saner replacement for debugfs_rename()

Existing primitive has several problems:
1) calling conventions are clumsy - it returns a dentry reference
that is either identical to its second argument or is an ERR_PTR(-E...);
in both cases no refcount changes happen. Inconvenient for users and
bug-prone; it would be better to have it return 0 on success and -E... on
failure.
2) it allows cross-directory moves; however, no such caller have
ever materialized and considering the way debugfs is used, it's unlikely
to happen in the future. What's more, any such caller would have fun
issues to deal with wrt interplay with recursive removal. It also makes
the calling conventions clumsier...
3) tautological rename fails; the callers have no race-free way
to deal with that.
4) new name must have been formed by the caller; quite a few
callers have it done by sprintf/kasprintf/etc., ending up with considerable
boilerplate.

Proposed replacement: int debugfs_change_name(dentry, fmt, ...). All callers
convert to that easily, and it's simpler internally.

IMO debugfs_rename() should go; if we ever get a real-world use case for
cross-directory moves in debugfs, we can always look into the right way
to handle that.

Signed-off-by: Al Viro <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/filesystems/debugfs.rst

···
 
 There are a couple of other directory-oriented helper functions::
 
-    struct dentry *debugfs_rename(struct dentry *old_dir,
-    				  struct dentry *old_dentry,
-		                  struct dentry *new_dir,
-				  const char *new_name);
+    struct dentry *debugfs_change_name(struct dentry *dentry,
+					  const char *fmt, ...);
 
     struct dentry *debugfs_create_symlink(const char *name,
                                           struct dentry *parent,
 				      	  const char *target);
 
-A call to debugfs_rename() will give a new name to an existing debugfs
-file, possibly in a different directory.  The new_name must not exist prior
-to the call; the return value is old_dentry with updated information.
+A call to debugfs_change_name() will give a new name to an existing debugfs
+file, always in the same directory.  The new_name must not exist prior
+to the call; the return value is 0 on success and -E... on failuer.
 Symbolic links can be created with debugfs_create_symlink().
 
 There is one important thing that all debugfs users must take into account:

drivers/net/bonding/bond_debugfs.c

···
 
 void bond_debug_reregister(struct bonding *bond)
 {
-	struct dentry *d;
-
-	d = debugfs_rename(bonding_debug_root, bond->debug_dir,
-			   bonding_debug_root, bond->dev->name);
-	if (!IS_ERR(d)) {
-		bond->debug_dir = d;
-	} else {
+	int err = debugfs_change_name(bond->debug_dir, "%s", bond->dev->name);
+	if (err) {
 		netdev_warn(bond->dev, "failed to reregister, so just unregister old one\n");
 		bond_debug_unregister(bond);
 	}

drivers/net/ethernet/amd/xgbe/xgbe-debugfs.c

···
 
 void xgbe_debugfs_rename(struct xgbe_prv_data *pdata)
 {
-	char *buf;
-
-	if (!pdata->xgbe_debugfs)
-		return;
-
-	buf = kasprintf(GFP_KERNEL, "amd-xgbe-%s", pdata->netdev->name);
-	if (!buf)
-		return;
-
-	if (!strcmp(pdata->xgbe_debugfs->d_name.name, buf))
-		goto out;
-
-	debugfs_rename(pdata->xgbe_debugfs->d_parent, pdata->xgbe_debugfs,
-		       pdata->xgbe_debugfs->d_parent, buf);
-
-out:
-	kfree(buf);
+	debugfs_change_name(pdata->xgbe_debugfs,
+			    "amd-xgbe-%s", pdata->netdev->name);
 }

drivers/net/ethernet/marvell/skge.c

···
 	skge = netdev_priv(dev);
 	switch (event) {
 	case NETDEV_CHANGENAME:
-		if (skge->debugfs)
-			skge->debugfs = debugfs_rename(skge_debug,
-						       skge->debugfs,
-						       skge_debug, dev->name);
+		debugfs_change_name(skge->debugfs, "%s", dev->name);
 		break;
 
 	case NETDEV_GOING_DOWN:

drivers/net/ethernet/marvell/sky2.c

···
 
 	switch (event) {
 	case NETDEV_CHANGENAME:
-		if (sky2->debugfs) {
-			sky2->debugfs = debugfs_rename(sky2_debug, sky2->debugfs,
-						       sky2_debug, dev->name);
-		}
+		debugfs_change_name(sky2->debugfs, "%s", dev->name);
 		break;
 
 	case NETDEV_GOING_DOWN:

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

···
 
 	switch (event) {
 	case NETDEV_CHANGENAME:
-		if (priv->dbgfs_dir)
-			priv->dbgfs_dir = debugfs_rename(stmmac_fs_dir,
-							 priv->dbgfs_dir,
-							 stmmac_fs_dir,
-							 dev->name);
+		debugfs_change_name(priv->dbgfs_dir, "%s", dev->name);
 		break;
 	}
 done:

drivers/opp/debugfs.c

···
 {
 	struct opp_device *new_dev = NULL, *iter;
 	const struct device *dev;
-	struct dentry *dentry;
+	int err;
 
