A basic Linux Kernel Driver.

vi hello.c

and populate as

hello.c

#include <linux/init.h>
#include <linux/module.h>
MODULE_LICENSE("Dual BSD/GPL");
 
static int hello_init(void)
{
  printk(KERN_ALERT "Hello, world\n");
  return 0;
}
 
static void hello_exit(void)
{
  printk(KERN_ALERT "Goodbye, cruel world\n");
}
 
module_init(hello_init);
module_exit(hello_exit);

Save the file.

vi Makefile

and populate as:

Makefile

# To build modules outside of the kernel tree, we run "make"
# in the kernel source tree; the Makefile these then includes this
# Makefile once again.
# This conditional selects whether we are being included from the
# kernel Makefile or not.
ifeq ($(KERNELRELEASE),)
 
    # Assume the source tree is where the running kernel was built
    # You should set KERNELDIR in the environment if it's elsewhere
    KERNELDIR ?= /lib/modules/$(shell uname -r)/build
    # The current directory is passed to sub-makes as argument
    PWD := $(shell pwd)
 
modules:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules
 
modules_install:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install
 
clean:
        rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions
 
.PHONY: modules modules_install clean
 
else
    # called from kernel build system: just declare what our modules are
    obj-m := hello.o hellop.o
endif

make

displays:

make -C /lib/modules/4.15.0-72-generic/build M=/home/peter/prog/kernel/helloworld modules
make[1]: Entering directory '/usr/src/linux-headers-4.15.0-72-generic'
  Building modules, stage 2.
  MODPOST 1 modules
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-72-generic'

ls hello,ko

displays:

hello.ko

sudo insmod hello.ko

This will not display anything.

lsmod

returns:

Module                  Size  Used by
hello                  16384  0
....

As mentioned above when the module was loaded it did not display that “Hello World” message.

This message is actually written to the system log.

tail /var/log/syslog

returns:

Dec 14 17:05:19 peter-bigmamba kernel: [74429.517770] Hello, world

sudo rmmod hello.ko

As mentioned above when the module was loaded it did not display that “Goodbye, Cruel World” message.

This message is actually written to the system log.

tail /var/log/syslog

returns:

Dec 14 17:07:07 peter-bigmamba kernel: [74537.686507] Goodbye, cruel world