title: Why does mmap(2) with PROT_WRITE only require a readable fd?
body: <p>From the POSIX (IEEE Std 1003.1-2008) <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html" rel="nofollow">section</a> on <code>mmap</code>:</p>
  
  <blockquote>
    <p>The file descriptor fildes shall have been opened with read permission, regardless of the protection options specified.</p>
  </blockquote>
  
  <p>Why is that? Seems like a descriptor opened <code>O_WRONLY</code> and mapped with <code>PROT_WRITE</code> and not <code>PROT_READ</code> shouldn't be problematic with respect to permissions, right?</p>