module compiling : asm/linkage.h file not found - linux

Question

I am trying to compile an example of "hello world" Kernel Module,
problems found on ubuntu 11.04, kernel 3.2.6, gcc 4.5.2 and fedora 16, kernel 3.2.7, gcc 4.6.7.
code:
#include <linux/module.h>
#include <linux/init.h>
MODULE_LICENSE("GPL");
static int __init hello_init (void)
{
printk("Hello module init\n");
return 0;
}
static void __exit hello_exit (void)
{
printk("Hello module exit\n");
}
module_init(hello_init);
module_exit(hello_exit);
compiled with:
gcc -D__KERNEL__ -I /usr/src/linux/include/ -DMODULE -Wall -O2 -c hello.c -o hello.o
error:
In file included from /usr/src/linux/include/linux/kernel.h:13:0,
from /usr/src/linux/include/linux/cache.h:4,
from /usr/src/linux/include/linux/time.h:7,
from /usr/src/linux/include/linux/stat.h:60,
from /usr/src/linux/include/linux/module.h:10,
from hello.c:1: /usr/src/linux/include/linux/linkage.h:5:25: fatal error:
asm/linkage.h: file not found
then I found in /usr/src/linux/include/ there is no folder named 'asm' but 'asm-generic';
so I made a soft link 'asm' to 'asm-generic', and compiled agail:
this time the error was:
In file included from /usr/src/linux/include/linux/preempt.h:9:0,
from /usr/src/linux/include/linux/spinlock.h:50,
from /usr/src/linux/include/linux/seqlock.h:29,
from /usr/src/linux/include/linux/time.h:8,
from /usr/src/linux/include/linux/stat.h:60,
from /usr/src/linux/include/linux/module.h:10,
from hello.c:1: /usr/src/linux/include/linux/thread_info.h:53:29: fatal error:
asm/thread_info.h: file not found
So I realized I was wrong, but why ? T_T

Answer 1

obj-m += hello.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
is a proper way to build modules see kbuild documentation
And to see difference beetween your compiler invocation you could
cat /lib/modules/$(shell uname -r)/build/Makefile
And analyze an output

Answer 2

obj-m += hello.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
Here hello.c is your kernel source file. just use make to build your hello.ko module.

Answer 3

asm should be a link to the actual architecture you're compiling for, not to asm-generic.
You can't compile a generic kernel module, that would work on a generic architecture. You have to compile it for the particular architecture you're going to use.
I don't know why the asm didn't exist. It should be created as part of the configuration process.
You might get other errors later, if configuration is incomplete in other ways.

Answer 4

The asm includes (such as linkage.h) are architecture specific. There should be a set of directories under:
/usr/src/kernels/(kernel version goes here)/arch
that provide specific includes for the specific CPU architecture you are targeting your code to be compiled for.
Try adding this to your Makefile:
KVERSION :=R(shell uname -r)
and add the kernel and architecture (x86 in this example):
INCDIRS = -I./include -I/usr/src/kernels/$(KVERSION)/include -I/usr/src/kernels/$(KVERSION)/arch/x86

Answer 5

module compiling : asm/linkage.h file not found
This means this particular file was not found in specified DIR, which gets specified when we use -I option with make.
We can either link that asm-generic to asm, if all headers are present in asm-generic, or we can use make utility.
Make utility is preferred in case of building kernel modules.
Create a 'Makefile' in working DIR.
obj-m += hello.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
Use of -C option will change to DIR specified before reading the makefiles or doing anything else.
So to avoid this error, use -C option with DIR/lib/modules/$(shell uname -r)/build
By this your program will be able to find required files, you will get hello.ko file.
You can add this to kernel modules by
sudo insmod hello.ko
Similarly you can remove by
sudo rmmod hello