[SOLVED] Does Writing Linux Kernel Module Require Compiling Own Kernel?

Issue

I have a simple hello world kernel module on Ubuntu x86_64:

#include <linux/module.h>

static int 
mod_init(void)
{
  printk(KERN_INFO "RYANhello world\n");
  return 0;
}

static void 
mod_exit(void)
{
  printk(KERN_INFO "RYANgoodbye world\n");
}

MODULE_LICENSE("GPL");

module_init(mod_init);
module_exit(mod_exit);

Makefile:

KERNEL_DIR := /lib/modules/$(shell uname -r)/build
CUR_DIR := $(shell pwd)
obj-m := module.o

default:
    $(MAKE) -C $(KERNEL_DIR) M=$(CUR_DIR) modules

When I sudo insmod module.ko I get insmod: ERROR: could not insert module module.ko: Invalid parameters. Inspecting dmesg:

loading out-of-tree module taints kernel
module verification failed: signature and/or required key missing - tainting kernel

Repeating insmod yields module is already loaded however /var/log/syslog shows no trace of it loading (i.e printk messages not present). Also, running sudo rmmod module.ko:

rmmod: ERROR: ../libkmod/libkmod-module.c:1941 kmod_module_get_holders() could not open '/sys/module/module/holders': No such file or directory
rmmod: ERROR: Module unloading is not supported

This seems to indicate it's not loaded, even though dmesg says it is?

Addressing common issues; my host kernel and gcc version are the same as ones I compiling with.

So, this leads me to think that the module not being signed is the issue. To disable this do I have to compile and install my own kernel with appropriate .config? In other words, to write and test your own kernel modules on a modern GNU/Linux OS with enforced signing, do you have to compile and install your own kernel?

EDIT CONFIG_MODULE_SIG_FORCE is not set in my /boot/config-5.8.0-53-generic, so it seems I should be able to load my module albeit with a tainted kernel message. So, why would I be getting Invalid parameters?

Solution

Inspecting dmesg on first insmod it was saying that the module was already loaded. As I had never loaded this before, this prompted me to think that this name was already taken. Low and behold, renaming module.c/module.o --> example.c/example.o fixed the problem. The invalid parameters message was what threw me.

Answered By - Ryan McClue