Re: [PATCH] virtiofs: Export filesystem tags through sysfs

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Oct 05, 2023 at 04:30:30PM -0400, Vivek Goyal wrote:
> virtiofs filesystem is mounted using a "tag" which is exported by the
> virtiofs device. virtiofs driver knows about all the available tags but
> these are not exported to user space.
> 
> People have asked 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 through sysfs. One tag is associated with each
> virtiofs device. A new "tag" file appears under virtiofs device dir.
> Actual filesystem tag can be obtained by reading this "tag" file.
> 
> For example, if a virtiofs device exports tag "myfs", a new file "tag"
> will show up here.
> 
> /sys/bus/virtio/devices/virtio<N>/tag
> 
> # cat /sys/bus/virtio/devices/virtio<N>/tag
> myfs
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
>  fs/fuse/virtio_fs.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 5f1be1da92ce..a5b11e18f331 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -107,6 +107,21 @@ static const struct fs_parameter_spec virtio_fs_parameters[] = {
>  	{}
>  };
>  
> +/* Forward Declarations */
> +static void virtio_fs_stop_all_queues(struct virtio_fs *fs);
> +
> +/* sysfs related */
> +static ssize_t tag_show(struct device *dev, struct device_attribute *attr,
> +			char *buf)
> +{
> +	struct virtio_device *vdev = container_of(dev, struct virtio_device,
> +						  dev);
> +	struct virtio_fs *fs = vdev->priv;
> +
> +	return sysfs_emit(buf, "%s", fs->tag);
> +}
> +static DEVICE_ATTR_RO(tag);
> +
>  static int virtio_fs_parse_param(struct fs_context *fsc,
>  				 struct fs_parameter *param)
>  {
> @@ -265,6 +280,15 @@ static int virtio_fs_add_instance(struct virtio_fs *fs)
>  	return 0;
>  }
>  
> +static void virtio_fs_remove_instance(struct virtio_fs *fs)
> +{
> +	mutex_lock(&virtio_fs_mutex);
> +	list_del_init(&fs->list);
> +	virtio_fs_stop_all_queues(fs);
> +	virtio_fs_drain_all_queues_locked(fs);
> +	mutex_unlock(&virtio_fs_mutex);
> +}
> +
>  /* Return the virtio_fs with a given tag, or NULL */
>  static struct virtio_fs *virtio_fs_find_instance(const char *tag)
>  {
> @@ -891,8 +915,15 @@ static int virtio_fs_probe(struct virtio_device *vdev)
>  	if (ret < 0)
>  		goto out_vqs;
>  
> +	/* Export tag through sysfs */
> +	ret = device_create_file(&vdev->dev, &dev_attr_tag);
> +	if (ret < 0)
> +		goto out_sysfs_attr;

You just raced with userspace and lost :(

Please use default groups for your device/bus, that is what they are
there for.

thanks,

greg k-h