Linux内核开发03：内核Hello World模块

环境

Linux Mint 21.3 + GCC 11.4 + Make 4.3

模块

内核模块的全称是动态可加载内核模块(Loadable Kernel Modul,KLM)，可以动态载入内核，让它成为内核代码的一部分。

一个模块一般由一组函数和数据结构组成。

开发

创建一个目录用于放置源码

mkdir hello
cd hello

模块源码hello.c为

#include<linux/module.h>   //每个模块都要包括的头文件
#include<linux/kernel.h>   //用到了printk()函数
#include<linux/init.h>

MODULE_LICENSE("GPL");  //没有指定license会出现error

static int hello_init(void)    //static使得该文件以外无法访问
{
	printk(KERN_ALERT "Hello World\n"); //只能使用内核里定义好的C函数，printk会根据日志级别将指定信息输出到控制台或日志文件中，KERN_ALERT会输出到控制台
	return 0;
}
static void hello_exit(void)
{
	printk(KERN_ALERT "Goodbye World\n");
}

module_init(hello_init);
module_exit(hello_exit);

Makefile内容为

obj-m := hello.o

KERNEL_DIR := /lib/modules/$(shell uname -r)/build   #指定内核源码
PWD := $(shell pwd)  #指向当前目录

all:
	make -C $(KERNEL_DIR) M=$(PWD) modules
clean:
	make -C $(KERNEL_DIR) M=$(PWD) clean

测试

编译模块

输出信息为

hyper372@hyper372-test:~/Documents/hello$ make
make -C /lib/modules/5.15.0-116-generic/build    M=/home/hyper372/Documents/hello   modules
make[1]: Entering directory '/usr/src/linux-headers-5.15.0-116-generic'
  CC [M]  /home/hyper372/Documents/hello/hello.o
  MODPOST /home/hyper372/Documents/hello/Module.symvers
  CC [M]  /home/hyper372/Documents/hello/hello.mod.o
  LD [M]  /home/hyper372/Documents/hello/hello.ko
  BTF [M] /home/hyper372/Documents/hello/hello.ko
Skipping BTF generation for /home/hyper372/Documents/hello/hello.ko due to unavailability of vmlinux
make[1]: Leaving directory '/usr/src/linux-headers-5.15.0-116-generic'

编译结果为

hyper372@hyper372-test:~/Documents/hello$ ll
total 240
drwxrwxr-x 2 hyper372 hyper372  4096 Jul 17 10:24 ./
drwxr-xr-x 7 hyper372 hyper372  4096 Jul 17 10:11 ../
-rw-rw-r-- 1 hyper372 hyper372   611 Jul 17 10:14 hello.c
-rw-rw-r-- 1 hyper372 hyper372 61736 Jul 17 10:24 hello.ko
-rw-rw-r-- 1 hyper372 hyper372   253 Jul 17 10:24 .hello.ko.cmd
-rw-rw-r-- 1 hyper372 hyper372    40 Jul 17 10:24 hello.mod
-rw-rw-r-- 1 hyper372 hyper372   856 Jul 17 10:24 hello.mod.c
-rw-rw-r-- 1 hyper372 hyper372   148 Jul 17 10:24 .hello.mod.cmd
-rw-rw-r-- 1 hyper372 hyper372 50048 Jul 17 10:24 hello.mod.o
-rw-rw-r-- 1 hyper372 hyper372 31035 Jul 17 10:24 .hello.mod.o.cmd
-rw-rw-r-- 1 hyper372 hyper372 13024 Jul 17 10:24 hello.o
-rw-rw-r-- 1 hyper372 hyper372 29739 Jul 17 10:24 .hello.o.cmd
-rw-rw-r-- 1 hyper372 hyper372   224 Jul 17 10:15 Makefile
-rw-rw-r-- 1 hyper372 hyper372    40 Jul 17 10:24 modules.order
-rw-rw-r-- 1 hyper372 hyper372   174 Jul 17 10:24 .modules.order.cmd
-rw-rw-r-- 1 hyper372 hyper372     0 Jul 17 10:24 Module.symvers
-rw-rw-r-- 1 hyper372 hyper372   224 Jul 17 10:24 .Module.symvers.cmd

查看模块信息

hyper372@hyper372-test:~/Documents/hello$ modinfo hello.ko
filename:       /home/hyper372/Documents/hello/hello.ko
license:        GPL
srcversion:     5F846D216D4D129E9B3246C
depends:        
retpoline:      Y
name:           hello
vermagic:       5.15.0-116-generic SMP mod_unload modversions

加载模块

sudo insmod hello.ko

获取已加载模块列表

查看已加载的模块，确定hello模块加载成功

hyper372@hyper372-test:~/Documents/hello$ lsmod
Module                  Size  Used by
hello                  16384  0
rfcomm                 81920  16
bnep                   28672  2
vsock_loopback         16384  0
vmw_vsock_virtio_transport_common    40960  1 vsock_loopback
vmw_vsock_vmci_transport    32768  2

查看内核信息

hyper372@hyper372-test:~/Documents/hello$ sudo dmesg
[    0.000000] Linux version 5.15.0-116-generic (buildd@lcy02-amd64-015) (gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, GNU ld (GNU Binutils for Ubuntu) 2.38) #126-Ubuntu SMP Mon Jul 1 10:14:24 UTC 2024 (Ubuntu 5.15.0-116.126-generic 5.15.158)
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.15.0-116-generic root=UUID=2f7aebd4-6e49-4772-977e-f7ad68635ecb ro quiet splash
[    0.000000] KERNEL supported cpus:
[    0.000000]   Intel GenuineIntel
...
[   30.588512] Bluetooth: RFCOMM ver 1.11
[  556.898154] hello: module verification failed: signature and/or required key missing - tainting kernel
[  556.898755] Hello World

卸载模块

执行命令，卸载后lsmod应该找不到hello模块了。

rmmod hello