Broken Linux kernel <linux/...> includes on Ubuntu 20.04

Question

I'm trying to write a kernel module in VS Code on Ubuntu 20.04

The problem is that my includes seem to be completely broken.

Firstly, I have no <linux/uaccess.h> or <asm/uaccess.h> headers. Secondly, I have put many includes on top of my file like:

#include <linux/module.h>
#include <linux/kernel.h>

However, no definitions for kernel stuff like printk() are found. Macros like bool or NULL are also not defined in my linux/types.h.

I tried sudo apt-get install linux-headers-generic.

Here's my linux/module.h:

/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef _LINUX_MODULE_H
#define _LINUX_MODULE_H

/* Flags for sys_finit_module: */
#define MODULE_INIT_IGNORE_MODVERSIONS  1
#define MODULE_INIT_IGNORE_VERMAGIC 2

#endif /* _LINUX_MODULE_H */

My linux/kernel.h:

/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef _LINUX_KERNEL_H
#define _LINUX_KERNEL_H

#include <linux/sysinfo.h>
#include <linux/const.h>

#endif /* _LINUX_KERNEL_H */

Answer 1

These headers are kernel-space headers, which means they cannot be found within the user-space headers (/usr/include/linux). In my Fedora machine, this is how I diagnose it.

1. Make sure those headers exist

I use a simple find /usr -name uaccess.h 2>/dev/null to list all files. The result might look like below:

/usr/src/kernels/6.6.4-200.fc39.x86_64/arch/x86/include/asm/uaccess.h
/usr/src/kernels/6.6.4-200.fc39.x86_64/include/asm-generic/uaccess.h
/usr/src/kernels/6.6.4-200.fc39.x86_64/include/linux/uaccess.h
/usr/src/kernels/6.6.6-200.fc39.x86_64/arch/x86/include/asm/uaccess.h
/usr/src/kernels/6.6.6-200.fc39.x86_64/include/asm-generic/uaccess.h
/usr/src/kernels/6.6.6-200.fc39.x86_64/include/linux/uaccess.h
/usr/src/kernels/6.6.7-200.fc39.x86_64/arch/x86/include/asm/uaccess.h
/usr/src/kernels/6.6.7-200.fc39.x86_64/include/asm-generic/uaccess.h
/usr/src/kernels/6.6.7-200.fc39.x86_64/include/linux/uaccess.h

2. How to use those kernel headers

An intuitive way to use those headers is to include them all when compiling. However, this is error-prone due to linking issues.

A (somewhat) standard way is to define it as module.

Source code:

// printk-example.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/uaccess.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple kernel module with printk");

static int __init hello_init(void)
{
    printk(KERN_INFO "Hello, this is your kernel module!\n");
    return 0;
}

static void __exit hello_exit(void)
{
    printk(KERN_INFO "Goodbye, exiting your kernel module!\n");
}

module_init(hello_init);
module_exit(hello_exit);

Makefile:

obj-m += printk-example.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

You might search for the rest on running kernel modules for your machine.

I hope this will help.