Build Linux Kernel Module using Makefile with different pathname - c

Question

I am trying to compile a Linux kernel module using the standard example Makefile specified in the Linux Kernel Module Programming Guide. If the Makefile is called Makefile, then everything works. If I rename the Makefile to Makefile.hello or something else, then it fails as it cannot find the path Makefile. I was wondering if there is a command or set of flags I can add to my Makefile to make this function properly. I need to rename the Makefile as I am calling it from CMake. Cmake creates its own Makefiles and will commonly overwrite what I already have.
I replaced my kernel module code with the hello world example and replicated the problem. I know its the makefile.
hello world example hello.c
/*
* hello−1.c − The simplest kernel module.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
int init_module(void)
{
printk(KERN_INFO "Hello world 1.\n");
/*
* A non 0 return means init_module failed; module can't be loaded.
*/
return 0;
}
void cleanup_module(void)
{
printk(KERN_INFO "Goodbye world 1.\n");
}
Makefile
obj-m += hello.o
ifeq (,$(KDIR))
KDIR := /lib/modules/$(shell uname -r)/build
endif
PWD := $(shell pwd)
all:
$(MAKE) -C $(KDIR) M=$(PWD) $(KCONFIG) modules
clean:
$(MAKE) -C $(KDIR) M=$(PWD) clean
If makefile is called Makefile. (Successfully builds)
$> make -f Makefile
make -C /lib/modules/4.4.0-21-generic/build M=/home/msmith/Desktop/kernel-test modules
make[1]: Entering directory '/usr/src/linux-headers-4.4.0-21-generic'
CC [M] /home/msmith/Desktop/kernel-test/hello.o
Building modules, stage 2.
MODPOST 1 modules
CC /home/msmith/Desktop/kernel-test/hello.mod.o
LD [M] /home/msmith/Desktop/kernel-test/hello.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.4.0-21-generic'
Make output if makefile is called Makefile.hello (Fails to build)
$> make -f Makefile.hello
make -C /lib/modules/4.4.0-21-generic/build M=/home/msmith/Desktop/kernel-test modules
make[1]: Entering directory '/usr/src/linux-headers-4.4.0-21-generic'
scripts/Makefile.build:44: /home/msmith/Desktop/kernel-test/Makefile: No such file or directory
make[2]: *** No rule to make target '/home/msmith/Desktop/kernel-test/Makefile'. Stop.
Makefile:1396: recipe for target '_module_/home/msmith/Desktop/kernel-test' failed
make[1]: *** [_module_/home/msmith/Desktop/kernel-test] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-4.4.0-21-generic'
Makefile.hello:10: recipe for target 'all' failed
make: *** [all] Error 2
I tried adding the -f to the internal MAKE parameters, however that just caused more issues.

Answer 1

Move all Kbuild-related logic into the file Kbuild. Kernel's build system checks file with this name first, so it won't look into Makefile, created by CMake. This feature is documented in Documentation/kbuild/makefiles.txt.
I use exactly this approach in my CMake projects, related with Linux kernel.

Answer 2

Open script/Makefile.build into kernel tree:
41 # The filename Kbuild has precedence over Makefile
42 kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
43 kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
44 include $(kbuild-file)
This part of code (43-44) include Makefile with name 'Makefile'.

Answer 3

The default Makefile. Name = Makefile ... and Makefile-hello, or Makefile.hello.
obj-m := hello.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
clean:
$(MAKE) -C $(KDIR) M=$(PWD) clean
$ make : OK
$ make -f Makefile-hello OK
$ make -f Makefile.hello Also OK.
Your Makefile : $ make -f Makefile.msmith OK, no errors.