linux-kernel-modules

What this skill does

# Linux Kernel Modules

## Purpose

Guide agents through writing loadable Linux kernel modules (LKMs): the Kbuild build system, module parameters, /proc and sysfs interfaces, character device implementation, kernel debugging with KGDB and `ftrace`, and module signing for Secure Boot.

## Triggers

- "How do I write a Linux kernel module?"
- "How do I add parameters to my kernel module?"
- "How do I create a /proc or sysfs entry?"
- "How do I implement a character device driver?"
- "How do I debug a kernel module with KGDB?"
- "How do I sign a kernel module for Secure Boot?"

## Workflow

### 1. Minimal kernel module

```c
// hello.c — minimal loadable kernel module
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("Minimal hello world module");
MODULE_VERSION("1.0");

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

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

module_init(hello_init);
module_exit(hello_exit);
```

```makefile
# Makefile — must be exactly this structure for Kbuild
obj-m := hello.o

KDIR := /lib/modules/$(shell uname -r)/build

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

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

```bash
# Build
make

# Load
sudo insmod hello.ko

# Check it loaded
lsmod | grep hello
dmesg | tail -5       # see printk output

# Unload
sudo rmmod hello

# Show module info
modinfo hello.ko
```

### 2. Module parameters

```c
#include <linux/moduleparam.h>

static int count = 1;
static char *name = "world";

// module_param(variable, type, permissions)
// permissions: 0 = no sysfs entry, S_IRUGO = readable, S_IWUSR = writable
module_param(count, int, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(count, "Number of times to print (default: 1)");

module_param(name, charp, S_IRUGO);
MODULE_PARM_DESC(name, "Name to greet (default: world)");

static int __init hello_init(void)
{
    int i;
    for (i = 0; i < count; i++)
        printk(KERN_INFO "hello: Hello, %s!\n", name);
    return 0;
}
```

```bash
# Pass parameters at load time
sudo insmod hello.ko count=3 name="kernel"

# Modify at runtime (if S_IWUSR set)
echo 5 > /sys/module/hello/parameters/count
```

### 3. /proc filesystem interface

```c
#include <linux/proc_fs.h>
#include <linux/seq_file.h>

static struct proc_dir_entry *proc_entry;

static int mymod_show(struct seq_file *m, void *v)
{
    seq_printf(m, "Counter: %d\n", my_counter);
    seq_printf(m, "Status: %s\n", my_status ? "active" : "idle");
    return 0;
}

static int mymod_open(struct inode *inode, struct file *file)
{
    return single_open(file, mymod_show, NULL);
}

static const struct proc_ops mymod_fops = {
    .proc_open    = mymod_open,
    .proc_read    = seq_read,
    .proc_lseek   = seq_lseek,
    .proc_release = single_release,
};

static int __init mymod_init(void)
{
    proc_entry = proc_create("mymod", 0444, NULL, &mymod_fops);
    if (!proc_entry)
        return -ENOMEM;
    return 0;
}

static void __exit mymod_exit(void)
{
    proc_remove(proc_entry);
}
```

```bash
cat /proc/mymod
```

### 4. sysfs interface

```c
#include <linux/kobject.h>
#include <linux/sysfs.h>

static struct kobject *mymod_kobj;
static int mymod_value = 42;

static ssize_t value_show(struct kobject *kobj,
                          struct kobj_attribute *attr, char *buf)
{
    return sprintf(buf, "%d\n", mymod_value);
}

static ssize_t value_store(struct kobject *kobj,
                           struct kobj_attribute *attr,
                           const char *buf, size_t count)
{
    sscanf(buf, "%d", &mymod_value);
    return count;
}

static struct kobj_attribute value_attr =
    __ATTR(value, 0664, value_show, value_store);

static int __init mymod_init(void)
{
    mymod_kobj = kobject_create_and_add("mymod", kernel_kobj);
    if (!mymod_kobj) return -ENOMEM;
    return sysfs_create_file(mymod_kobj, &value_attr.attr);
}

static void __exit mymod_exit(void)
{
    sysfs_remove_file(mymod_kobj, &value_attr.attr);
    kobject_put(mymod_kobj);
}
```

```bash
cat /sys/kernel/mymod/value
echo 100 > /sys/kernel/mymod/value
```

### 5. Character device

```c
#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#define DEVICE_NAME "mydev"
#define BUF_SIZE 1024

static int major;
static struct cdev my_cdev;
static char kernel_buf[BUF_SIZE];

static int mydev_open(struct inode *inode, struct file *file) { return 0; }
static int mydev_release(struct inode *inode, struct file *file) { return 0; }

static ssize_t mydev_read(struct file *f, char __user *buf, size_t len, loff_t *off)
{
    size_t to_copy = min(len, (size_t)BUF_SIZE);
    if (copy_to_user(buf, kernel_buf, to_copy)) return -EFAULT;
    return to_copy;
}

static ssize_t mydev_write(struct file *f, const char __user *buf, size_t len, loff_t *off)
{
    size_t to_copy = min(len, (size_t)(BUF_SIZE - 1));
    if (copy_from_user(kernel_buf, buf, to_copy)) return -EFAULT;
    kernel_buf[to_copy] = '\0';
    return to_copy;
}

static const struct file_operations mydev_fops = {
    .owner   = THIS_MODULE,
    .open    = mydev_open,
    .release = mydev_release,
    .read    = mydev_read,
    .write   = mydev_write,
};

static int __init mydev_init(void)
{
    major = register_chrdev(0, DEVICE_NAME, &mydev_fops);
    if (major < 0) return major;
    printk(KERN_INFO "mydev: registered with major %d\n", major);
    return 0;
}
```

```bash
# Create device node (after loading module)
sudo mknod /dev/mydev c $(cat /proc/devices | grep mydev | awk '{print $1}') 0
echo "test" > /dev/mydev
cat /dev/mydev
```

### 6. Debugging with KGDB and ftrace

```bash
# KGDB — kernel GDB via serial/network
# Boot with: kgdboc=ttyS0,115200 kgdbwait
# Or over network: [email protected]/,@192.168.1.11/

# On debug host:
gdb vmlinux
(gdb) target remote /dev/ttyS0
(gdb) set architecture i386:x86-64:intel
(gdb) info registers

# ftrace — kernel function tracer
echo function > /sys/kernel/debug/tracing/current_tracer
echo mymod_write > /sys/kernel/debug/tracing/set_ftrace_filter
echo 1 > /sys/kernel/debug/tracing/tracing_on
cat /sys/kernel/debug/tracing/trace

# Dynamic debug — enable pr_debug() output
echo "module hello +p" > /sys/kernel/debug/dynamic_debug/control
```

### 7. Module signing (Secure Boot)

```bash
# Generate signing key
openssl req -new -x509 -newkey rsa:2048 \
    -keyout signing_key.pem -out signing_cert.pem \
    -days 365 -subj "/CN=Module Signing Key/" -nodes

# Sign the module
/usr/src/linux-headers-$(uname -r)/scripts/sign-file \
    sha256 signing_key.pem signing_cert.pem hello.ko

# Import certificate to MOK database
sudo mokutil --import signing_cert.pem
# (requires reboot and MOK enrollment at UEFI)
```

For Kbuild system details, see [references/kbuild-basics.md](references/kbuild-basics.md).

## Related skills

- Use `skills/observability/ebpf` for userspace kernel tracing without modules
- Use `skills/debuggers/gdb` for GDB session management with KGDB
- Use `skills/binaries/elf-inspection` for inspecting module ELF structure