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MODULE(9) Kernel Developer's Manual MODULE(9)

module, module_load, module_autoload, module_unload, module_init_class, module_hold, module_rele, module_find_section, module_kernel, module_name, module_source, module_register_callbacks, module_unregister_callbacks, module_specific_key_create, module_specific_key_delete, module_getspecific, module_setspecific — kernel module loader

#include <sys/module.h>

MODULE(class, name, required);

int
module_load(const char *name, int flags, prop_dictionary_t props, modclass_t class);

int
module_autoload(const char *name, modclass_t class);

int
module_unload(const char *name);

void
module_init_class(modclass_t class);

int
module_hold(module_t *module);

void
module_rele(module_t *module);

int
module_find_section(const char *, void **, size_t *);

module_t *
module_kernel(void);

const char *
module_name(struct module *module);

modsrc_t
module_source(struct module *module);

void
module_init(void);

void
module_start_unload_thread(void);

void
module_builtin_require_force(void);

void
module_load_vfs_init(void);

void *
module_register_callbacks(void (*)(struct module *), void (*unload)(struct module *));

void
module_unregister_callbacks(void *);

specificdata_key_t
module_specific_key_create(specificdata_key_t *keyp, specificdata_dtor_t dtor);

void
module_specific_key_delete(specificdata_key_t key);

void *
module_getspecific(module_t *mod, specificdata_key_t key);

void *
module_setspecific(module_t *mod, specificdata_key_t key, void *data);

Modules are sections of code that can be independently linked and selectively loaded into or unloaded from a running kernel. This provides a mechanism to update the module without having to relink the kernel and reboot. Modules can be loaded from within the kernel image, provided by the boot loader, or loaded from the file system.

The module subsystem includes two data types:

  1. The module_t type provides storage to describe a module.
  2. The modinfo_t type resides within module_t and contains module header info.

The module subsystem is protected by the global kernconfig_mutex.

(class, name, required)
The MODULE() macro creates and initializes a modinfo_t structure. The class argument identifies the class of module, and must be one of the following (note that MODULE_CLASS_ANY should not be used here):
The module provide a virtual file system - see vfs(9)
The module is a device driver - see driver(9)
The module provides an alternate execution environment - see the various COMPAT_xxx options in options(4)
The module provides a security model - see secmodel(9)
The module provides a buffer queue strategy - see bufq(9)
The module provides miscellaneous kernel services

The name argument provides the name of the module. Loaded modules, including those that are built-in to the kernel, must all have unique names.

The required argument is a quoted string containing a comma-separated list of module names that are required by this module. The list must not contain any white-space. When a module is loaded, all of its required modules are auto-loaded and initialized before the module itself is loaded. Loading of required modules is a recursive operation.

If there are no required modules, this argument should be specified as NULL.

In addition to the explicit arguments, the () macro creates a reference to the module's modcmd() function. This function is defined as:

modcmd_t
(modcmd_t cmd, void *data)

(where xxx is the name of the module, from the MODULE macro).

The cmd argument requests one of the following operations:

Perform module-specific initialization when the module is loaded.
Perform module-specific clean-up before the module is unloaded.
Notify the module that it is about to be unloaded.
Request the module to provide status information (not currently implemented).

All modules' () functions must implement the MODULE_CMD_INIT and MODULE_CMD_FINI commands. The other commands are optional, and should return ENOTTY if not implemented.

For the MODULE_CMD_INIT command, the data argument is used to pass a pointer to the module's prop_dictionary(3). For the MODULE_CMD_STAT command, the data argument points to a buffer where the status information should be placed.

The __link_set mechanism is used to enable the module subsystem to locate the modinfo_t structure.

(name, flags, props, class)
Load a module, link it into the running kernel, and call the module's modcmd() routine with a cmd argument of MODULE_CMD_INIT. If the specified module requires other modules, they are loaded first; if any required module cannot be loaded or if any of their modcmd() control routines returns a non-zero status, loading of this module and the specific required module will fail. The required modules are marked for automatic unloading. Thus, if the loading of the module failed, the required modules will be automatically unloaded after a short delay.

The loader will look first for a built-in module with the specified name that has not been disabled (see () below). If a built-in module with that name is not found, the list of modules prepared by the boot loader is searched. If the named module is still not found, an attempt is made to locate the module within the file system, provided it has been mounted by the initialization code.

The flags argument can include:

When loading a module from the file system, do not attempt to locate a corresponding prop_dictionary file.
Force loading of disabled built-in modules and modules built for a different version of the operating system.

The props argument points to an externalized property list which is passed to the module's () routine. If a module is being loaded from the file system, and the MODCTL_NO_PROP flag is not set, the system searches for a file with the same name as the module file, but with the suffix “.plist”. If this file is found, the prop_dictionary it contains is loaded and merged with the prop_dictionary from the props argument.

The class argument can be any of:

 
Device driver
Executable image handler
Miscellaneous module
Security model (see secmodel(9) for more details)
Virtual file system

If the class is not MODULE_CLASS_ANY, the class of the module being loaded must match the requested class. Except when verifying a module's class when it is being loaded, module classes other than MODULE_CLASS_SECMODEL are transparent to the module subsystem. They are provided only for the benefit of the subsystem's clients. Modules with class MODULE_CLASS_SECMODEL are automatically registered with () after being successfully loaded, and automatically deregistered with () when being unloaded.

