【超入門】デバイスドライバ

$ uname -r
4.18.0-13-generic

$ cat /etc/lsb-release | head -n2
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.10

$ gcc --version
gcc (Ubuntu 8.2.0-7ubuntu1) 8.2.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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

static int m_init(void)
{
    printk(KERN_ALERT "Hello World\n");
    return 0;
}

static void m_exit(void)
{
    printk(KERN_ALERT "driver unloaded\n");
}

// モジュール開始と終了のマクロ
module_init(m_init);
module_exit(m_exit);
MODULE_LICENSE("GPL2");

/**
 * module_init() - driver initialization entry point
 * @x: function to be run at kernel boot time or module insertion
 *
 * module_init() will either be called during do_initcalls() (if
 * builtin) or at module insertion time (if a module).  There can only
 * be one per module.
 */
#define module_init(x)  __initcall(x);

/**
 * module_exit() - driver exit entry point
 * @x: function to be run when driver is removed
 *
 * module_exit() will wrap the driver clean-up code
 * with cleanup_module() when used with rmmod when
 * the driver is a module.  If the driver is statically
 * compiled into the kernel, module_exit() has no effect.
 * There can only be one per module.
 */
#define module_exit(x)  __exitcall(x);

/*
 * initcalls are now grouped by functionality into separate
 * subsections. Ordering inside the subsections is determined
 * by link order. 
 * For backwards compatibility, initcall() puts the call in 
 * the device init subsection.
 *
 * The `id' arg to __define_initcall() is needed so that multiple initcalls
 * can point at the same handler without causing duplicate-symbol build errors.
 *
 * Initcalls are run by placing pointers in initcall sections that the
 * kernel iterates at runtime. The linker can do dead code / data elimination
 * and remove that completely, so the initcall sections have to be marked
 * as KEEP() in the linker script.
 */

#define __define_initcall(fn, id) \
    static initcall_t __initcall_##fn##id __used \
    __attribute__((__section__(".initcall" #id ".init"))) = fn;

/*
 * Early initcalls run before initializing SMP.
 *
 * Only for built-in code, not modules.
 */
#define early_initcall(fn)      __define_initcall(fn, early)

/*
 * A "pure" initcall has no dependencies on anything else, and purely
 * initializes variables that couldn't be statically initialized.
 *
 * This only exists for built-in code, not for modules.
 * Keep main.c:initcall_level_names[] in sync.
 */
#define pure_initcall(fn)       __define_initcall(fn, 0)

#define core_initcall(fn)       __define_initcall(fn, 1)
#define core_initcall_sync(fn)      __define_initcall(fn, 1s)
#define postcore_initcall(fn)       __define_initcall(fn, 2)
#define postcore_initcall_sync(fn)  __define_initcall(fn, 2s)
#define arch_initcall(fn)       __define_initcall(fn, 3)
#define arch_initcall_sync(fn)      __define_initcall(fn, 3s)
#define subsys_initcall(fn)     __define_initcall(fn, 4)
#define subsys_initcall_sync(fn)    __define_initcall(fn, 4s)
#define fs_initcall(fn)         __define_initcall(fn, 5)
#define fs_initcall_sync(fn)        __define_initcall(fn, 5s)
#define rootfs_initcall(fn)     __define_initcall(fn, rootfs)
#define device_initcall(fn)     __define_initcall(fn, 6)
#define device_initcall_sync(fn)    __define_initcall(fn, 6s)
#define late_initcall(fn)       __define_initcall(fn, 7)
#define late_initcall_sync(fn)      __define_initcall(fn, 7s)

#define __initcall(fn) device_initcall(fn)

#define __exitcall(fn)                      \
    static exitcall_t __exitcall_##fn __exit_call = fn

obj-m := hello.o

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

$ make
make -C /lib/modules/4.18.0-13-generic/build M=/root modules
make[1]: ディレクトリ '/usr/src/linux-headers-4.18.0-13-generic' に入ります
Makefile:970: "Cannot use CONFIG_STACK_VALIDATION=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel"
  CC [M]  /root/hello.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /root/hello.mod.o
  LD [M]  /root/hello.ko
make[1]: ディレクトリ '/usr/src/linux-headers-4.18.0-13-generic' から出ま

$ ls -l
合計 28
-rw-r--r-- 1 root root  155 Jan 11 07:14 Makefile
-rw-r--r-- 1 root root    0 Jan 11 07:16 Module.symvers
-rw-r--r-- 1 root root  350 Jan 11 07:14 hello.c
-rw-r--r-- 1 root root 3784 Jan 11 07:16 hello.ko
-rw-r--r-- 1 root root  596 Jan 11 07:16 hello.mod.c
-rw-r--r-- 1 root root 2568 Jan 11 07:16 hello.mod.o
-rw-r--r-- 1 root root 2104 Jan 11 07:16 hello.o
-rw-r--r-- 1 root root   22 Jan 11 07:16 modules.order

$ sudo insmod hello.ko

# メッセージの確認
$ sudo dmesg
[    0.000000] Linux version 4.18.0-13-generic (buildd@lgw01-amd64-048) (gcc version 8.2.0 (Ubuntu 8.2.0-7ubuntu1)) #14-Ubuntu SMP Wed Dec 5 09:04:24 UTC 2018 (Ubuntu 4.18.0-13.14-generic 4.18.17)
(中略)
[ 1572.805848] Hello World

# モジュールの確認
$ sudo lsmod | grep -E "hello|Mod"
Module                  Size  Used by
hello                  16384  0

$ sudo rmmod hello

$ dmesg | tail -n 3
[ 1572.805215] Disabling lock debugging due to kernel taint
[ 1572.805848] Hello World
[ 2577.303835] driver unloaded