virtiofs: export filesystem tags through sysfs

The virtiofs filesystem is mounted using a "tag" which is exported by
the virtiofs device:

# mount -t virtiofs <tag> /mnt

The virtiofs driver knows about all the available tags but these are
currently not exported to user space.

People have asked for these tags to be exported to user space. Most
recently Lennart Poettering has asked for it as he wants to scan the
tags and mount virtiofs automatically in certain cases.

https://gitlab.com/virtio-fs/virtiofsd/-/issues/128

This patch exports tags at /sys/fs/virtiofs/<N>/tag where N is the id of
the virtiofs device. The filesystem tag can be obtained by reading this
"tag" file.

There is also a symlink at /sys/fs/virtiofs/<N>/device that points to
the virtiofs device that exports this tag.

This patch converts the existing struct virtio_fs into a full kobject.
It already had a refcount so it's an easy change. The virtio_fs objects
can then be exposed in a kset at /sys/fs/virtiofs/. Note that virtio_fs
objects may live slightly longer than we wish for them to be exposed to
userspace, so kobject_del() is called explicitly when the underlying
virtio_device is removed. The virtio_fs object is freed when all
references are dropped (e.g. active mounts) but disappears as soon as
the virtiofs device is gone.

Originally-by: Vivek Goyal <[e-mail]>
Signed-off-by: Stefan Hajnoczi <[e-mail]>
Reviewed-by: Vivek Goyal <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>

Documentation/ABI/testing/sysfs-fs-virtiofs

···
+What:		/sys/fs/virtiofs/<n>/tag
+Date:		Feb 2024
+Contact:	[e-mail]
+Description:
+		[RO] The mount "tag" that can be used to mount this filesystem.
+
+What:		/sys/fs/virtiofs/<n>/device
+Date:		Feb 2024
+Contact:	[e-mail]
+Description:
+		Symlink to the virtio device that exports this filesystem.

fs/fuse/virtio_fs.c

···
 static DEFINE_MUTEX(virtio_fs_mutex);
 static LIST_HEAD(virtio_fs_instances);
 
+/* The /sys/fs/virtio_fs/ kset */
+static struct kset *virtio_fs_kset;
+
 enum {
 	VQ_HIPRIO,
 	VQ_REQUEST
···
 
 /* A virtio-fs device instance */
 struct virtio_fs {
-	struct kref refcount;
+	struct kobject kobj;
 	struct list_head list;    /* on virtio_fs_instances */
 	char *tag;
 	struct virtio_fs_vq *vqs;
···
 		complete(&fsvq->in_flight_zero);
 }
 
-static void release_virtio_fs_obj(struct kref *ref)
+static ssize_t tag_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
 {
-	struct virtio_fs *vfs = container_of(ref, struct virtio_fs, refcount);
+	struct virtio_fs *fs = container_of(kobj, struct virtio_fs, kobj);
+
+	return sysfs_emit(buf, fs->tag);
+}
+
+static struct kobj_attribute virtio_fs_tag_attr = __ATTR_RO(tag);
+
+static struct attribute *virtio_fs_attrs[] = {
+	&virtio_fs_tag_attr.attr,
+	NULL
+};
+ATTRIBUTE_GROUPS(virtio_fs);
+
+static void virtio_fs_ktype_release(struct kobject *kobj)
+{
+	struct virtio_fs *vfs = container_of(kobj, struct virtio_fs, kobj);
 
 	kfree(vfs->vqs);
 	kfree(vfs);
 }
 
+static const struct kobj_type virtio_fs_ktype = {
+	.release = virtio_fs_ktype_release,
+	.sysfs_ops = &kobj_sysfs_ops,
+	.default_groups = virtio_fs_groups,
+};
+
 /* Make sure virtiofs_mutex is held */
 static void virtio_fs_put(struct virtio_fs *fs)
 {
-	kref_put(&fs->refcount, release_virtio_fs_obj);
+	kobject_put(&fs->kobj);
 }
 
 static void virtio_fs_fiq_release(struct fuse_iqueue *fiq)
···
 }
 
 /* Add a new instance to the list or return -EEXIST if tag name exists*/
-static int virtio_fs_add_instance(struct virtio_fs *fs)
+static int virtio_fs_add_instance(struct virtio_device *vdev,
+				  struct virtio_fs *fs)
 {
 	struct virtio_fs *fs2;
-	bool duplicate = false;
+	int ret;
 
 	mutex_lock(&virtio_fs_mutex);
 
 	list_for_each_entry(fs2, &virtio_fs_instances, list) {
-		if (strcmp(fs->tag, fs2->tag) == 0)
-			duplicate = true;
+		if (strcmp(fs->tag, fs2->tag) == 0) {
+			mutex_unlock(&virtio_fs_mutex);
+			return -EEXIST;
+		}
 	}
 
