usb: gadget: configfs: Disallow empty function instance name

[Felipe Balbi <balbi@kernel.org>]

Hi,

Krzysztof Opasiak <k.opasiak@samsung.com> writes:
> Every function should have a type and instance name.
> Unfortunately in most cases instance name was left unused and unchecked.
> This may lead to situations like FunctionFS device name identified by ""
> or some misleading debug messages from TCM like:
>
> tcm: Activating
>
> To avoid this let's add a check that instance name should have at least
> one character.
>
> Reported-by: Stefan Agner <stefan@agner.ch>
> Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
> ---
>  drivers/usb/gadget/configfs.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
> index aeb9f3c40521..bdc9ec597d6a 100644
> --- a/drivers/usb/gadget/configfs.c
> +++ b/drivers/usb/gadget/configfs.c
> @@ -548,6 +548,11 @@ static struct config_group *function_make(
>  	*instance_name = '\0';
>  	instance_name++;
>  
> +	if (*instance_name == '\0') {
> +		pr_err("Instance name (after .) should not be empty\n");
> +		return ERR_PTR(-EINVAL);
> +	}

aaaaaand just like that you break potentially existing scripts :-)

We need to find a better way of enforcing a name which doesn't break
existing users.