Understanding the Linux Kernel, Second Edition

Linking and Unlinking Modules

A user can link a module into the running kernel by executing the insmod external program. This program performs the following operations:

1. Reads from the command line the name of the module to be linked.

2. Locates the file containing the module’s object code in the system directory tree. The file is usually placed in some subdirectory below /lib/modules.

3. Computes the size of the memory area needed to store the module code, its name, and the module object.

4. Invokes the create_module( ) system call, passing to it the name and size of the new module. The corresponding sys_create_module( ) service routine performs the following operations:

   1. Checks whether the user is allowed to link the module (the current process must have the CAP_SYS_MODULE capability). In any situation where one is adding functionality to a kernel, which has access to all data and processes on the system, security is a paramount concern.

   2. Invokes the find_module( ) function to scan the module_list list of module objects looking for a module with the specified name. If it is found, the module has already been linked, so the system call terminates.

   3. Invokes vmalloc( ) to allocate a memory area for the new module.

   4. Initializes the fields of the module object at the beginning of the memory area and copies the name of the module right below the object.

   5. Inserts the module object into the list pointed to by module_list.

   6. Returns the starting address of the memory area allocated to the module.

5. Invokes ...