-	if (!duplicate)
-		list_add_tail(&fs->list, &virtio_fs_instances);
+	/* Use the virtio_device's index as a unique identifier, there is no
+	 * need to allocate our own identifiers because the virtio_fs instance
+	 * is only visible to userspace as long as the underlying virtio_device
+	 * exists.
+	 */
+	fs->kobj.kset = virtio_fs_kset;
+	ret = kobject_add(&fs->kobj, NULL, "%d", vdev->index);
+	if (ret < 0) {
+		mutex_unlock(&virtio_fs_mutex);
+		return ret;
+	}
+
+	ret = sysfs_create_link(&fs->kobj, &vdev->dev.kobj, "device");
+	if (ret < 0) {
+		kobject_del(&fs->kobj);
+		mutex_unlock(&virtio_fs_mutex);
+		return ret;
+	}
+
+	list_add_tail(&fs->list, &virtio_fs_instances);
 
 	mutex_unlock(&virtio_fs_mutex);
 
-	if (duplicate)
-		return -EEXIST;
 	return 0;
 }
 
···
 
 	list_for_each_entry(fs, &virtio_fs_instances, list) {
 		if (strcmp(fs->tag, tag) == 0) {
-			kref_get(&fs->refcount);
+			kobject_get(&fs->kobj);
 			goto found;
 		}
 	}
···
 	fs = kzalloc(sizeof(*fs), GFP_KERNEL);
 	if (!fs)
 		return -ENOMEM;
-	kref_init(&fs->refcount);
+	kobject_init(&fs->kobj, &virtio_fs_ktype);
 	vdev->priv = fs;
 
 	ret = virtio_fs_read_tag(vdev, fs);
···
 	 */
 	virtio_device_ready(vdev);
 
-	ret = virtio_fs_add_instance(fs);
+	ret = virtio_fs_add_instance(vdev, fs);
 	if (ret < 0)
 		goto out_vqs;
 
···
 out_vqs:
 	virtio_reset_device(vdev);
 	virtio_fs_cleanup_vqs(vdev);
-	kfree(fs->vqs);
 
 out:
 	vdev->priv = NULL;
-	kfree(fs);
+	kobject_put(&fs->kobj);
 	return ret;
 }
 
···
 	mutex_lock(&virtio_fs_mutex);
 	/* This device is going away. No one should get new reference */
 	list_del_init(&fs->list);
+	sysfs_remove_link(&fs->kobj, "device");
+	kobject_del(&fs->kobj);
 	virtio_fs_stop_all_queues(fs);
 	virtio_fs_drain_all_queues_locked(fs);
 	virtio_reset_device(vdev);
···
 	.kill_sb	= virtio_kill_sb,
 };
 
+static int __init virtio_fs_sysfs_init(void)
+{
+	virtio_fs_kset = kset_create_and_add("virtiofs", NULL, fs_kobj);
+	if (!virtio_fs_kset)
+		return -ENOMEM;
+	return 0;
+}
+
+static void __exit virtio_fs_sysfs_exit(void)
+{
+	kset_unregister(virtio_fs_kset);
+	virtio_fs_kset = NULL;
+}
+
 static int __init virtio_fs_init(void)
 {
 	int ret;
 
-	ret = register_virtio_driver(&virtio_fs_driver);
+	ret = virtio_fs_sysfs_init();
 	if (ret < 0)
 		return ret;
 
+	ret = register_virtio_driver(&virtio_fs_driver);
+	if (ret < 0)
+		goto sysfs_exit;
+
 	ret = register_filesystem(&virtio_fs_type);
-	if (ret < 0) {
-		unregister_virtio_driver(&virtio_fs_driver);
-		return ret;
-	}
+	if (ret < 0)
+		goto unregister_virtio_driver;
 
 	return 0;
+
+unregister_virtio_driver:
+	unregister_virtio_driver(&virtio_fs_driver);
+sysfs_exit:
+	virtio_fs_sysfs_exit();
+	return ret;
 }
 module_init(virtio_fs_init);
 
···
 {
 	unregister_filesystem(&virtio_fs_type);
 	unregister_virtio_driver(&virtio_fs_driver);
+	virtio_fs_sysfs_exit();
 }
 module_exit(virtio_fs_exit);