Ubuntu Manpage: init_module, finit_module

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(unsigned long size;
                   SYS_init_module, void module_image[size], unsigned long size,
                   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; size 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.

MODULE_INIT_COMPRESSED_FILE (since Linux 5.17)
Use in-kernel module decompression.

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.

If the kernel was build with CONFIG_MODULE_DECOMPRESS, the in-kernel decompression feature can be used.
User-space code can check if the kernel supports decompression by reading the /sys/module/compression
attribute. If the kernel supports decompression, the compressed file can directly be passed to
finit_module() using the MODULE_INIT_COMPRESSED_FILE flag. The in-kernel module decompressor supports
the following compression algorithms:

• gzip (since Linux 5.17)
• xz (since Linux 5.17)
• zstd (since Linux 6.2)

The kernel only implements a single decompression method. This is selected during module generation
accordingly to the compression method chosen in the kernel configuration.

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.

EINVAL The decompressor sanity checks failed, while loading a compressed module with flag
MODULE_INIT_COMPRESSED_FILE set.

ENOEXEC
fd does not refer to an open file.

EOPNOTSUPP (since Linux 5.17)
The flag MODULE_INIT_COMPRESSED_FILE is set to load a compressed module, and the kernel was built
without CONFIG_MODULE_DECOMPRESS.

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;
               typeof(int (void))   *init;
               typeof(void (void))  *cleanup;
               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.