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NAME
init_module, finit_module - load a kernel module
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <linux/module.h> /* Definition of MODULE_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_init_module, void module_image[.len], unsigned long len,
const char *param_values);
int syscall(SYS_finit_module, int fd,
const char *param_values, int flags);
Note: glibc provides no wrappers for these system calls, necessitating the use of
syscall(2).
DESCRIPTION
init_module() loads an ELF image into kernel space, performs any necessary symbol
relocations, initializes module parameters to values provided by the caller, and then runs
the module's init function. This system call requires privilege.
The module_image argument points to a buffer containing the binary image to be loaded; len
specifies the size of that buffer. The module image should be a valid ELF image, built
for the running kernel.
The param_values argument is a string containing space-delimited specifications of the
values for module parameters (defined inside the module using module_param() and
module_param_array()). The kernel parses this string and initializes the specified
parameters. Each of the parameter specifications has the form:
name[=value[,value...]]
The parameter name is one of those defined within the module using module_param() (see the
Linux kernel source file include/linux/moduleparam.h). The parameter value is optional in
the case of bool and invbool parameters. Values for array parameters are specified as a
comma-separated list.
finit_module()
The finit_module() system call is like init_module(), but reads the module to be loaded
from the file descriptor fd. It is useful when the authenticity of a kernel module can be
determined from its location in the filesystem; in cases where that is possible, the
overhead of using cryptographically signed modules to determine the authenticity of a
module can be avoided. The param_values argument is as for init_module().
The flags argument modifies the operation of finit_module(). It is a bit mask value
created by ORing together zero or more of the following flags:
MODULE_INIT_IGNORE_MODVERSIONS
Ignore symbol version hashes.
MODULE_INIT_IGNORE_VERMAGIC
Ignore kernel version magic.
There are some safety checks built into a module to ensure that it matches the kernel
against which it is loaded. These checks are recorded when the module is built and
verified when the module is loaded. First, the module records a "vermagic" string
containing the kernel version number and prominent features (such as the CPU type).
Second, if the module was built with the CONFIG_MODVERSIONS configuration option enabled,
a version hash is recorded for each symbol the module uses. This hash is based on the
types of the arguments and return value for the function named by the symbol. In this
case, the kernel version number within the "vermagic" string is ignored, as the symbol
version hashes are assumed to be sufficiently reliable.
Using the MODULE_INIT_IGNORE_VERMAGIC flag indicates that the "vermagic" string is to be
ignored, and the MODULE_INIT_IGNORE_MODVERSIONS flag indicates that the symbol version
hashes are to be ignored. If the kernel is built to permit forced loading (i.e.,
configured with CONFIG_MODULE_FORCE_LOAD), then loading continues, otherwise it fails with
the error ENOEXEC as expected for malformed modules.
RETURN VALUE
On success, these system calls return 0. On error, -1 is returned and errno is set to
indicate the error.
ERRORS
EBADMSG (since Linux 3.7)
Module signature is misformatted.
EBUSY Timeout while trying to resolve a symbol reference by this module.
EFAULT An address argument referred to a location that is outside the process's accessible
address space.
ENOKEY (since Linux 3.7)
Module signature is invalid or the kernel does not have a key for this module.
This error is returned only if the kernel was configured with
CONFIG_MODULE_SIG_FORCE; if the kernel was not configured with this option, then an
invalid or unsigned module simply taints the kernel.
ENOMEM Out of memory.
EPERM The caller was not privileged (did not have the CAP_SYS_MODULE capability), or
module loading is disabled (see /proc/sys/kernel/modules_disabled in proc(5)).
The following errors may additionally occur for init_module():
EEXIST A module with this name is already loaded.
EINVAL param_values is invalid, or some part of the ELF image in module_image contains
inconsistencies.
ENOEXEC
The binary image supplied in module_image is not an ELF image, or is an ELF image
that is invalid or for a different architecture.
The following errors may additionally occur for finit_module():
EBADF The file referred to by fd is not opened for reading.
EFBIG The file referred to by fd is too large.
EINVAL flags is invalid.
ENOEXEC
fd does not refer to an open file.
ETXTBSY (since Linux 4.7)
The file referred to by fd is opened for read-write.
In addition to the above errors, if the module's init function is executed and returns an
error, then init_module() or finit_module() fails and errno is set to the value returned
by the init function.
STANDARDS
Linux.
HISTORY
finit_module()
Linux 3.8.
The init_module() system call is not supported by glibc. No declaration is provided in
glibc headers, but, through a quirk of history, glibc versions before glibc 2.23 did
export an ABI for this system call. Therefore, in order to employ this system call, it is
(before glibc 2.23) sufficient to manually declare the interface in your code;
alternatively, you can invoke the system call using syscall(2).
Linux 2.4 and earlier
In Linux 2.4 and earlier, the init_module() system call was rather different:
#include <linux/module.h>
int init_module(const char *name, struct module *image);
(User-space applications can detect which version of init_module() is available by calling
query_module(); the latter call fails with the error ENOSYS on Linux 2.6 and later.)
The older version of the system call loads the relocated module image pointed to by image
into kernel space and runs the module's init function. The caller is responsible for
providing the relocated image (since Linux 2.6, the init_module() system call does the
relocation).
The module image begins with a module structure and is followed by code and data as
appropriate. Since Linux 2.2, the module structure is defined as follows:
struct module {
unsigned long size_of_struct;
struct module *next;
const char *name;
unsigned long size;
long usecount;
unsigned long flags;
unsigned int nsyms;
unsigned int ndeps;
struct module_symbol *syms;
struct module_ref *deps;
struct module_ref *refs;
int (*init)(void);
void (*cleanup)(void);
const struct exception_table_entry *ex_table_start;
const struct exception_table_entry *ex_table_end;
#ifdef __alpha__
unsigned long gp;
#endif
};
All of the pointer fields, with the exception of next and refs, are expected to point
within the module body and be initialized as appropriate for kernel space, that is,
relocated with the rest of the module.
NOTES
Information about currently loaded modules can be found in /proc/modules and in the file
trees under the per-module subdirectories under /sys/module.
See the Linux kernel source file include/linux/module.h for some useful background
information.
SEE ALSO
create_module(2), delete_module(2), query_module(2), lsmod(8), modprobe(8)