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@xxxxxxxxxx>
---
fs/namei.c | 52 +++++++++++++++++++++++++++++++++++++++++++
include/linux/namei.h | 2 ++
2 files changed, 54 insertions(+)
diff --git a/fs/namei.c b/fs/namei.c
index 4bb889fc980b..7d5bf2bb604f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1424,6 +1424,58 @@ static bool choose_mountpoint(struct mount *m, const struct path *root,
return found;
}
+/**
+ * path_walk_parent - Walk to the parent of path
+ * @path: input and output path.
+ * @root: root of the path walk, do not go beyond this root. If @root is
+ * zero'ed, walk all the way to real root.
+ *
+ * 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.
+ *
+ * The logic of path_walk_parent() is similar to follow_dotdot(), except
+ * that path_walk_parent() will continue walking for !path_connected case.
+ * This effectively means we are walking from disconnectedbind mount to the
+ * original mount point. If this behavior is not desired, the caller can
+ * add a check like:
+ *
+ * if (path_walk_parent(&path) && !path_connected(path.mnt, path.dentry)
+ * // continue walking
+ * else
+ * // stop walking
+ *
+ * Returns:
+ * true - if @path is updated to its parent.
+ * false - if @path is already the root (real root or @root).
+ */
+bool path_walk_parent(struct path *path, const struct path *root)
+{
+ struct dentry *parent;
+
+ if (path_equal(path, root))
+ return false;
+
+ if (unlikely(path->dentry == path->mnt->mnt_root)) {
+ struct path p;
+
+ if (!choose_mountpoint(real_mount(path->mnt), root, &p))
+ return false;
+ path_put(path);
+ *path = p;
+ return true;
+ }
+
+ if (unlikely(IS_ROOT(path->dentry)))
+ return false;
+
+ parent = dget_parent(path->dentry);
+ dput(path->dentry);
+ path->dentry = parent;
+ return true;
+}
+EXPORT_SYMBOL_GPL(path_walk_parent);
+
/*
* Perform an automount
* - return -EISDIR to tell follow_managed() to stop and return the path we
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 5d085428e471..cba5373ecf86 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -85,6 +85,8 @@ extern int follow_down_one(struct path *);
extern int follow_down(struct path *path, unsigned int flags);
extern int follow_up(struct path *);
+bool path_walk_parent(struct path *path, const struct path *root);
+
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
