struct path *const path,
-				const u32 access_request, u64 layer_mask)
+				const access_mask_t access_request,
+				u64 layer_mask)
 {
 	const struct landlock_rule *rule;
 	const struct inode *inode;
···
 }
 
 static int check_access_path(const struct landlock_ruleset *const domain,
-			     const struct path *const path, u32 access_request)
+			     const struct path *const path,
+			     const access_mask_t access_request)
 {
 	bool allowed = false;
 	struct path walker_path;
···
 }
 
 static inline int current_check_access_path(const struct path *const path,
-					    const u32 access_request)
+					    const access_mask_t access_request)
 {
 	const struct landlock_ruleset *const dom =
 		landlock_get_current_domain();
···
 
 /* Path hooks */
 
-static inline u32 get_mode_access(const umode_t mode)
+static inline access_mask_t get_mode_access(const umode_t mode)
 {
 	switch (mode & S_IFMT) {
 	case S_IFLNK:
···
 		get_mode_access(d_backing_inode(old_dentry)->i_mode));
 }
 
-static inline u32 maybe_remove(const struct dentry *const dentry)
+static inline access_mask_t maybe_remove(const struct dentry *const dentry)
 {
 	if (d_is_negative(dentry))
 		return 0;
···
 
 /* File hooks */
 
-static inline u32 get_file_access(const struct file *const file)
+static inline access_mask_t get_file_access(const struct file *const file)
 {
-	u32 access = 0;
+	access_mask_t access = 0;
 
 	if (file->f_mode & FMODE_READ) {
 		/* A directory can only be opened in read mode. */

security/landlock/fs.h

···
 
 int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
 			    const struct path *const path,
-			    u32 access_hierarchy);
+			    access_mask_t access_hierarchy);
 
 #endif /* _SECURITY_LANDLOCK_FS_H */

security/landlock/limits.h

···
 #ifndef _SECURITY_LANDLOCK_LIMITS_H
 #define _SECURITY_LANDLOCK_LIMITS_H
 
+#include <linux/bitops.h>
 #include <linux/limits.h>
 #include <uapi/linux/landlock.h>
 
···
 
 #define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_MAKE_SYM
 #define LANDLOCK_MASK_ACCESS_FS		((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
+#define LANDLOCK_NUM_ACCESS_FS		__const_hweight64(LANDLOCK_MASK_ACCESS_FS)
 
 /* clang-format on */
 

security/landlock/ruleset.c

···
 	return new_ruleset;
 }
 
-struct landlock_ruleset *landlock_create_ruleset(const u32 fs_access_mask)
+struct landlock_ruleset *
+landlock_create_ruleset(const access_mask_t fs_access_mask)
 {
 	struct landlock_ruleset *new_ruleset;
 
···
 
 /* @ruleset must be locked by the caller. */
 int landlock_insert_rule(struct landlock_ruleset *const ruleset,
-			 struct landlock_object *const object, const u32 access)
+			 struct landlock_object *const object,
+			 const access_mask_t access)
 {
 	struct landlock_layer layers[] = { {
 		.access = access,

security/landlock/ruleset.h

···
 #ifndef _SECURITY_LANDLOCK_RULESET_H
 #define _SECURITY_LANDLOCK_RULESET_H
 
+#include <linux/bitops.h>
+#include <linux/build_bug.h>
 #include <linux/mutex.h>
 #include <linux/rbtree.h>
 #include <linux/refcount.h>
 #include <linux/workqueue.h>
 
+#include "limits.h"
 #include "object.h"
+
+typedef u16 access_mask_t;
+/* Makes sure all filesystem access rights can be stored. */
+static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS);
 
 /**
  * struct landlock_layer - Access rights for a given layer
···
 	 * @access: Bitfield of allowed actions on the kernel object.  They are
 	 * relative to the object type (e.g. %LANDLOCK_ACTION_FS_READ).
 	 */
-	u16 access;
+	access_mask_t access;
 };
 
 /**
···
 			 * layers are set once and never changed for the
 			 * lifetime of the ruleset.
 			 */
-			u16 fs_access_masks[];
+			access_mask_t fs_access_masks[];
 		};
 	};
 };
 
-struct landlock_ruleset *landlock_create_ruleset(const u32 fs_access_mask);
+struct landlock_ruleset *
+landlock_create_ruleset(const access_mask_t fs_access_mask);
 
 void landlock_put_ruleset(struct landlock_ruleset *const ruleset);
 void landlock_put_ruleset_deferred(struct landlock_ruleset *const ruleset);
 
 int landlock_insert_rule(struct landlock_ruleset *const ruleset,
 			 struct landlock_object *const object,
-			 const u32 access);
+			 const access_mask_t access);
 
 struct landlock_ruleset *
 landlock_merge_ruleset(struct landlock_ruleset *const parent,