Re: [PATCH 1/5] Input: joystick - replace strlcat() with strscpy()

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Hi Shihuang,

On Sat, Oct 17, 2026 at 01:04:07AM +0800, Shihuang Liu wrote:
> The pxrc and xpad drivers build the physical path by appending a
> constant "/input0" suffix with strlcat() after usb_make_path().
>
> In preparation for removing the deprecated strlcat() API[1],
> replace it with strnlen() followed by strscpy() into the remainder
> of the buffer, as commit 34b228b739f5 ("HID: usbhid: replace
> strlcat with better alternatives") did for the usbhid drivers.
...
>  	usb_make_path(udev, pxrc->phys, sizeof(pxrc->phys));
> -	strlcat(pxrc->phys, "/input0", sizeof(pxrc->phys));
> +	len = strnlen(pxrc->phys, sizeof(pxrc->phys));
> +	strscpy(pxrc->phys + len, "/input0", sizeof(pxrc->phys) - len);

I really dislike this conversion pattern, and I do not want to see it
open-coded across 18 drivers in this series.

usb_make_path() is just a trivial wrapper around snprintf() that already
returns the length of the formatted string. Calling usb_make_path(),
ignoring its return value, rescanning the buffer with strnlen(), and
then open-coding pointer and size arithmetic for strscpy() (along with
an extra local variable in every probe function) makes the code much
uglier and harder to read than what we had before.

All of these USB input drivers already include <linux/usb/input.h>, and
drivers across other subsystems (drivers/hid/usbhid/, drivers/media/rc/,
drivers/media/usb/, sound/usb/caiaq/, drivers/usb/storage/onetouch.c)
all use the exact same usb_make_path() + suffix pattern ("/input0",
"/input%d", "/ir0", "/button", etc.).

If we want to get rid of strlcat() here, please introduce a generic
helper in include/linux/usb.h (or a USB input helper in
include/linux/usb/input.h) that formats the USB path together with the
subpath in a single pass, for example:

__printf(4, 5)
int usb_make_subpath(struct usb_device *dev, char *buf, size_t size,
		     const char *fmt, ...);

(or a simple const char *suffix variant if formatting is not needed).
Then each driver can do:

	usb_make_subpath(udev, pxrc->phys, sizeof(pxrc->phys), "input0");

and even drivers/hid/usbhid/hid-core.c can simply do:

	usb_make_subpath(dev, hid->phys, sizeof(hid->phys),
			 "input%d", intf->altsetting[0].desc.bInterfaceNumber);

instead of repeating this strnlen() + strscpy() boilerplate everywhere.

Thanks.

-- 
Dmitry