title: "insmod: invalid module format" when trying to use operaFS
body: <p>I want to build and install the Linux filesystem driver <a href="https://github.com/gameblabla/operafs/blob/master/INSTALL" rel="nofollow noreferrer">OperaFS</a>. After compiling the source codes, "insmod" shows the error message "module invalid format", although I did download the package "linux-source", so I expect that the source is the source of my currently used kernel.</p>
  
  <p>I did following so far:</p>
  
  <pre><code>$ uname -a
  Linux debianstretch 4.9.0-3-686-pae #1 SMP Debian 4.9.30-2+deb9u2 (2017-06-26) i686 GNU/Linux
  
  $ sudo su
  $ aptitude update
  $ aptitude upgrade (so I have the latest kernel now)
  $ aptitude install linux-source libssl-dev
  $ cd /usr/src
  $ tar xf linux-source-4.9.tar.xz
  $ cd linux-source-4.9
  $ make oldconfig &amp;&amp; make prepare
  $ make scripts
  
  Download ZIP from github.com/gameblabla/operafs ... and unzip ...
  $ cd ~/Downloads/operafs
  $ sudo su
  $ make -C /usr/src/linux-source-4.9/ M=$PWD modules
  $ make -C /usr/src/linux-source-4.9/ M=$PWD modules_install
  $ modinfo operafs.ko
  filename:     ...
  license:      GPLv2
  author:       Serge van den Boom
  description:  3DO 'Opera' filesystem support
  depends:
  vermagic:     4.9.30 SMP mod_unload modversions 686
  $ insmod operafs.ko
  insmod: ERROR: could not insert module operafs.ko: Invalid module format
  $ modprobe operafs
  modprobe: FATAL: Module operafs not found in directory /lib/modules/4.9.0-3-686-pae
  </code></pre>
  
  <p>What am I doing wrong?</p>