Changes to module blacklisting

Since the dawn of time we have shipped our homegrown system for blacklisting kernel modules (in various incarnations). This has served us well, but we (or rather our various upstreams) have now reached a point where it is no longer necessary.

We have therefore decided to remove all our module blacklisting logic, and rely entirely on the blacklisting provided by modprobe. This means a change in syntax, and a minor change in functionality. We hope that the short-term inconvenience will be justified by the long-term gain.

Due to an unrelated change in our udev package, we no longer load as many modules as we used to by default, so some problematic modules (such as pcspkr and snd-seq-oss) might no longer need to be blacklisted.

The different notions of blacklisting

The notion of module blacklisting that we have used is (sadly) slightly different from what is used by modprobe.

  • our blacklisting: a blacklisted module, or any module that depends on it, will never be loaded automatically by udev, but may be loaded manually.
  • modprobe's blacklisting: a blacklisted module will not be loaded automatically, but may be loaded if another non-blacklisted module depends on it, or if it is loaded manually.
  • disabling a module: one can get similar behavior to what we used to provide by forcing a module to always fail loading, it will not be possible to load it manually.

Boot time blacklisting or disabling

Modules can be blacklisted at boot time by appending modprobe.blacklist=mod1,mod2,mod3 to the kernel command line (in grub, lilo or syslinux). Alternatively, the modules can be disabled altogether by appending mod1.disable=1 mod2.disable=1 mod3.disable=1 instead. This is useful in an emergency where a broken module makes it impossible to boot your system.

Persistent blacklisting or disabling

Rather than blacklisting modules in rc.conf, use modprobe's native configuration files. These can be found in /etc/modprobe.d/, and are documented in man modprobe.d.

The replacement of rc.conf's MODULES=(!mod1 !mod2 !mod3) is to add a .conf file to /etc/modprobe.d/ with the contents:

blacklist mod1
blacklist mod2
blacklist mod3

Alternatively, to ensure that the modules are never inserted, even if they are needed by other modules you load:

install mod1 /bin/false
install mod2 /bin/false
install mod3 /bin/false