[PATCH v7 01/10] landlock: Add path walk helper

[Justin Suess <utilityemal77@gmail.com>]

Add path walk helper landlock_walk_path_up. This helper takes a pointer
to a struct path and walks the path upward towards the VFS root, and
returns an enum corresponding whether the current position in the walk
is an internal mountpoint, the real root, or neither.

Cc: Tingmao Wang <m@maowtm.org>
Signed-off-by: Justin Suess <utilityemal77@gmail.com>
---

Notes:
    v6..v7 changes:
    
      * New patch split out from the v6 core NO_INHERIT implementation.
      * Added enum comments.

 security/landlock/fs.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index bd7554d0b65a..644637a8c0b5 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -320,6 +320,38 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
 	LANDLOCK_ACCESS_FS_RESOLVE_UNIX)
 /* clang-format on */
 
+/**
+ * enum landlock_walk_result - Result codes for landlock_walk_path_up()
+ * @LANDLOCK_WALK_CONTINUE: Path is now neither the real root nor an internal mount point.
+ * @LANDLOCK_WALK_STOP_REAL_ROOT: Path has reached the real VFS root.
+ * @LANDLOCK_WALK_INTERNAL: Path has reached an internal mount point.
+ */
+enum landlock_walk_result {
+	LANDLOCK_WALK_CONTINUE,
+	LANDLOCK_WALK_STOP_REAL_ROOT,
+	LANDLOCK_WALK_INTERNAL,
+};
+
+static enum landlock_walk_result landlock_walk_path_up(struct path *const path)
+{
+	struct dentry *old;
+
+	while (path->dentry == path->mnt->mnt_root) {
+		if (!follow_up(path))
+			return LANDLOCK_WALK_STOP_REAL_ROOT;
+	}
+	old = path->dentry;
+	if (unlikely(IS_ROOT(old))) {
+		if (likely(path->mnt->mnt_flags & MNT_INTERNAL))
+			return LANDLOCK_WALK_INTERNAL;
+		path->dentry = dget(path->mnt->mnt_root);
+	} else {
+		path->dentry = dget_parent(old);
+	}
+	dput(old);
+	return LANDLOCK_WALK_CONTINUE;
+}
+
 /*
  * @path: Should have been checked by get_path_from_fd().
  */
-- 
2.53.0