The () routine is primarily intended as the implementation of the MODCTL_LOAD option of the modctl(2) system call.

module_autoload(name, class)
Auto-load a module, making it available for automatic unloading. The name and class arguments are the same as for the module_load() routine.

The module subsystem uses a kernel thread to attempt to automatically unload modules a short time (currently, 10 seconds) after being loaded by (). Before the module is unloaded, its modcmd() is called with the cmd argument specified as MODULE_CMD_AUTOUNLOAD. A module can prevent itself from being unloaded by returning a non-zero value.

The () function is intended for use by kernel components to locate and load optional system components. The function is also used to load modules that are required by other modules.

The directory from which the module is loaded will be searched for a file with the same name as the module file, but with the suffix “.plist”. If this file is found, the prop_dictionary it contains will be loaded and passed to the module's () routine. If this prop_dictionary contains a “noautoload” property which is set to “true” then the system will refuse to load the module.

module_unload(name)
Unload a module. If the module's reference count is non-zero, the function returns EBUSY. Otherwise, the module's modcmd() routine is called with a cmd argument of MODULE_CMD_FINI. If the modcmd() routine returns with an error, then the error is returned to the caller otherwise the module is unloaded.

The reference counts of all modules that were required by this module are decremented, but the required modules are not unloaded by the call to (). Instead, the required modules may be unloaded by subsequent calls to module_unload().

Unloading a built-in module causes the module to be marked as disabled. This prevents the module from being re-loaded, except by the () function with the flags argument set to MODULE_FORCE_LOAD.

The () function may be called by the modctl(2) system call, by the module subsystem's internal auto-unload thread, or by other kernel facilities. Generally, other kernel facilities should not be calling this function.

(class)
Load and initialize all available modules of the specified class. Any built-in modules that have not been disabled, and any modules provided by the boot loader are loaded.
(module)
Increment the reference count of a module. A module cannot be unloaded if its reference count is non-zero.
(module)
Decrement the reference count of a module.
(name, addr, size)
Find the start address and size of linker section name within a module. The miniroot module uses this routine to find the address and size of the embedded file system image. This routine can only examine the linker data for the module that is currently being initialized; it cannot examine data for any other module.
(void)
Returns a pointer to a module_t structure describing the kernel module.
(module)
Returns a pointer to the module's name.
(module)
Returns the source of the module, one of MODULE_SOURCE_KERNEL, MODULE_SOURCE_BOOT, or MODULE_SOURCE_FILESYS.
(void)
Initialize the module subsystem. Creates and initializes various data structures, locates all built-in modules, and establishes the sub-system's sysctl(8) tree. module_init() is called early in system initialization to facilitate use of security model modules.
(void)
Create the thread that attempts to automatically unload modules that were loaded via the module_autoload() routine. The function is called only once, after the scheduler and timer functions are initialized.
(void)
Mark as "disabled" any built-in modules that have not been successfully initialized. Modules marked "disabled" can only be loaded if the MODCTL_LOAD_FORCE is specified. module_builtin_require_force() is called near the end of system initialization, after the init(8) process is created.
(void)
The module subsystem is initialized early, long before any file systems are available. After the root file system is mounted, module_load_vfs_init(void) is used to enable loading modules from the file system. Until this routine is called, modules can only be loaded if they were built-in to the kernel image or provided by the boot loader.
(void (*load)(struct module *), void (*unload)(struct module *))
Register a new set of callbacks. The load callback is invoked after any module (including this module) is successfully loaded; the unload callback is invoked before any module is unloaded. Each load or unload event can result in multiple invocations of the callback routines. An opaque cookie is returned which can be passed to ().
module_unregister_callbacks(void *opaque)
Unregister a set of callback routines previously registered with module_register_callbacks(). The opaque argument should be the return value from the previous module_register_callbacks() call.
(specificdata_key_t *keyp, specificdata_dtor_t dtor)
Creates a new specificdata_key for use within the module domain. The key identifier is returned in keyp.
(specificdata_key_t key)
Deletes the specified specificdata_key key from the module domain.
(module_t *mod, specificdata_key_t key)
Retrieves the value associated with key from module mod.
(module_t *mod, specificdata_key_t key, void *data)
Stores data as the value associated with key for module mod.

The module subsystem is designed to be called recursively, but only within a single LWP. This permits one module's modcmd() routine to load or unload other modules.

Additional considerations:

A set of example modules is available in the src/sys/modules/examples directory hierarchy.

The core of the kernel module implementation is in sys/kern/kern_module.c and sys/kern/kern_module_vfs.c.

The routines for linking the module are in sys/kern/subr_kobj.c.

The routines for reading a module from the file system are in sys/kern/subr_kobj_vfs.c.

The header file <sys/sys/module.h> describes the public interface.

In addition, each architecture is expected to provide (), (), and (). kobj_machdep() is for any machine dependent actions, such as flushing caches, that are needed when a module is loaded or unloaded. kobj_reloc() deals with resolution of relocatable symbols. module_init_md() is for finding modules passed in by the boot loader.

modctl(2), module(7), specificdata(9), intro(9lua)

The kernel module subsystem first appeared in NetBSD 5.0. It replaces the “LKM” subsystem from earlier releases.

The module system was written by Andrew Doran <ad@NetBSD.org>. This manual page was written by Paul Goyette <pgoyette@NetBSD.org>.

April 8, 2019 NetBSD-9.2