How modprobe works

What is modprobe and how it works

It is a program that intelligently load and remove modules from the system.
modprobe looks in the module directory “lib/modules/$(KERNEL_RELEASE)/” for the modules and load them according to the rules defined directory /etc/modprobe.d.
In earlier distros, modprobe.conf was used instead.

While loading a specific module via modprobe, modprobe will also load all the dependent modules of a given module.
The list of modules and its dependent modules is stored in modules.dep file.

One can check the list of loaded modules using “lsmod” command

What is depmod

depmod is a program that generates modules.dep and map files.
module.dep is a database of modprobe. It helps modprobe to know a list of dependent modules, while modprobe is loading a speciic module.
Incase we have written a some new modules, to add new modules in the database (modules.dep file), we need to run following command
$ depmod -a.
Generally depmod is executed internally when we install the modules in “lib/modules/$(KERNEL_RELEASE)/” directory by issuing “make modules_install”.

Example

In previous post, I explain kmod subsystem and significance of request_module(). The following outputs highlights how modules of previous posts are loaded and how one can exploit modprobe effectively


[root@kashish kmod_subsystem_usage]# make
make -C /lib/modules/2.6.33.3-85.fc13.i686.PAE/build M=/root/git_client/reps/kmod_subsystem_usage modules
make[1]: Entering directory `/usr/src/kernels/2.6.33.3-85.fc13.i686.PAE'
  CC [M]  /root/git_client/reps/kmod_subsystem_usage/send_signal.o
  CC [M]  /root/git_client/reps/kmod_subsystem_usage/signal_handling.o
  Building modules, stage 2.
  MODPOST 2 modules
  CC      /root/git_client/reps/kmod_subsystem_usage/send_signal.mod.o
  LD [M]  /root/git_client/reps/kmod_subsystem_usage/send_signal.ko
  CC      /root/git_client/reps/kmod_subsystem_usage/signal_handling.mod.o
  LD [M]  /root/git_client/reps/kmod_subsystem_usage/signal_handling.ko
make[1]: Leaving directory `/usr/src/kernels/2.6.33.3-85.fc13.i686.PAE'

The following command will install the modules to /lib/modules/$(KREL). It will also invoke depmod


[root@kashish kmod_subsystem_usage]# make modules_install
make -C /lib/modules/2.6.33.3-85.fc13.i686.PAE/build INSTALL_MOD_DIR=extra M=/root/git_client/reps/kmod_subsystem_usage modules_install
make[1]: Entering directory `/usr/src/kernels/2.6.33.3-85.fc13.i686.PAE'
INSTALL /root/git_client/reps/kmod_subsystem_usage/send_signal.ko
INSTALL /root/git_client/reps/kmod_subsystem_usage/signal_handling.ko
DEPMOD  2.6.33.3-85.fc13.i686.PAE                                        <=======
make[1]: Leaving directory `/usr/src/kernels/2.6.33.3-85.fc13.i686.PAE'                  

One can check whether modules are copied to following directory successfully or not


[root@kashish kmod_subsystem_usage]# ls /lib/modules/2.6.33.3-85.fc13.i686.PAE/extra/
send_signal.ko  signal_handling.ko

Now load the module using modprobe


[root@kashish kmod_subsystem_usage]# modprobe signal_handling

As signal_module.ko loads send_signal.ko as well using request_module().
Module send_signal is using/dependent on module signal_handling.


[root@kashish kmod_subsystem_usage]# lsmod | head -n 3
Module                  Size  Used by
send_signal             1561  0
signal_handling         1429  1 send_signal


[root@kashish kmod_subsystem_usage]# modinfo signal_handling.ko
filename:       signal_handling.ko
license:        GPL
author:         @Kashish_Bhatia
description:    dynamically loading module through kmod subsystem and receiving signal from another module
srcversion:     594684C2DBE8A44CCC1D9A6
depends:
vermagic:       2.6.33.3-85.fc13.i686.PAE SMP mod_unload 686
[root@kashish kmod_subsystem_usage]# modinfo send_signal.ko
filename:       send_signal.ko
license:        GPL
author:         @Kashish_Bhatia
description:    sending signal from 1 kthread to another and using default signal handler
srcversion:     049F7612A1256598DA9F4FD
depends:        signal_handling
vermagic:       2.6.33.3-85.fc13.i686.PAE SMP mod_unload 686

Hence signal_handling module cannot be removed prior send_signal


[root@kashish kmod_subsystem_usage]# modprobe -r signal_handling
FATAL: Module signal_handling is in use.

modprobe can remove dependent modules also (signal_handling)


[root@kashish kmod_subsystem_usage]# modprobe -r send_signal
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