 	/* Look for next opp-dev */
 	list_for_each_entry(iter, &opp_table->dev_list, node)
···
 
 	opp_set_dev_name(dev, opp_table->dentry_name);
 
-	dentry = debugfs_rename(rootdir, opp_dev->dentry, rootdir,
-				opp_table->dentry_name);
-	if (IS_ERR(dentry)) {
+	err = debugfs_change_name(opp_dev->dentry, "%s", opp_table->dentry_name);
+	if (err) {
 		dev_err(dev, "%s: Failed to rename link from: %s to %s\n",
 			__func__, dev_name(opp_dev->dev), dev_name(dev));
 		return;
 	}
 
-	new_dev->dentry = dentry;
-	opp_table->dentry = dentry;
+	new_dev->dentry = opp_table->dentry = opp_dev->dentry;
 }
 
 /**

fs/debugfs/inode.c

···
 EXPORT_SYMBOL_GPL(debugfs_lookup_and_remove);
 
 /**
- * debugfs_rename - rename a file/directory in the debugfs filesystem
- * @old_dir: a pointer to the parent dentry for the renamed object. This
- *          should be a directory dentry.
- * @old_dentry: dentry of an object to be renamed.
- * @new_dir: a pointer to the parent dentry where the object should be
- *          moved. This should be a directory dentry.
- * @new_name: a pointer to a string containing the target name.
+ * debugfs_change_name - rename a file/directory in the debugfs filesystem
+ * @dentry: dentry of an object to be renamed.
+ * @fmt: format for new name
  *
  * This function renames a file/directory in debugfs.  The target must not
  * exist for rename to succeed.
  *
- * This function will return a pointer to old_dentry (which is updated to
- * reflect renaming) if it succeeds. If an error occurs, ERR_PTR(-ERROR)
- * will be returned.
+ * This function will return 0 on success and -E... on failure.
  *
  * If debugfs is not enabled in the kernel, the value -%ENODEV will be
  * returned.
  */
-struct dentry *debugfs_rename(struct dentry *old_dir, struct dentry *old_dentry,
-		struct dentry *new_dir, const char *new_name)
+int __printf(2, 3) debugfs_change_name(struct dentry *dentry, const char *fmt, ...)
 {
-	int error;
-	struct dentry *dentry = NULL, *trap;
+	int error = 0;
+	const char *new_name;
 	struct name_snapshot old_name;
+	struct dentry *parent, *target;
+	struct inode *dir;
+	va_list ap;
 
-	if (IS_ERR(old_dir))
-		return old_dir;
-	if (IS_ERR(new_dir))
-		return new_dir;
-	if (IS_ERR_OR_NULL(old_dentry))
-		return old_dentry;
+	if (IS_ERR_OR_NULL(dentry))
+		return 0;
 
-	trap = lock_rename(new_dir, old_dir);
-	/* Source or destination directories don't exist? */
-	if (d_really_is_negative(old_dir) || d_really_is_negative(new_dir))
-		goto exit;
-	/* Source does not exist, cyclic rename, or mountpoint? */
-	if (d_really_is_negative(old_dentry) || old_dentry == trap ||
-	    d_mountpoint(old_dentry))
-		goto exit;
-	dentry = lookup_one_len(new_name, new_dir, strlen(new_name));
-	/* Lookup failed, cyclic rename or target exists? */
-	if (IS_ERR(dentry) || dentry == trap || d_really_is_positive(dentry))
-		goto exit;
+	va_start(ap, fmt);
+	new_name = kvasprintf_const(GFP_KERNEL, fmt, ap);
+	va_end(ap);
+	if (!new_name)
+		return -ENOMEM;
 
-	take_dentry_name_snapshot(&old_name, old_dentry);
+	parent = dget_parent(dentry);
+	dir = d_inode(parent);
+	inode_lock(dir);
 
-	error = simple_rename(&nop_mnt_idmap, d_inode(old_dir), old_dentry,
-			      d_inode(new_dir), dentry, 0);
-	if (error) {
-		release_dentry_name_snapshot(&old_name);
-		goto exit;
+	take_dentry_name_snapshot(&old_name, dentry);
+
+	if (WARN_ON_ONCE(dentry->d_parent != parent)) {
+		error = -EINVAL;
+		goto out;
 	}
-	d_move(old_dentry, dentry);
-	fsnotify_move(d_inode(old_dir), d_inode(new_dir), &old_name.name,
-		d_is_dir(old_dentry),
-		NULL, old_dentry);
+	if (strcmp(old_name.name.name, new_name) == 0)
+		goto out;
+	target = lookup_one_len(new_name, parent, strlen(new_name));
+	if (IS_ERR(target)) {
+		error = PTR_ERR(target);
+		goto out;
+	}
+	if (d_really_is_positive(target)) {
+		dput(target);
+		error = -EINVAL;
+		goto out;
+	}
+	simple_rename_timestamp(dir, dentry, dir, target);
+	d_move(dentry, target);
+	dput(target);
+	fsnotify_move(dir, dir, &old_name.name, d_is_dir(dentry), NULL, dentry);
+out:
 	release_dentry_name_snapshot(&old_name);
-	unlock_rename(new_dir, old_dir);
-	dput(dentry);
-	return old_dentry;
-exit:
-	if (dentry && !IS_ERR(dentry))
-		dput(dentry);
-	unlock_rename(new_dir, old_dir);
-	if (IS_ERR(dentry))
-		return dentry;
-	return ERR_PTR(-EINVAL);
+	inode_unlock(dir);
+	dput(parent);
+	kfree_const(new_name);
+	return error;
 }
-EXPORT_SYMBOL_GPL(debugfs_rename);
+EXPORT_SYMBOL_GPL(debugfs_change_name);
 
 /**
  * debugfs_initialized - Tells whether debugfs has been registered

include/linux/debugfs.h

···
 ssize_t debugfs_attr_write_signed(struct file *file, const char __user *buf,
 			size_t len, loff_t *ppos);
 
-struct dentry *debugfs_rename(struct dentry *old_dir, struct dentry *old_dentry,
-                struct dentry *new_dir, const char *new_name);
+int debugfs_change_name(struct dentry *dentry, const char *fmt, ...) __printf(2, 3);
 
 void debugfs_create_u8(const char *name, umode_t mode, struct dentry *parent,
 		       u8 *value);
···
 	return -ENODEV;
 }
 
-static inline struct dentry *debugfs_rename(struct dentry *old_dir, struct dentry *old_dentry,
-                struct dentry *new_dir, char *new_name)
+static inline int __printf(2, 3) debugfs_change_name(struct dentry *dentry,
+					const char *fmt, ...)
 {
-	return ERR_PTR(-ENODEV);
+	return -ENODEV;
 }
 
 static inline void debugfs_create_u8(const char *name, umode_t mode,

mm/shrinker_debug.c

···
 
 int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
 {
-	struct dentry *entry;
-	char buf[128];
 	const char *new, *old;
 	va_list ap;
 	int ret = 0;
···
 	old = shrinker->name;
 	shrinker->name = new;
 
-	if (shrinker->debugfs_entry) {
-		snprintf(buf, sizeof(buf), "%s-%d", shrinker->name,
-			 shrinker->debugfs_id);
-
-		entry = debugfs_rename(shrinker_debugfs_root,
-				       shrinker->debugfs_entry,
-				       shrinker_debugfs_root, buf);
-		if (IS_ERR(entry))
-			ret = PTR_ERR(entry);
-		else
-			shrinker->debugfs_entry = entry;
-	}
+	ret = debugfs_change_name(shrinker->debugfs_entry, "%s-%d",
+			shrinker->name, shrinker->debugfs_id);
 
 	mutex_unlock(&shrinker_mutex);
 

net/hsr/hsr_debugfs.c

···
 void hsr_debugfs_rename(struct net_device *dev)
 {
 	struct hsr_priv *priv = netdev_priv(dev);
-	struct dentry *d;
+	int err;
 
-	d = debugfs_rename(hsr_debugfs_root_dir, priv->node_tbl_root,
-			   hsr_debugfs_root_dir, dev->name);
-	if (IS_ERR(d))
+	err = debugfs_change_name(priv->node_tbl_root, "%s", dev->name);
+	if (err)
 		netdev_warn(dev, "failed to rename\n");
-	else
-		priv->node_tbl_root = d;
 }
 
 /* hsr_debugfs_init - create hsr node_table file for dumping

net/mac80211/debugfs_netdev.c

···
 
 void ieee80211_debugfs_rename_netdev(struct ieee80211_sub_if_data *sdata)
 {
-	struct dentry *dir;
-	char buf[10 + IFNAMSIZ];
-
-	dir = sdata->vif.debugfs_dir;
-
-	if (IS_ERR_OR_NULL(dir))
-		return;
-
-	sprintf(buf, "netdev:%s", sdata->name);
-	debugfs_rename(dir->d_parent, dir, dir->d_parent, buf);
+	debugfs_change_name(sdata->vif.debugfs_dir, "netdev:%s", sdata->name);
 }
 
 void ieee80211_debugfs_recreate_netdev(struct ieee80211_sub_if_data *sdata,

net/wireless/core.c

···
 	if (result)
 		return result;
 
-	if (!IS_ERR_OR_NULL(rdev->wiphy.debugfsdir))
-		debugfs_rename(rdev->wiphy.debugfsdir->d_parent,
-			       rdev->wiphy.debugfsdir,
-			       rdev->wiphy.debugfsdir->d_parent, newname);
+	debugfs_change_name(rdev->wiphy.debugfsdir, "%s", newname);
 
 	nl80211_notify_wiphy(rdev, NL80211_CMD_NEW_WIPHY);