Writing a Linux driver in the D ecosystem

[Eduard Staniloiu]

On Thursday, 25 October 2018 at 12:35:56 UTC, Eduard Staniloiu 
wrote:
> On Wednesday, 24 October 2018 at 06:36:51 UTC, sarn wrote:
>> On Tuesday, 23 October 2018 at 12:06:48 UTC, Eduard Staniloiu 
>> wrote:
>>> Hello, everyone!
>>>
>>> We, here at UPB, were thinking if it would be possible to 
>>> write a simple Linux driver.
>>
>> Hi, I gave this a casual try once.  The difficulty is the same 
>> difficulty you get trying to write Linux kernel modules in 
>> C++: the Linux build system is a mess of make files that are 
>> only designed for C, and don't represent a stable API between 
>> kernel source versions.
>>
>> If I had to write a Linux module in D, I'd write an interface 
>> layer in C that gets built using the normal Linux build system 
>> and can be linked to my D code.  The interface would have to 
>> provide alternatives to the various Linux preprocessor macros.
>>  (I think trying to refactor the build system to work with dpp 
>> would be a total waste of time.)
>
> Thank you all for your replies!
>
> We too, had the same feeling that it should be possible to do, 
> based on the ABI and `-betterC`.
>
> Our concern was the build system, and how everything can be 
> glued together.
> Thank you for the thin-layer C interface suggestion; I'll try 
> to give it a go ("Hello, insmod") in the weekend.
>
> I'll give an update as soon as I have one.
>
> Cheers,
> Edi

Hello, everyone!

I am trying to get a simple dummy module to work.
I have a simple function written in D, named `call_d`, that I 
want to call
from the `module_init` C function.

I had the following plan:
   * build a .o from the D source file, using `dmd -c -betterC 
dsrc.d`
   * add `dsrc.o` to the list of objects in Kbuild
   * let make do it's magic

Based on this stackoverflow Q/A [0], in order to tell Kbuild to 
use a prebuilt object,
the object needs to end in `.o_shipped`; so, I named my `dsrc.o` 
accordingly -> dsrc.o_shipped

Here are my files:

// C Module
```
// hellomod.c

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>

MODULE_DESCRIPTION("My kernel module");
MODULE_AUTHOR("Me");
MODULE_LICENSE("GPL");

static int dummy_init(void)
{
     int r = call_d();
     printk( KERN_ALERT "Hi. got %d from D\n", r);
     return 0;
}

static void dummy_exit(void)
{
     printk( KERN_ALERT "Bye\n" );
}

module_init(dummy_init);
module_exit(dummy_exit);
```

// D Source
```
// dsrc.d

extern(C) int call_d()
{
     return 10;
}
```

// Kbuild
```
EXTRA_CFLAGS = -Wall -g

obj-m = hellomod.o

hellomod-y = dsrc.o_shipped
```

// Makefile
```
KDIR = /lib/modules/`uname -r`/build

kbuild:
»   make -C $(KDIR) M=`pwd`

clean:
»   make -C $(KDIR) M=`pwd` clean
```

I'm building my dsrc.o_shipped with `dmd -c -betterC dsrc.d 
-of=dsrc.o_shipped`

When I `make` the build, I get the following warning:
```
WARNING: could not find 
/home/fawkes/ws/dlang/hello/.dsrc.o_shipped.cmd for 
/home/fawkes/ws/dlang/hello/dsrc.o_shipped
```

The hellomod.ko get's built, but when I `insmod` it, it doesn't 
print anything at `dmesg`.
I don't get a Kernel panic/oops either.

I know it's a long post, but any suggestions?

Cheers,
Edi