init_module(2) System Calls Manual 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(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 neces-
sary symbol relocations, initializes module parameters to values pro-
vided 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 speci-
fications 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 mod-
ule_param() (see the Linux kernel source file include/linux/mod-
uleparam.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 mod-
ule to be loaded from the file descriptor fd. It is useful when the au-
thenticity of a kernel module can be determined from its location in the
filesystem; in cases where that is possible, the overhead of using cryp-
tographically 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 configura-
tion option enabled, a version hash is recorded for each symbol the mod-
ule 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 in-
dicates that the symbol version hashes are to be ignored. If the kernel
is built to permit forced loading (i.e., configured with CONFIG_MOD-
ULE_FORCE_LOAD), then loading continues, otherwise it fails with the er-
ror ENOEXEC as expected for malformed modules.
If the kernel was build with CONFIG_MODULE_DECOMPRESS, the in-kernel de-
compression feature can be used. User-space code can check if the ker-
nel supports decompression by reading the /sys/module/compression at-
tribute. If the kernel supports decompression, the compressed file can
directly be passed to finit_module() using the MODULE_INIT_COM-
PRESSED_FILE flag. The in-kernel module decompressor supports the fol-
lowing 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 se-
lected 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 mod-
ule.
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 con-
figured with CONFIG_MODULE_SIG_FORCE; if the kernel was not con-
figured with this option, then an invalid or unsigned module sim-
ply 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/ker-
nel/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 mod-
ule_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_DECOM-
PRESS.
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 exe-
cuted 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; alter-
natively, 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 dif-
ferent:
#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 er-
ror 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 func-
tion. 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 de-
fined 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 ex-
pected 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), mod-
probe(8)
Linux man-pages 6.9.1 2024-05-02 init_module(2)
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