[PATCH v3 bpf-next 1/5] namei: Introduce new helper function path_walk_parent()

[NeilBrown]

On Sat, 07 Jun 2025, Song Liu wrote:
> 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 at kernel.org>
> ---
>  fs/namei.c            | 51 +++++++++++++++++++++++++++++++++++++++++++
>  include/linux/namei.h |  2 ++
>  2 files changed, 53 insertions(+)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 4bb889fc980b..f02183e9c073 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1424,6 +1424,57 @@ 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 disconnected bind mount to
> + * the original mount. 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;
> +	}
> +
> +	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);

The above looks a lot like follow_dotdot().  This is good because it
means that it is likely correct.  But it is bad because it means there
are two copies of essentially the same code - making maintenance harder.

I think it would be good to split the part that you want out of
follow_dotdot() and use that.  Something like the following.

You might need a small wrapper in landlock which would, for example,
pass LOOKUP_BENEATH and replace path->dentry with the parent on success.

NeilBrown

diff --git a/fs/namei.c b/fs/namei.c
index 4bb889fc980b..b81d07b4417b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2048,36 +2048,65 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd)
 	return nd->path.dentry;
 }
 
-static struct dentry *follow_dotdot(struct nameidata *nd)
+/**
+ * path_walk_parent - Find the parent of the given struct path
+ * @path  - The struct path to start from
+ * @root  - A struct path which serves as a boundary not to be crosses
+ * @flags - Some LOOKUP_ flags
+ *
+ * Find and return the dentry for the parent of the given path (mount/dentry).
+ * If the given path is the root of a mounted tree, it is first updated to
+ * the mount point on which that tree is mounted.
+ *
+ * If %LOOKUP_NO_XDEV is given, then *after* the path is updated to a new mount,
+ * the error EXDEV is returned.
+ * If no parent can be found, either because the tree is not mounted or because
+ * the @path matches the @root, then @path->dentry is returned unless @flags
+ * contains %LOOKUP_BENEATH, in which case -EXDEV is returned.
+ *
+ * Returns: either an ERR_PTR() or the chosen parent which will have had the
+ * refcount incremented.
+ */
+struct dentry *path_walk_parent(struct path *path, struct path *root, int flags)
 {
 	struct dentry *parent;
 
-	if (path_equal(&nd->path, &nd->root))
+	if (path_equal(path, root))
 		goto in_root;
-	if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
-		struct path path;
+	if (unlikely(path->dentry == path->mnt->mnt_root)) {
+		struct path new_path;
 
-		if (!choose_mountpoint(real_mount(nd->path.mnt),
-				       &nd->root, &path))
+		if (!choose_mountpoint(real_mount(path->mnt),
+				       root, &new_path))
 			goto in_root;
-		path_put(&nd->path);
-		nd->path = path;
-		nd->inode = path.dentry->d_inode;
-		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+		path_put(path);
+		*path = new_path;
+		if (unlikely(flags & LOOKUP_NO_XDEV))
 			return ERR_PTR(-EXDEV);
 	}
 	/* rare case of legitimate dget_parent()... */
-	parent = dget_parent(nd->path.dentry);
+	parent = dget_parent(path->dentry);
+	return parent;
+
+in_root:
+	if (unlikely(flags & LOOKUP_BENEATH))
+		return ERR_PTR(-EXDEV);
+	return dget(path->dentry);
+}
+EXPORT_SYMBOL(path_walk_parent);
+
+static struct dentry *follow_dotdot(struct nameidata *nd)
+{
+	struct dentry *parent = path_walk_parent(&nd->path, &nd->root, nd->flags);
+
+	if (IS_ERR(parent))
+		return parent;
 	if (unlikely(!path_connected(nd->path.mnt, parent))) {
 		dput(parent);
 		return ERR_PTR(-ENOENT);
 	}
+	nd->inode = nd->path.dentry->d_inode;
 	return parent;
-
-in_root:
-	if (unlikely(nd->flags & LOOKUP_BENEATH))
-		return ERR_PTR(-EXDEV);
-	return dget(nd->path.dentry);
 }
 
 static const char *handle_dots(struct nameidata *nd, int type)
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 5d085428e471..4cc15a58d900 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -80,6 +80,7 @@ struct dentry *lookup_one_unlocked(struct mnt_idmap *idmap,
 struct dentry *lookup_one_positive_unlocked(struct mnt_idmap *idmap,
 					    struct qstr *name,
 					    struct dentry *base);
+struct dentry *path_walk_parent(struct path *path, struct path *root, int flags);
 
 extern int follow_down_one(struct path *);
 extern int follow_down(struct path *path, unsigned int flags);