l_strnstart(const char *tstr1, u_int tl1, const char *str2, u_int l2)
 {
 	if (tl1 > l2)
 		return 0;
 
	return (strncmp(tstr1, str2, tl1) == 0 ? 1 : 0);
}

The product reads data past the end, or before the beginning, of the intended buffer.