sysfs: Fix internal_create_group() for named group updates

There are a couple of problems with named group updates in the code
today:

* sysfs_update_group() will always fail for a named group, because
  internal_create_group() will try to create a new sysfs directory
  unconditionally, which will ofcourse fail with -EEXIST.

* We can leak the kernfs_node for grp->name if some one tries to:
  - rename a group (change grp->name), or
  - update a named group, to an unnamed group

It appears that the whole purpose of sysfs_update_group() was to
allow changing the permissions or visibility of attributes and not
the names. So make it clear in the comments, and allow it to update
an existing named group.
Signed-off-by: Rajat Jain <rajatja@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/sysfs/group.c

@@ -119,13 +119,23 @@ static int internal_create_group(struct kobject *kobj, int update,
 		return -EINVAL;
 	}
 	if (grp->name) {
+		if (update) {
+			kn = kernfs_find_and_get(kobj->sd, grp->name);
+			if (!kn) {
+				pr_warn("Can't update unknown attr grp name: %s/%s\n",
+					kobj->name, grp->name);
+				return -EINVAL;
+			}
+		} else {
 			kn = kernfs_create_dir(kobj->sd, grp->name,
-				       S_IRWXU | S_IRUGO | S_IXUGO, kobj);
+					       S_IRWXU | S_IRUGO | S_IXUGO,
+					       kobj);
 			if (IS_ERR(kn)) {
 				if (PTR_ERR(kn) == -EEXIST)
 					sysfs_warn_dup(kobj->sd, grp->name);
 				return PTR_ERR(kn);
 			}
+		}
 	} else
 		kn = kobj->sd;
 	kernfs_get(kn);
@@ -135,6 +145,10 @@ static int internal_create_group(struct kobject *kobj, int update,
 			kernfs_remove(kn);
 	}
 	kernfs_put(kn);
+
+	if (grp->name && update)
+		kernfs_put(kn);
+
 	return error;
 }
 
@@ -199,7 +213,8 @@ EXPORT_SYMBOL_GPL(sysfs_create_groups);
  * of the attribute files being created already exist.  Furthermore,
  * if the visibility of the files has changed through the is_visible()
  * callback, it will update the permissions and add or remove the
- * relevant files.
+ * relevant files. Changing a group's name (subdirectory name under
+ * kobj's directory in sysfs) is not allowed.
  *
  * The primary use for this function is to call it after making a change
  * that affects group visibility.