ovl: fix regression caused by lookup helpers API changes

The lookup helpers API was changed by merge of vfs-6.16-rc1.async.dir to
pass a non-const qstr pointer argument to lookup_one*() helpers.

All of the callers of this API were changed to pass a pointer to temp
copy of qstr, except overlays that was passing a const pointer to
dentry->d_name that was changed to pass a non-const copy instead
when doing a lookup in lower layer which is not the fs of said dentry.

This wrong use of the API caused a regression in fstest overlay/012.

Fix the regression by making a non-const copy of dentry->d_name prior
to calling the lookup API, but the API should be fixed to not allow this
class of bugs.

Cc: NeilBrown <neilb@suse.de>
Fixes: 5741909 ("VFS: improve interface for lookup_one functions")
Fixes: 390e34b ("VFS: change lookup_one_common and lookup_noperm_common to take a qstr")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/20250605101530.2336320-1-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: amir73il

fs/overlayfs/namei.c

@@ -1371,7 +1371,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 bool ovl_lower_positive(struct dentry *dentry)
 {
 	struct ovl_entry *poe = OVL_E(dentry->d_parent);
-	struct qstr *name = &dentry->d_name;
+	const struct qstr *name = &dentry->d_name;
 	const struct cred *old_cred;
 	unsigned int i;
 	bool positive = false;
@@ -1394,9 +1394,15 @@ bool ovl_lower_positive(struct dentry *dentry)
 		struct dentry *this;
 		struct ovl_path *parentpath = &ovl_lowerstack(poe)[i];
 
+		/*
+		 * We need to make a non-const copy of dentry->d_name,
+		 * because lookup_one_positive_unlocked() will hash name
+		 * with parentpath base, which is on another (lower fs).
+		 */
 		this = lookup_one_positive_unlocked(
 				mnt_idmap(parentpath->layer->mnt),
-				name, parentpath->dentry);
+				&QSTR_LEN(name->name, name->len),
+				parentpath->dentry);
 		if (IS_ERR(this)) {
 			switch (PTR_ERR(this)) {
 			case -ENOENT: