[PATCH bpf-next 1/4] namei: Introduce new helper function path_parent()

[Song Liu <song@kernel.org>]

This helper walks an input path to its parent. Logic are added to handle
walking across mount tree.

This will be used by landlock, and BPF LSM.

Signed-off-by: Song Liu <song@kernel.org>
---
 fs/namei.c            | 39 +++++++++++++++++++++++++++++++++++++++
 include/linux/namei.h |  8 ++++++++
 2 files changed, 47 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 84a0e0b0111c..475f4188a0e6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1379,6 +1379,45 @@ int follow_up(struct path *path)
 }
 EXPORT_SYMBOL(follow_up);
 
+/**
+ * path_parent - Find the parent of path
+ * @path: input and output path.
+ *
+ * Given a path, find the parent path. Replace @path with the parent path.
+ * If we were already at the real root or a disconnected root, @path is
+ * not changed.
+ *
+ * Returns:
+ *   PATH_PARENT_SAME_MOUNT         - if we walked up within the same mount;
+ *   PATH_PARENT_CHANGED_MOUNT      - if we walked up a mount point;
+ *   PATH_PARENT_REAL_ROOT          - if we were already at real root;
+ *   PATH_PARENT_DISCONNECTED_ROOT  - if we were at the root of a disconnected
+ *                                    filesystem.
+ */
+enum path_parent_status path_parent(struct path *path)
+{
+	struct dentry *parent;
+
+	if (path->dentry == path->mnt->mnt_root) {
+		if (!follow_up(path))
+			return PATH_PARENT_REAL_ROOT;
+		return PATH_PARENT_CHANGED_MOUNT;
+	}
+	if (unlikely(IS_ROOT(path->dentry)))
+		return PATH_PARENT_DISCONNECTED_ROOT;
+
+	parent = dget_parent(path->dentry);
+	if (unlikely(!path_connected(path->mnt, parent))) {
+		dput(parent);
+		return PATH_PARENT_DISCONNECTED_ROOT;
+	}
+
+	dput(path->dentry);
+	path->dentry = parent;
+	return PATH_PARENT_SAME_MOUNT;
+}
+EXPORT_SYMBOL_GPL(path_parent);
+
 static bool choose_mountpoint_rcu(struct mount *m, const struct path *root,
 				  struct path *path, unsigned *seqp)
 {
diff --git a/include/linux/namei.h b/include/linux/namei.h
index bbaf55fb3101..dc6e9096eb15 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -86,6 +86,14 @@ extern int follow_down_one(struct path *);
 extern int follow_down(struct path *path, unsigned int flags);
 extern int follow_up(struct path *);
 
+enum path_parent_status {
+	PATH_PARENT_SAME_MOUNT = 0,
+	PATH_PARENT_CHANGED_MOUNT,
+	PATH_PARENT_REAL_ROOT,
+	PATH_PARENT_DISCONNECTED_ROOT,
+};
+enum path_parent_status path_parent(struct path *path);
+
 extern struct dentry *lock_rename(struct dentry *, struct dentry *);
 extern struct dentry *lock_rename_child(struct dentry *, struct dentry *);
 extern void unlock_rename(struct dentry *, struct dentry *);
-- 
2.47.1