ovl: use is_subdir() for testing if one thing is a subdir of another

Rather than using lock_rename(), use the more obvious is_subdir() for
ensuring that neither upper nor workdir contain the other.
Also be explicit in the comment that the two directories cannot be the
same.

As this is a point-it-time sanity check and does not provide any
on-going guarantees, the removal of locking does not introduce any
interesting races.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260224222542.3458677-14-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 109643930b9fcee1f268581ed4d26d56884e81a5..58adefb1c5b8a0d624a01951746bdd6d52ef5e6d 100644 (file)
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -451,18 +451,13 @@ static int ovl_lower_dir(const char *name, const struct path *path,
        return 0;
 }
 
-/* Workdir should not be subdir of upperdir and vice versa */
+/*
+ * Workdir should not be subdir of upperdir and vice versa, and
+ * they should not be the same.
+ */
 static bool ovl_workdir_ok(struct dentry *workdir, struct dentry *upperdir)
 {
-       bool ok = false;
-
-       if (workdir != upperdir) {
-               struct dentry *trap = lock_rename(workdir, upperdir);
-               if (!IS_ERR(trap))
-                       unlock_rename(workdir, upperdir);
-               ok = (trap == NULL);
-       }
-       return ok;
+       return !is_subdir(workdir, upperdir) && !is_subdir(upperdir, workdir);
 }
 
 static int ovl_setup_trap(struct super_block *sb, struct dentry *dir,