[RFC PATCH bpf-next 01/12] namei: introduce __path_walk_parent()

[Justin Suess <utilityemal77@gmail.com>]

From: Song Liu <song@kernel.org>

Factor the step-to-parent logic out of follow_dotdot() into
__path_walk_parent(), which walks a plain struct path to its parent,
crossing mount trees via choose_mountpoint().

This will be used by a VFS ancestor walker serving landlock and BPF.

Suggested-by: Neil Brown <neil@brown.name>
Signed-off-by: Song Liu <song@kernel.org>
[justin: rebased, kept the helper static to namei.c, dropped the
 path_walk_parent() wrapper and the namei.h export]
Signed-off-by: Justin Suess <utilityemal77@gmail.com>
---
 fs/namei.c | 62 ++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 44 insertions(+), 18 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 20a6534ea3ef..808fb4bed7c4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2192,36 +2192,62 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd)
 	return nd->path.dentry;
 }
 
-static struct dentry *follow_dotdot(struct nameidata *nd)
+/**
+ * __path_walk_parent - step towards the parent of the given struct path
+ * @path: position to step up from; if it is the root of a mounted tree, it
+ *        is first updated to the mount point that tree is mounted on
+ * @root: boundary not to be crossed; if zero'ed, walk all the way to the
+ *        global root
+ * @flags: %LOOKUP_NO_XDEV returns -EXDEV once @path was updated to a new
+ *         mount; %LOOKUP_BENEATH returns -EXDEV at @root or at the root of
+ *         an unmounted tree, where @path->dentry itself is returned
+ *         otherwise
+ *
+ * Returns: the parent dentry with its refcount incremented, or an ERR_PTR().
+ */
+static struct dentry *__path_walk_parent(struct path *path, const 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);
+	return dget_parent(path->dentry);
+
+in_root:
+	if (unlikely(flags & LOOKUP_BENEATH))
+		return ERR_PTR(-EXDEV);
+	return dget(path->dentry);
+}
+
+static struct dentry *follow_dotdot(struct nameidata *nd)
+{
+	struct dentry *parent;
+
+	if (path_equal(&nd->path, &nd->root)) {
+		/* nd->root is never path_connected()-checked. */
+		if (unlikely(nd->flags & LOOKUP_BENEATH))
+			return ERR_PTR(-EXDEV);
+		return dget(nd->path.dentry);
+	}
+	parent = __path_walk_parent(&nd->path, &nd->root, nd->flags);
+	nd->inode = nd->path.dentry->d_inode;
+	if (IS_ERR(parent))
+		return parent;
 	if (unlikely(!path_connected(nd->path.mnt, parent))) {
 		dput(parent);
 		return ERR_PTR(-ENOENT);
 	}
 	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, enum last_type type)
-- 
2.55.0