LKM - can't compile module - missing headers files but header packages are installed

So I have been trying to compile a simple "Hello world" LKM on my Ubuntu 18.04 running 4.15.0-43-generic and the linux-headers-4.15.0-43 are installed.

This is the code:

#define MODULE#define LINUX#define __KERNEL__#include <linux/init.h>#include <linux/module.h>#include <linux/kernel.h>int init_module(void){    printk(KERN_DEBUG "Hello World!");    return 0;}module_init( init_module );MODULE_LICENSE("GPL");MODULE_DESCRIPTION("Hello world");

This is my Makefile:

CC      := gccWARN    := -W -Wall -Wstrict-prototypes -Wmissing-prototypesINCLUDE := -isystem /lib/modules/`uname -r`/build/includeCFLAGS  := -O2 -c $(WARN) $(INCLUDE)SOURCES := helloworld.cTARGET  := helloworld.koall:    $(CC) $(CFLAGS) $(SOURCES) -o $(TARGET)

These are the contents of /lib/modules/$(uname -r)/build/include

acpi  asm-generic  clocksource  config  crypto  drm  dt-bindings  generated  keys  kvm  linux  math-emu  media  memory  misc  net  pcmcia  ras  rdma  scsi  soc  sound  target  trace  uapi  video  xen

and I got a fatal error: asm/barrier.h: No such file or directory which is to be expected because the asm dir isn't there. See below the full output:

gcc -O2 -c -W -Wall -Wstrict-prototypes -Wmissing-prototypes -isystem /lib/modules/`uname -r`/build/include helloworld.c -o helloworld.koIn file included from /usr/src/linux-headers-4.15.0-43/include/linux/init.h:5:0,                 from helloworld.c:5:/usr/src/linux-headers-4.15.0-43/include/linux/compiler.h:247:10: fatal error: asm/barrier.h: No such file or directory #include <asm/barrier.h>          ^~~~~~~~~~~~~~~compilation terminated.Makefile:9: recipe for target 'all' failedmake: *** [all] Error 1

Right...so I thought what if I create a symlink of asm-generic to asm?

sudo ln -s /lib/modules/$(uname -r)/build/include/asm-generic /lib/modules/$(uname -r)/build/include/asm

ls -ld /lib/modules/$(uname -r)/build/include/asm confirms the link is there:

lrwxrwxrwx 1 root root 56 Jan 24 19:51 /lib/modules/4.15.0-43-generic/build/include/asm -> /lib/modules/4.15.0-43-generic/build/include/asm-generic

Everything should work right? No :( Now it's complaining asm/thread_info.h cannot be found (after I did the symlink).

gcc -O2 -c -W -Wall -Wstrict-prototypes -Wmissing-prototypes -isystem /lib/modules/`uname -r`/build/include helloworld.c -o helloworld.koIn file included from /lib/modules/4.15.0-43-generic/build/include/asm/preempt.h:5:0,                 from /usr/src/linux-headers-4.15.0-43/include/linux/preempt.h:81,                 from /usr/src/linux-headers-4.15.0-43/include/linux/spinlock.h:51,                 from /usr/src/linux-headers-4.15.0-43/include/linux/seqlock.h:36,                 from /usr/src/linux-headers-4.15.0-43/include/linux/time.h:6,                 from /usr/src/linux-headers-4.15.0-43/include/linux/stat.h:19,                 from /usr/src/linux-headers-4.15.0-43/include/linux/module.h:10,                 from helloworld.c:6:/usr/src/linux-headers-4.15.0-43/include/linux/thread_info.h:38:10: fatal error: asm/thread_info.h: No such file or directory#include <asm/thread_info.h>         ^~~~~~~~~~~~~~~~~~~compilation terminated.Makefile:9: recipe for target 'all' failedmake: *** [all] Error 1

I knew from the start that the symlink was a bad idea.

I also tried including /usr/src/linux-headers-4.15.0-43/include instead of /lib/modules/... but long story short, the "asm" dir isn't there in /usr/src/linux-headers-4.15.0-43/include/ and symlinking /usr/src/linux-headers-4.15.0-43/include/asm-generic to /usr/src/linux-headers-4.15.0-43/include/asm produced the same issues as above.

I know what the problem is, obviously the header files are missing but the problem is I cannot find them, where do I get them from? The packages are installed:

$ dpkg -l | grep linux-headers-ii  linux-headers-4.15.0-23        ii  linux-headers-4.15.0-43        ii  linux-headers-4.15.0-43-genericii  linux-headers-generic          

I tried all of those, all appear to have the same issue.