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

[Greg KH <greg@kroah.com>]

On Fri, Oct 16, 2026 at 12:17:05PM -0700, Dmitry Torokhov wrote:
> 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.

I'm all for that too, and putting it in the USB core for everyone to use
would be great.

thanks,

greg k-h