FreeBSD kernel comes with some basic features. It could be necessary, however, to widen them with additional drivers or functionalities: for this purpose, kernel modules allow the system administrator to integrate new code inside a running system. In FreeBSD, a module is provided as an object file with extension .ko. It

contains code to extend the running kernel, or so-called base kernel […]. [Modules] are typically used to add support for new hardware (as device drivers) and/or filesystems, or for adding system calls. When [their functionalities are] no longer required, [they] can be unloaded in order to free memory and other resources.

(From Wikipedia)

Again, modules are used by

system administrators to dynamically add and remove functionality from a running system.

They also allow

device driver writers to load their new changes into a running kernel without constantly rebooting to test changes.

(From the FreeBSD Handbook)

To add, list and remove FreeBSD kernel modules, the Dynamic Kernel Linker kld interface is available: these operations are respectively performed by the tools kldload(8), kldstat(8) and kldunload(8).


kldstat

More specifically, kldstat lists the modules currently loaded and in use by the kernel. For example:

$ kldstat
Id Refs Address                Size Name
 1   47 0xffffffff80200000  243cd00 kernel
 2    1 0xffffffff8263d000    265d0 ext2fs.ko
 3    1 0xffffffff82665000     2be8 coretemp.ko
 4    1 0xffffffff82821000   154cc8 radeonkms.ko
 5    1 0xffffffff82976000    858b0 drm.ko
 6    4 0xffffffff829fc000    104f0 linuxkpi.ko
 7    3 0xffffffff82a0d000    13f30 linuxkpi_gplv2.ko
 8    2 0xffffffff82a21000      6c0 debugfs.ko
 9    1 0xffffffff82a22000      a75 radeon_RV635_pfp_bin.ko
10    1 0xffffffff82a23000     5573 radeon_RV635_me_bin.ko
11    1 0xffffffff82a29000      d73 radeon_R600_rlc_bin.ko
12    1 0xffffffff82a2a000    12343 radeon_R600_uvd_bin.ko
13    1 0xffffffff82a3d000     1800 uhid.ko
14    1 0xffffffff82a3f000     23a8 ums.ko
15    1 0xffffffff82a42000      acf mac_ntpd.ko
16    1 0xffffffff82a43000     9c08 fuse.ko

Each module has an id. Kernel, and so its modules, are loaded into RAM: therefore, the memory address of the beginning of each module is shown in this output. Size is by default an hexadecimal number of bytes: option -h can be used to display a human-readable value instead.

Consider for example module ext2fs.ko: it begins at byte 0xffffffff8263d000 in memory, its size is 0x265d0 bytes. Note that the sum

0xffffffff8263d000 + 0x265d0 = 0xffffffff826635D0

(which denotes the end of the module) is lower than the beginning byte 0xffffffff82665000 of the next module coretemp.ko. This occurs for each module. In ext2fs.ko, the difference amounts to 0x1a30 (6704 in decimal units) bytes: this value is not constant across modules.

Use options -vd to display more information and to list the modules already included in kernel.


kldload

kldload(8) manpage in FreeBSD provides very exhaustive information about modules and their loading procedure. Their name can be used, with or without the extension .ko. It can be provided their full path or just their name: in this latter case, the module will be searched in the paths provided by the kernel parameter kern.module_path. To obtain its value, use sysctl. For example:

$ sysctl kern.module_path
kern.module_path: /boot/kernel;/boot/modules;/boot/dtb;/boot/dtb/overlays

Usually, modules which are part of the base system (shipped with the OS) are in /boot/kernel, while modules installed by ports or packages, or proprietary ones, are in /boot/modules.

For example, ext2fs.ko, coretemp.ko, fuse.ko are stored in /boot/kernel, while radeonkms.ko, drm.ko and the modules radeon_R*.ko are stored in /boot/modules.

kldload(8) also states that:

To load a module from the current directory it must be specified as a full
or relative path.

To load the example module ext2fs.ko, which is already in /boot/kernel, use

# kldload ext2fs.ko


kldunload

As anticipated, each kernel module requires an amount of RAM, according to its size. If its functionality is no more required, a module can be removed from the kernel, so freeing its related portion of memory. For example:

# kldunload ext2fs.ko

See kldunload(8) for more options.


Load at boot

Modules can be loaded at any moment with kldload(8), but if their use is always required, they can alternatively be loaded at boot time. Two methods are available, with a different syntax.

  • In /etc/rc.conf, add the following line:

    kld_list="/full/path/to/module.ko"

  • In /boot/loader.conf, add the following line:

    module_name_without_ko="YES"

As specified in this message, both of them are «correct»; the first method, however, may be preferred because, according to rc.conf(5), kld_list specifies «a list of kernel modules to load right after the local disks are mounted». Not only: «loading modules at this point in the boot process is much faster than doing it via /boot/loader.conf», but obviously they can not be modules necessary to mount a local disk.

If for example modules ext2fs and/or fuse are required to mount one or more local disks (or partitions), they must be loaded using /boot/loader.conf.