title: Poll/select copying to kernelspace from userspace
body: <p>Why do we need to copy data from user space to kernel space when calling poll/select if in kernel mode CPU can access user space memory?</p>
  
  <p>Quote from Linux Programming Interface:</p>
  
  <blockquote>
    <p>When running in user mode, the CPU can access only memory that is marked as being in user space; attempts to access memory in kernel space result in a hardware exception. When running in kernel mode, the CPU can access both user and kernel memory space.</p>
  </blockquote>