Writing to a const pointer: is this supposed to happen?

["Kars Mulder" <kerneldev@karsmulder.nl>]

In the file drivers/usb/core/quirks.c, I noticed a couple of odd things about the function "quirks_param_set", and I'd like to check whether those are ok according to the kernel programming practices. Here are the relevant lines from the function (several lines omitted):

	static int quirks_param_set(const char *val, const struct kernel_param *kp) {
		char *p, *field;
		for (i = 0, p = (char *)val; p && *p;) {
			field = strsep(&p, ":");
			if (!field)
				break;

In here a const pointer *val is cast into a non-const pointer and then written to by the function strsep, which replaces the first occurrence of the ':' token by a null-byte. Is this allowed?

On a minor side note, this function immediately checks whether the first call to strsep(&p, ":") returned a nullpointer. From what I can learn from the documentation, strsep always returns what *&p was when the strsep was called, and p is verified to be nonzero in the loop condition right before the call to strsep. Is this check actually necessary? Is it a good idea to add a return-value check anyway even if it is not necessary, as an abundance of caution?