Loadable Kernel Modules and Kerneld

Begin excerpt from kerneld mini-HOWTO

What is kerneld?

kerneld is a feature introduced in the 1.3 development kernels by Bjorn Ekwall. It allows kernel "modules" - i.e. device drivers, network drivers, filesystems - to be loaded automatically when they are needed, rather than having to do it manually with modprobe or insmod.

kerneld also has some other functions:

It can start a program whenever you attempt to access another node on the network, for instance - this makes it easy to implement dial-on-demand networking if you are connected through SLIP or PPP.

And for the more amusing aspects, although these are not (yet ?) integrated with the standard kernel:

It can be setup to run a user-program instead of the default screen blanker, thus letting you use any program as a screen-saver.
Similar to the screen-blanker support, you can also change the standard console "beep" into something completely different ...

kerneld consists of two separate entities:

Support in the Linux kernel for sending requests to a daemon that a module is needed for a certain task.
A user-level daemon that can figure out what modules must be loaded to fulfill the request from the kernel.

Both pieces must be working for the kerneld support to function - it is not enough that only one or the other has been setup.

Why do I want to use it ?

There are some good reasons for using kerneld. The ones I will mention are mine - others may want to use it for other reasons.

If you have to build kernels for several systems that only differ slightly - different kind of network card, for instance - then you can build a single kernel and some modules, instead of having to build individual kernels for each system.
Modules are easier for developers to test - you don't need to reboot the system to load and unload the driver. (This applies to all modules, not just kerneld-loaded ones).
It cuts down on the kernel memory usage, meaning you have more memory available for applications. Memory used by the kernel is *never* swapped out, so if you have 100Kb worth of unused drivers compiled into your kernel, they are simply wasting RAM.
Note: 724k kernel code without modules, 568k kernel code with - for the math challenged, that's a 160k savings.
Some of the things I use - the ftape floppy-tape driver, for instance, or iBCS - are only available as modules. But I don't want to bother with loading and unloading them whenever I need them.

End excerpt from kerneld mini-HOWTO

Links to necessary components

Documentation

kerneld Mini-HOWTO - in its entirety
Bjorn Ekwall's web page - author of modules/kerneld
/usr/src/linux/README - kernel README file (look for this on your system)
/usr/src/linux/Documentation/modules.txt - kernel module README file (again, on your system)

Source Code

Installing Linux loadable modules and kerneld

Build & install module utilities

     [root@bluenote modules]# tar xzf modules-2.0.0.tar.gz 
     [root@bluenote modules]# cd modules-2.0.0
     [root@bluenote modules-2.0.0]# make install

Build & install Linux kernel with module support

Build & install kernel modules

     [root@bluenote linux]# make modules
     [root@bluenote linux]# make modules_install

Modules are installed in /lib/modules/`uname -r`

Edit rc scripts. Add the following to /etc/rc.d/rc.S for Slackware or /etc/rc.d/rc.sysinit for Red Hat:

     # Start kerneld - this should happen very early in the
     # boot process, certainly BEFORE you run fsck on filesystems
     # that might need to have disk drivers autoloaded
     if [ -x /sbin/kerneld ]
     then
             /sbin/kerneld
     fi

     .
     .
     .

     # Update kernel-module dependencies file
     # Your root-fs MUST be mounted read-write by now
     if [ -x /sbin/depmod ]
     then
             /sbin/depmod -a
     fi

Boot the new kernel
Create an /etc/conf.modules file
```
     [root@bluenote /]# /sbin/modprobe -c | grep -v '^path' > /etc/conf.modules
```
Loading/unloading modules manually
Inserting modules
insmod module_name
modprobe module_name
(modprobe is smarter about module dependencies)

Remove modules
rmmod module_name
modprobe -r module_name
(again modprobe is smarter about module dependencies)

List loaded modules
lsmod
cat /proc/modules
Example:
```
[root@bluenote /]# lsmod
Module:        #pages:  Used by:
serial             7            1 (autoclean)
binfmt_aout        1            2 (autoclean)
[root@bluenote /]# modprobe msdos
[root@bluenote /]# lsmod
Module:        #pages:  Used by:
msdos              2            0
fat                6    [msdos] 0
serial             7            1 (autoclean)
binfmt_aout        1            2 (autoclean)
[root@bluenote /]# rmmod msdos
[root@bluenote /]# rmmod fat
[root@bluenote /]# lsmod
Module:        #pages:  Used by:
serial             7            1 (autoclean)
binfmt_aout        1            2 (autoclean)
```
What kerneld does
- The kernel notices that a feature is requested that is not resident in the kernel.
- The kernel sends a message to kerneld, with a symbolic description of the requested feature.
- The kerneld daemon asks e.g. modprobe to load a module that fits this symbolic description.
- modprobe looks into its internal "alias" translation table to see if there is a match. This table can be reconfigured and expanded by having "alias" lines in "/etc/conf.modules".
- insmod is then asked to insert the module(s) that modprobe has decided that the kernel needs. Every module will be configured according to the "options" lines in "/etc/conf.modules".
- modprobe exits and kerneld tells the kernel that the request succeded (or failed...)
- The kernel uses the freshly installed feature just as if it had been configured into the kernel as a "resident" part.
Configuring kerneld
Options for the /etc/conf.modules file.
```
     keep
     parameter=value
     options module symbol=value ...
     alias module real_name
     pre-install module command ...
     install module command ...
     post-install module command ...
     pre-remove module command ...
     remove module command ...
     post-remove module command ...
```
ifconfig messages

Begin excerpt from kerneld mini-HOWTO

Around kernel version 1.3.80, the networking code was changed to allow loading protocol families (e.g. IPX, AX.25 and AppleTalk) as modules. This caused the addition of a new kerneld request: net-pf-X, where X is a number identifying the protocol (see /usr/src/linux/include/linux/socket.h for the meaning of the various numbers).
Unfortunately, ifconfig accidentally triggers these messages, so a lot of people get a couple of messages logged when the system boots and runs ifconfig to setup the loopback device. The messages are harmless, and you can disable them by adding the lines
```
        alias net-pf-3 off      # Forget AX.25
        alias net-pf-4 off      # Forget IPX
        alias net-pf-5 off      # Forget AppleTalk
```
to /etc/conf.modules. Of course, if you do use IPX as a module, you should not add a line to disable IPX.

End excerpt from kerneld mini-HOWTO

Last Modified: 17 September 1996
St. Louis Unix Users Group - Linux SIG

Loadable Kernel Modules and Kerneld

What is kerneld?

Why do I want to use it ?

Links to necessary components

Installing Linux loadable modules and kerneld

Loading/unloading modules manually

What kerneld does

Configuring kerneld

ifconfig messages