fs: add may_copy_tree()

Add a helper that verifies whether a caller may copy a given mount tree.

Link: https://lore.kernel.org/r/20250221-brauner-open_tree-v1-5-dbcfcb98c676@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 37 insertions, 7 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 8cb15539aaaf..631404fa623c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2808,6 +2808,41 @@ static int do_change_type(struct path *path, int ms_flags)
 	return err;
 }
 
+/* may_copy_tree() - check if a mount tree can be copied
+ * @path: path to the mount tree to be copied
+ *
+ * This helper checks if the caller may copy the mount tree starting
+ * from @path->mnt. The caller may copy the mount tree under the
+ * following circumstances:
+ *
+ * (1) The caller is located in the mount namespace of the mount tree.
+ *     This also implies that the mount does not belong to an anonymous
+ *     mount namespace.
+ * (2) The caller tries to copy an nfs mount referring to a mount
+ *     namespace, i.e., the caller is trying to copy a mount namespace
+ *     entry from nsfs.
+ * (3) The caller tries to copy a pidfs mount referring to a pidfd.
+ *
+ * Returns true if the mount tree can be copied, false otherwise.
+ */
+static inline bool may_copy_tree(struct path *path)
+{
+	struct mount *mnt = real_mount(path->mnt);
+	const struct dentry_operations *d_op;
+
+	if (check_mnt(mnt))
+		return true;
+
+	d_op = path->dentry->d_op;
+	if (d_op == &ns_dentry_operations)
+		return true;
+
+	if (d_op == &pidfs_dentry_operations)
+		return true;
+
+	return false;
+}
+
 static struct mount *__do_loopback(struct path *old_path, int recurse)
 {
 	struct mount *mnt = ERR_PTR(-EINVAL), *old = real_mount(old_path->mnt);
@@ -2815,13 +2850,8 @@ static struct mount *__do_loopback(struct path *old_path, int recurse)
 	if (IS_MNT_UNBINDABLE(old))
 		return mnt;
 
-	if (!check_mnt(old)) {
-		const struct dentry_operations *d_op = old_path->dentry->d_op;
-
-		if (d_op != &ns_dentry_operations &&
-		    d_op != &pidfs_dentry_operations)
-			return mnt;
-	}
+	if (!may_copy_tree(old_path))
+		return mnt;
 
 	if (!recurse && has_locked_children(old, old_path->dentry))
 		return mnt;