Understanding the Linux Kernel

Module Implementation

Modules are stored in the filesystem as ELF object files. The kernel considers only modules that have been loaded into RAM by the /sbin/insmod program (see Section B.3) and for each of them it allocates a memory area containing the following data:

• A module object

• A null-terminated string that represents the name of the module (all modules should have unique names)

• The code that implements the functions of the module

The module object describes a module; its fields are shown in Table B-1. A simply linked list collects all module objects, where the next field of each object points to the next element in the list. The first element of the list is addressed by the module_list variable. But actually, the first element of the list is always the same: it is named kernel_module and refers to a fictitious module representing the statically linked kernel code.

Table The module Object

Type	Name	Description
unsigned long	size_of_struct	Size of module object
struct module *	next	Next list element
const char *	name	Pointer to module name
unsigned long	size	Module size
atomic_t	uc.usecount	Module usage counter
unsigned long	flags	Module flags
unsigned int	nsyms	Number of exported symbols
unsigned int	ndeps	Number of referenced modules
struct module_symbol *	syms	Table of exported symbols
struct module_ref *	deps	List of referenced modules
struct module_ref *	refs	List of referencing modules
int (*)(void)	init	Initialization method
void (*)(void)	cleanup	Cleanup method