Makefile: Kernel module in subfolders - c

Question

I have a more or less complicated LKM. This LKM contains the directories as shown below:
├── core.c
├── headers
│   ├── core.h
│   └── server.h
├── include
│   ├── headers
│   │   └── utils.h
│   └── utils.c
├── libs
│   ├── headers
│   │   └── syscalltable.h
│   └── syscalltable.c
├── LICENSE
├── Makefile
├── README.md
├── server.c
├── src
│   ├── getdents_hook.c
│   ├── headers
│   │   ├── getdents_hook.h
│   │   ├── module_hiding.h
│   │   ├── network_keylog.h
│   │   ├── packet_hiding.h
│   │   ├── port_knocking.h
│   │   ├── privilege_escalation.h
│   │   └── socket_hiding.h
│   ├── module_hiding.c
│   ├── network_keylog.c
│   ├── packet_hiding.c
│   ├── port_knocking.c
│   ├── privilege_escalation.c
│   └── socket_hiding.c
└── TASKS.md
It seems like a overhead for this small project, but the goal was to create a LKM that could be extended easily.
The problem lies in the Makefile. As every header file is in its respective headers subdirectory, I created a Makefile that looks like this:
# Module name
ROOTKIT = rootkit
# Build
CC = gcc
PWD = $(shell pwd)
UNAME = $(shell uname -r)
MODULEDIR = /lib/modules/$(UNAME)
BUILDDIR = $(MODULEDIR)/build
KERNELDIR = $(MODULEDIR)/kernel
# Headers
CORE_H = headers
LIBS_H = libs/headers
SRCS_H = src/headers
INCL_H = include/headers
# Files
CORE = $(wildcard *.c)
CORE_OBJS = $(patsubst %.c, %.o, $(CORE))
LIBS = $(wildcard libs/*.c)
LIBS_OBJS = $(patsubst %.c, %.o, $(LIBS))
INCL = $(wildcard include/*.c)
INCL_OBJS = $(patsubst %.c, %.o, $(INCL))
# Exception for this one file
NETW = src/network_keylog.c
NETW_OBJS = src/network_keylog.o
SRC = $(wildcard src/*.c)
SRCS = $(filter-out $(NETW), $(SRC))
SRCS_OBJS = $(patsubst %.c, %.o, $(SRCS))
# Objects
# $(CORE_OBJS): $(CORE)
# $(CC) -c -o $# $< -I$(CORE_H) -I$(INCL_H)
# $(LIBS_OBJS): $(LIBS)
# $(CC) -c -o $# $< -I$(LIBS_H) -I$(INCL_H)
# $(SRCS_OBJS): $(SRCS)
# $(CC) -c -o $# $< -I$(SRCS_H) -I$(INCL_H)
# $(NETW_OBJS): $(NETW)
# $(CC) -c -o $# $< -I$(SRCS_H) -I$(CORE_H) -I$(INCL_H)
# Module
obj-m += $(ROOTKIT).o
$(ROOTKIT)-y = $(CORE_OBJS) $(LIBS_OBJS) $(SRCS_OBJS) $(NETW_OBJS)
EXTRA_CFLAGS = -I$(CORE_H) -I$(INCL_H) -I$(LIBS_H) -I$(SRCS_H)
# Recipes
all:
$(MAKE) -C $(BUILDDIR) M=$(PWD) modules
load:
insmod $(KERNELDIR)/net/ipv4/netfilter/nf_reject_ipv4.ko
insmod $(KERNELDIR)/net/ipv6/netfilter/nf_reject_ipv6.ko
insmod rootkit.ko
clean:
$(MAKE) -C $(BUILDDIR) M=$(PWD) clean
However I only get this error:
make -C /lib/modules/4.9.0-3-amd64/build M=/home/croemheld/Repositories/lkm-rootkit modules
make[1]: Verzeichnis „/usr/src/linux-headers-4.9.0-3-amd64“ wird betreten
/bin/sh: 1: Syntax error: "(" unexpected
[...]
I know that the Makefile executes its procedure in a shell, but I can't understand what command the shell can't execute or why this is failing in general. Or is this approach completely wrong?
EDIT: Seems like I missed a missing $ in a -I flag...
However, it still does not compile. The problem is that every file includes the utils.h in include/headers/, and I don't know how to specify the compile procedure so that it works.
Currently:
all source files include my own header files (meaning in the *.c files are only includes like "xyz.h" and in the headers only <....h >)
the core.c and server.c include all other header files.
every source file includes the utils.h header and its "own" header file, except for network_keylog.c: this one also includes server.h.
Current output:
make -C /lib/modules/4.9.0-3-amd64/build M=/home/croemheld/Repositories/lkm-rootkit modules
make[1]: Verzeichnis „/usr/src/linux-headers-4.9.0-3-amd64“ wird betreten
CC [M] /home/croemheld/Repositories/lkm-rootkit/src/network_keylog.o
/home/croemheld/Repositories/lkm-rootkit/src/network_keylog.c:1:19: fatal error: utils.h: Datei oder Verzeichnis nicht gefunden
#include "utils.h"
^
compilation terminated.
/usr/src/linux-headers-4.9.0-3-common/scripts/Makefile.build:298: die Regel für Ziel „/home/croemheld/Repositories/lkm-rootkit/src/network_keylog.o“ scheiterte
make[4]: *** [/home/croemheld/Repositories/lkm-rootkit/src/network_keylog.o] Fehler 1