Linux Kernel Modules

Linux Kernel Modules (LKM)

Practices for writing correct, loadable kernel modules for embedded Linux targets such as automotive IVI, HUD, and RSE devices.

Source of truth: Linux Kernel Module Programming Guide and the Linux kernel source tree (Documentation/).

When to Use This Skill

• Writing a new out-of-tree kernel module for a device driver.
• Adding a lightweight kernel-space component that communicates via /proc, /sys, or ioctl.
• Building a module against a vendor kernel for Android/Linux embedded targets.

Minimal Kernel Module

// hello.c
#include <linux/module.h>   // module_init, module_exit, MODULE_*
#include <linux/kernel.h>   // printk, KERN_INFO

static int __init hello_init(void)
{
    printk(KERN_INFO "hello: module loaded\n");
    return 0;  // return 0 = success; non-zero = refuse to load
}

static void __exit hello_exit(void)
{
    printk(KERN_INFO "hello: module unloaded\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("Minimal example LKM");

Rules:

• __init marks the function reclaimable after the module loads (reduces kernel memory).
• __exit is discarded if the module is compiled into the kernel (not loadable).
• MODULE_LICENSE("GPL") is required for using GPL-exported kernel symbols. Without it, the kernel will mark itself as "tainted".
• module_init / module_exit register the init and cleanup entry points.

Kbuild File

# Kbuild — used by the kernel build system
obj-m += hello.o

For a module built from multiple source files:

obj-m += mydriver.o
mydriver-objs := mydriver_core.o mydriver_hw.o mydriver_irq.o

Out-of-Tree Build (Makefile)

# Makefile — top-level build script invoked by the developer
KDIR ?= /lib/modules/$(shell uname -r)/build

all:
	$(MAKE) -C $(KDIR) M=$(PWD) modules

clean:
	$(MAKE) -C $(KDIR) M=$(PWD) clean

For cross-compilation (e.g., targeting ARM64 Android kernel):

KDIR ?= /path/to/android/kernel/out/arch/arm64
ARCH := arm64
CROSS_COMPILE := aarch64-linux-gnu-

all:
	$(MAKE) -C $(KDIR) M=$(PWD) ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE) modules

Module Parameters

#include <linux/moduleparam.h>

static int debug_level = 0;
static char *device_name = "mydevice";
static int buffer_size = 4096;

module_param(debug_level, int, 0644);   // readable and writable via sysfs
module_param(device_name, charp, 0444); // read-only via sysfs
module_param(buffer_size, int, 0444);

MODULE_PARM_DESC(debug_level, "Debug verbosity level (0=off, 1=basic, 2=verbose)");
MODULE_PARM_DESC(device_name, "Name of the target device");
MODULE_PARM_DESC(buffer_size, "Internal buffer size in bytes");

Parameters can be set at load time:

insmod mymodule.ko debug_level=2 device_name=sensor0

Or at boot via /etc/modprobe.d/mymodule.conf:

options mymodule debug_level=1

The third argument to module_param is the sysfs permissions:

• 0 — not exposed in sysfs.
• 0444 — world-readable, not writable.
• 0644 — root-writable, world-readable.

printk and Log Levels

printk(KERN_EMERG   "system is unusable\n");          // 0
printk(KERN_ALERT   "action must be taken immediately\n"); // 1
printk(KERN_CRIT    "critical condition\n");           // 2
printk(KERN_ERR     "error condition\n");              // 3
printk(KERN_WARNING "warning condition\n");            // 4
printk(KERN_NOTICE  "normal but significant\n");       // 5
printk(KERN_INFO    "informational message\n");        // 6
printk(KERN_DEBUG   "debug-level message\n");          // 7

Use the pr_* shortcuts for shorter code:

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt  // adds "mymodule: " prefix

pr_info("driver initialized, version %s\n", DRIVER_VERSION);
pr_err("failed to allocate buffer: %d\n", ret);
pr_debug("entering %s\n", __func__);  // only active when dynamic debug or DEBUG defined

Loading and Unloading

# Load a module (with dependencies resolved)
modprobe mymodule

# Load a specific .ko file (no dependency resolution)
insmod ./mymodule.ko

# Unload
rmmod mymodule

# Show loaded modules
lsmod | grep my

# Show module info (license, params, etc.)
modinfo mymodule.ko

# View kernel log
dmesg | tail -20

Exporting Symbols

Other modules can call functions from your module if you export them:

int my_public_function(int arg)
{
    return arg * 2;
}
EXPORT_SYMBOL(my_public_function);       // available to all modules
EXPORT_SYMBOL_GPL(my_public_function);   // available only to GPL-licensed modules

Common Kernel APIs in Modules

GFP flags for kmalloc:

• GFP_KERNEL — may sleep; use in process context.
• GFP_ATOMIC — cannot sleep; use in interrupt context.

Prerequisites

• Linux host machine (Ubuntu 20.04 LTS or Debian 11 recommended).
• Cross-compilation toolchain: arm-linux-gnueabihf-gcc or aarch64-linux-gnu-gcc.
• Target board with serial console access (USB-to-UART adapter).
• ssh access to target (optional but recommended).

Step-by-Step Workflows

Step 1: Create the module source file

Add module_init(), module_exit(), MODULE_LICENSE("GPL"), and module metadata macros.

Step 2: Create the Kbuild file

Add obj-m += mymodule.o for out-of-tree builds or obj-$(CONFIG_MY_MODULE) += mymodule.o for in-tree.

Step 3: Build the module

Run make -C /lib/modules/$(uname -r)/build M=$(pwd) modules for out-of-tree builds.

Step 4: Load and test

Run insmod mymodule.ko; check dmesg for init messages; exercise the module functionality.

Step 5: Unload and clean up

Run rmmod mymodule; confirm cleanup log messages appear; run make clean.

Troubleshooting

• Unknown symbol in module — the symbol is defined but not exported; add EXPORT_SYMBOL(symbol_name) in the defining module.
• module verification failed: signature and/or required key missing — boot without CONFIG_MODULE_SIG_FORCE or sign the module with the kernel signing key.
• Module loads but device not created — device_create() or misc_register() may have failed silently; check the return value and dmesg.
• Kernel oops on insmod — enable CONFIG_KASAN in the kernel config; build the module with debug info and analyze the oops stack trace.

Pre-Commit Checklist

• MODULE_LICENSE declared (usually "GPL" or "Dual MIT/GPL").
• module_init and module_exit macros present.
• Init function returns 0 on success and a negative errno on failure.
• Cleanup function frees all resources allocated in init and in any other paths.
• No GFP_KERNEL allocation in interrupt context — use GFP_ATOMIC.
• Exported symbols use EXPORT_SYMBOL_GPL if they require GPL module consumers.
• Module parameters have MODULE_PARM_DESC.
• Cross-compilation Makefile sets ARCH and CROSS_COMPILE.

References

• Linux Kernel Module Programming Guide
• Linux kernel — Documentation/kbuild/modules.rst
• Kernel printk and dynamic debug