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Unit 13: Case Study: Linux

Let us revisit the working of the insmod to get a clear picture of the module loading operation. Notes
The insmod depends on some critical system calls to load the module to the kernel. It uses the
sys_create_module to allocate kernel memory to hold module. It uses get_kernel_syms system
call to get the kernel symbol table in order to link the module. It then calls the sys_init_module
system call to copy the relocatable object code of the module to the kernel space. And soon after
this, insmod calls the initialization function of the concerned module i.e. init_module. All of these
system calls can be found in kernel/module.c.

13.2.3 Unloading Modules

The modules can be unloaded using rmmod command. While removing modules, rmmod ensures
the restriction that the modules are not in use and they are not referred by any other module or
the part of the kernel. The demand loaded modules (i.e. the modules loaded by kerneld) are
automatically removed from the system by `kerneld’ when they are no longer used. Every time
it’s idle, timer expires and kerneld makes a system call requesting for all the demand loaded

kernels, which are not busy to be removed. The modules, whose visited flags are cleared and
marked as AUTOCLEAN, are `unloaded’.
Assuming that the module can be unloaded, the cleanup_module function of the concerned
module is called to freeup the kernel resources it has allocated. After the successful execution of
the cleanup_module, the module datastructure is marked DELETED and it is unlinked from the
kernel and unlisted from the list of kernel modules. The reference list of the modules on which
it (module removed) is dependent is modified and dependency is released. The kernel memory

allocated to the concerned module is deallocated and returned to the kernel memory spool.
Version Dependency of modules Version dependency of the module is one of the trickiest parts
of the Linux Kernel Module programming. Typically, the modules are required to be compiled
for each version of the kernel. Each module defines a symbol called kernel_version, which

insmod matches against the version number of the current kernel. The kernel 2.2.x/2.4.x defi ne
the symbol in. Hence if the module is made up of multiple source files, should be included in

only one of the source fi les.
Though typically, modules should be recompiled for each kernel version, it is not always possible
to recompile module when it is run on as a commercial module distributed in binary form. Kernel
developers have provided a flexible way to deal with the version problem. The idea is that a

module is incompatible with a different kernel version only if the software interface offered by
the kernel is changed. The software interface is represented by the function prototype and the

exact definition of all the data structures involved in the function call. The CRC algorithm can be
used to map all the information about software interface to the single 32bit number. The issue of
version dependency is handled by using the name of the each symbol exported.

13.3 Process Management

Any application that runs on a Linux system is assigned a process ID or PID. This is a numerical
representation of the instance of the application on the system. In most situations this information
is only relevant to the system administrator who may have to debug or terminate processes by
referencing the PID. Process Management is the series of tasks a System Administrator completes
to monitor, manage, and maintain instances of running applications.

Multitasking

Process Management beings with an understanding concept of Multitasking. Linux is what is
referred to as a preemptive multitasking operating system. Preemptive multitasking systems rely
on a scheduler. The function of the scheduler is to control the process that is currently using the
CPU. In contrast, symmetric multitasking systems such as Windows 3.1 relied on each running
process to voluntary relinquish control of the processor. If an application in this system hung or

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