init_module(2)

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.

SEE ALSO
create_module(2), delete_module(2), query_module(2), lsmod(8), modprobe(8)