How to compile a single source file with the correct flags?

alkhimey · 04-22-2018, 02:31 PM

I am developing kernel module and want to perform the following:
1. Compile a source file into an object file with all correct flags. The source file includes kernel headers.
2. Produce a loadable kernel module by linking the object file with a 'main' of my kernel module.

To simplify, I want a statically linked library for loadable modules.

I guess that for step 1 I need to use kbuild makefile. The problem is that I was not able to figure out how to do it.

Here is the most obvious attempt:

Code:

obj-m: myfile.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$PWD/ myfile.o

Delcaran · 04-23-2018, 07:37 AM

Code:

make

is a tool for executing actions in a specific order. In your case this is overkill.

If you want to compile a single source, you should use the compiler. So, assuming your source is

Code:

myfile.c

and kernel sources are in

Code:

/src/$(shell uname -r)/

if you launch this command

gcc -I/src/$(shell uname -r) myfile.c

you should obtain

Code:

myfile.o

in current dir.

This should answer point 1.

I don't know anything about kernel programming, or creating modules, etc. so for point 2 you have to wait someone else.

alkhimey · 04-23-2018, 01:34 PM

Delcaran, unfortunately this will not quite cut it.

Code:

$ gcc -I/usr/src/linux-headers-4.13.0-39-generic/include myfile.c 
In file included from /usr/src/linux-headers-4.13.0-39-generic/include/linux/printk.h:7:0,
                 from myfile.c:42:
/usr/src/linux-headers-4.13.0-39-generic/include/linux/linkage.h:7:25: fatal error: asm/linkage.h: No such file or directory
 #include <asm/linkage.h>
                         ^
compilation terminated.

One needs to pass gcc a set of correct flags and preprocessor macro flags to properly compile for the kernel.
When building the whole tree or a loadable kernel module then kbuild system handles all these.

Delcaran · 04-24-2018, 05:11 AM

gcc is only complaining about a missing inclusion file, not about macros or other things.
If you can find the file linkage.h in the tree of kernel sources, you can pass the correct value to the -I parameter of gcc.
Then maybe other problems will emerge, but you can tackle them one by one.

EDIT:
looking at the tree in https://git.kernel.org/pub/scm/linux...h=linux-4.13.y I see that directory asm does not exists, but exists asm-generic... Maybe yes, kbuild is doing some voodoo, but you can try to create a symlink like

Quote:

cd /usr/src/linux-headers-4.13.0-39-generic/include/ ; ln -s asm-generic asm

EDIT2:
Looking at Makefile the directory asm will be create inside the directory /usr/src/linux-headers-4.13.0-39-generic/include/generated by Makefile+kbuild... That means that waht I told you is useless and I can't help you. Sorry.

sundialsvcs · 04-24-2018, 06:57 AM

If you modify a single file, make should detect it and recompile only the necessary modules ... which should include perhaps not only the file that you modified but any others that are dependent on it. Then it should perform the remaining steps needed. You should not have to – and, I think, should not – individually run the compiler.

Delcaran · 04-26-2018, 02:06 AM

Quote:

Originally Posted by alkhimey

I am developing kernel module

As I understood, OP wants to compile a source file, so unless the Makefile contains some globbing instruction he needs to modify the Makefile or equivalent...