Can't build hello world kernel module on Android JellyBean

Question

I'm trying to build a simple kernel module on Android JellyBean.

Code:

#include <linux/module.h>  /* Needed by all modules */
#include <linux/kernel.h>  /* Needed for KERN_ALERT */

MODULE_LICENSE("GPL");
MODULE_AUTHOR("test");
MODULE_DESCRIPTION("Android ko test");

int init_module(void)
{
   printk(KERN_ALERT, "Hello world\n");

   // A non 0 return means init_module failed; module can't be loaded.
   return 0;
}

void cleanup_module(void)
{
  printk(KERN_ALERT "Goodbye world 1.\n");
}

Makefile:

obj-m +=hello.o

KERNELDIR ?= ~/android/kernel
PWD := $(shell pwd)
CROSS_COMPILE=~/android/prebuilt/linux-x86/toolchain/arm-eabi-4.4.3/bin/arm-eabi-
ARCH=arm

default:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) ARCH=arm CROSS_COMPILE=$(CROSS_COMPILE) modules

clean:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) clean

Output:

make -C ~/android/kernel M=/home/test/testmod ARCH=arm CROSS_COMPILE=~/android/prebuilt/linux-x86/toolchain/arm-eabi-4.4.3/bin/arm-eabi- modules
make[1]: Entering directory `/home/test/android/kernel'

  ERROR: Kernel configuration is invalid.
         include/generated/autoconf.h or include/config/auto.conf are missing.
         Run 'make oldconfig && make prepare' on kernel src to fix it.


  WARNING: Symbol version dump /home/test/android/kernel/Module.symvers
           is missing; modules will have no dependencies and modversions.

  CC [M]  /home/test/testmod/hello.o
In file included from <command-line>:0:
/home/test/android/kernel/include/linux/kconfig.h:4:32: error: generated/autoconf.h: No such file or directory
In file included from /home/test/android/kernel/arch/arm/include/asm/types.h:4,
                 from include/linux/types.h:4,
                 from include/linux/list.h:4,
                 from include/linux/module.h:9,
                 from /home/test/testmod/hello.c:1:
include/asm-generic/int-ll64.h:11:29: error: asm/bitsperlong.h: No such file or directory
In file included from /home/test/android/kernel/arch/arm/include/asm/posix_types.h:38,
                 from include/linux/posix_types.h:47,
                 from include/linux/types.h:17,
                 from include/linux/list.h:4,
                 from include/linux/module.h:9,
                 from /home/test/testmod/hello.c:1:
include/asm-generic/posix_types.h:70:5: warning: "__BITS_PER_LONG" is not defined
error, forbidden warning: posix_types.h:70
make[2]: *** [/home/test/testmod/hello.o] Error 1
make[1]: *** [_module_/home/test/testmod] Error 2
make[1]: Leaving directory `/home/test/android/kernel'
make: *** [default] Error 2

If I follow the suggestion in the output, and run 'make oldconfig && make prepare' on the kernel, it leads me through dozens of kernel config yes / no questions. After that, the compile still fails on the next error, which is about bitsperlong.h.

Answer 1

Android puts output binaries under out directory. So for example one can have out/target/product/<target name>/obj/KERNEL_OBJ/ or $ANDROID_PRODUCT_OUT/obj/KERNEL_OBJ/ if $ANDROID_PRODUCT_OUT is defined. This directory may have a different name from different vendors but simply that's the directory containing vmlinux .

So when you compile a kernel module under Android repo, you should submit make command like below inside your module's directory.

make -C $ANDROID_PRODUCT_OUT/obj/KERNEL_OBJ/ M=`pwd` ARCH=arm CROSS_COMPILE=arm-eabi- modules

Answer 2

Make modules should be done after compiling the kernel at least once. You have not compiled the kernel, that's why Module.symvers is missing. During compilation certain header files like asm/bitsperlong.h are created.

Answer 3

Firstly make sure that you have compiled kernel in the specified path. that is

" /home/test/android/kernel" but you are using 
" /home/android/kernel "     during compilation of module
KERNELDIR ?= ~/android/kernel has to be KERNELDIR ?= ~/test/android/kernel

If not then in the ~/android/kernel directory run the below command to compile the kernel.

make ARCH=arm CROSS_COMPILE=~/test/android/prebuilt/linux-x86/toolchain/arm-eabi-4.4.3/bin/arm-eabi

After your kernel is compiled you will get this "__BITS_PER_LONG" variable defined in System.map file of kernel i,e

~/test/android/kernel/System.map

After this you will be able to compile your module without any hurdle