Modern NetBSD allows adding and removing kernel functionality dynamically through kernel modules. Kernel modules can be loaded and unloaded without needing to recompile or reboot the kernel. They are placed in directories under /stand and can be loaded at boot time via boot.cfg or using the modload command. Writing kernel modules is straightforward, requiring only MODULE and _modcmd declarations. However, there are some limitations around passing parameters and creating device nodes.

1. Modern NetBSD
Kernel Module
Mar 14, 2014
Masaru OKI (@masaru0714)

2. Contents
Self introduction
Introduction of kernel module
Using kernel module
Creating kernel module
Problems

3. Self introduction
Name 沖 勝 (Masaru OKI) oki@n.o
History
1993 Port NetBSD to Sharp X68030
2001- IIJ SEIL team
2013- Stratosphere SDN related job
Last week touch kernel module :-)

4. What is kernel module?
Add or remove kernel function dynamically.
● filesystem
● device driver
● etc.
No kernel module case, we need edit kernel
configuration, compile kernel, replacement
kernel binary, and reboot.

5. modern?
historical: LKM (Loadable Kernel Module)
reesrved entry in the table.
fixed limit number of entries.
no loading at bootstrap.
modern: options MODULAR framework
no reserved entry
loading at bootstrap (see boot.cfg(5))

6. placement of kernel module
/starnd/<arch>/<ver>/modules/<name>/<name>.kmod
<arch> amd64, i386,
<ver> 6.0, 6.1,
<name> iscsi, zfs, compat_linux, wbsio,
kernel module is also in NetBSD kernel binary,
there are named ‘builtin module’.

7. Show status
modstat(8)
NAME CLASS SOURCE REFS SIZE REQUIRES
accf_dataready misc builtin 0 - -
accf_httpready misc builtin 0 - -
acpiacad driver builtin 0 - -
acpibat driver builtin 0 - -
acpibut driver builtin 0 - -
acpicpu driver builtin 0 - -
acpidalb driver builtin 0 - -
acpifan driver builtin 0 - -

8. Load kernel module
modload <name>
loading /stand/…/<name>/<name>.kmod.
modload example
modload <path>
loading specified file.
modload ~/src/example/example.kmod
securitylevel <= 0 only.

9. Unload module
modunload <name>
unloading module specified name.
modunload example
Unloading module in use?
error … fine, no problem.
but no error, caused panic.

10. Loading at bootstrap
/boot.cfg (amd64, i386 only)
menu=Boot normally:rndseed /var/db/entropy-file;boot netbsd
menu=Boot single user:rndseed /var/db/entropy-file;boot netbsd -s
menu=Boot with module foo:load /foo.kmod;boot
# always load example module
load=/foo/bar/example.kmod

11. Writing kernel module, easy?
Yes, easy!
also easy panic reboot...
but don’t worry.
kernel module needs two definitions
MODULE(<class>, <name>, <required>);
<name>_modcmd()

12. MODULE()
Declare kernel module.
MODULE(<class>, <name>, <required>)
class Module class.
name name of module
required required module string if needed
MODULE(MODULE_CLASS_DRIVER, vnd, ”zlib”);

13. MODULE(): class
MODULE_CLASS_DRIVER device driver
MODULE_CLASS_EXEC executable image handler
MODULE_CLASS_MISC moscellaneous module
MODULE_CLASS_SECMODEL security model
MODULE_CLASS_VFS virtual filesystem

14. <name>_modcmd()
module command API.
<name>_modcmd(<cmd>, <data>)
value of <cmd>
MODULE_CMD_INIT Initialize
MODULE_CMD_FINI Clean-up
MODULE_CMD_AUTOUNLOAD Notify (option)
MODULE_CMD_STAT Status (not implemented)

15. Writing Makefile
Simple.
KMOD= example
SRCS= example.c
.include <bsd.kmodule.mk>

16. Other item requires for compile
Kernel source needed.
● extract to /usr/src/sys
● other place, specify e.g. S=/foo/src

17. Example (do nothing)
test by modload, modstat and modunload.
#include <sys/module.h>
MODULE(MODULE_CLASS_MISC, example, NULL);
static int
example_modcmd(modcmd_t cmd, void *arg)
{
return 0;
}

18. Add sysctl
static struct sysctl_log *sysctl_log;
static int testvar;
MODULE(MODULE_CLASS_MISC, example, NULL);
static int example_modcmd(modcmd_t cmd, void *arg) {
switch (cmd) {
case MODULE_CMD_INIT:
sysctl_createv(&sysctl_log, 0, NULL, NULL,
CTLFLAG_READWRITE, CTLTYPE_INT, “example”,
SYSCTL_DESCR(“Test.”), NULL, 0, &testvar, 0,
CTL_HW, CTL_CREATE, CTL_EOL);
break;
case MODULE_CMD_FINI:
sysctl_teardown(&sysctl_log);
break;
}
return 0;
}
hw.example is created by
load module.
it can read/write by sysctl
hw.example is destroyed
by unload module.

19. Problems
No parameter with loading module
static resource allocation at bootstrap case
workaround: after bootstrap, use sysctl?
kenv is available at FreeBSD (/boot/loader.conf)
No API for create device node (/dev/<name>)
major is fixed at loading module
mknod in module? (not recommended by module(7))
mknod -l?

20. Not only API, but commands are different.
Appendix: compares with other OSs
OS NetBSD FreeBSD Linux
suffix .kmod .ko .ko
load modload kldload insmod
unload modunload kldunload rmmod
status modstat kldstat lsmod
placement /stand/... /boot/kernel/ /lib/modules/...