Specifications

CHAPTER 5. IMPLEMENTATION 40

pass all checks performed by the module loader’s functions to be accepted as a valid Input

Abstraction Layer module.

mod_scan()

mod_verify() dl_function()

filename

filename, symbol

handle, symbol

dlopen()

Failure

Success

Failure

Success

Failure

Success

filename

SuccessFailure

dlsym()

SuccessFailure

Figure 5.5: Input Abstraction Layer Module Veriﬁcation

Veriﬁed modules are stored in a double linked list. This double linked list is used by the

daemon to manage the available modules at run time. An element of the list represents one

module and consists of three pointers: a pointer data to a structure of type ModuleData, a

pointer prev to the previous element of the list and a pointer next to the next element of the

list. The data structure for an element of the list is declared in libial mod.h:

typedef struct IalModule_s {

ModuleData *data;

struct IalModule_s *prev;

struct IalModule_s *next;

} IalModule;

Each veriﬁed mo dule is added to the double linked list. To access the double linked list,

the daemon deﬁnes the global variable modules list head, which is initialized with the value

NULL:

IalModule *modules_list_head = NULL;

Once a veriﬁed module is found, the module loader adds it to the list by invoking the

function module add(). The function module add() allocates memory for the new element

(IalModule module) and sets up its members. To obtain the pointer to the module’s structure

ModuleData, the already discussed function dl function() is used. The following source code

is an excerpt of module add():

/* Define function returning a pointer to ModuleData. */

ModuleData *(*function) (void);

/* Obtain function pointer. */

function = dl_function(filename, "mod_get_data");

Now the function pointer returned by dl function() is assigned to function. It can