作者:Hcamael@知道创宇404实验室

最近在搞IoT的时候,因为没有设备,模拟跑固件经常会缺/dev/xxx,所以我就开始想,我能不能自己写一个驱动,让固件能跑起来?因此,又给自己挖了一个很大坑,不管最后能不能达到我的初衷,能学到怎么开发Linux驱动,也算是有很大的收获了。

前言

我写的这个系列以实践为主,不怎么谈理论,理论可以自己去看书,我是通过《Linux Device Drivers》这本书学的驱动开发,Github上有这本书中讲解的实例的代码[1]

虽然我不想谈太多理论,但是关于驱动的基本概念还是要有的。Linux系统分为内核态和用户态,只有在内核态才能访问到硬件设备,而驱动可以算是内核态中提供出的API,供用户态的代码访问到硬件设备。

有了基本概念以后,我就产生了一系列的问题,而我就是通过我的这一系列的问题进行学习的驱动开发:

  1. 一切代码的学习都是从Hello World开始的,怎么写一个Hello World的程序?
  2. 驱动是如何在/dev下生成设备文件的?
  3. 驱动怎么访问实际的硬件?
  4. 因为我毕竟是搞安全的,我会在想,怎么获取系统驱动的代码?或者没有代码那能逆向驱动吗?驱动的二进制文件储存在哪?以后有机会可能还可以试试搞驱动安全。

Everything start from Hello World

提供我的Hello World代码[2]

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

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Hcamal");

int hello_init(void)
{
    printk(KERN_INFO "Hello World\n");
    return 0;
}

void hello_exit(void)
{
    printk(KERN_INFO "Goodbye World\n");
}

module_init(hello_init);
module_exit(hello_exit);

Linux下的驱动是使用C语言进行开发的,但是和我们平常写的C语言也有不同,因为我们平常写的C语言使用的是Libc库,但是驱动是跑在内核中的程序,内核中却不存在libc库,所以要使用内核中的库函数。

比如printk可以类比为libc中的printf,这是在内核中定义的一个输出函数,但是我觉得更像Python里面logger函数,因为printk的输出结果是打印在内核的日志中,可以使用dmesg命令进行查看

驱动代码只有一个入口点和一个出口点,把驱动加载到内核中,会执行module_init函数定义的函数,在上面代码中就是hello_init函数。当驱动从内核被卸载时,会调用module_exit函数定义的函数,在上面代码中就是hello_exit函数。

上面的代码就很清晰了,当加载驱动时,输出Hello World,当卸载驱动时,输出Goodbye World

PS:MODULE_LICENSEMODULE_AUTHOR这两个不是很重要,我又不是专业开发驱动的,所以不用关注这两个

PSS: printk输出的结果要加一个换行,要不然不会刷新缓冲区

编译驱动

驱动需要通过make命令进行编译,Makefile如下所示:

ifneq ($(KERNELRELEASE),)

    obj-m := hello.o

else

    KERN_DIR ?= /usr/src/linux-headers-$(shell uname -r)/
    PWD := $(shell pwd)

default:
    $(MAKE) -C $(KERN_DIR) M=$(PWD) modules

endif


clean:
    rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions

一般情况下,内核的源码都存在与/usr/src/linux-headers-$(shell uname -r)/目录下

比如:

$ uname -r
4.4.0-135-generic

/usr/src/linux-headers-4.4.0-135/  --> 该内核源码目录
/usr/src/linux-headers-4.4.0-135-generic/    --> 该内核编译好的源码目录

而我们需要的是编译好后的源码的目录,也就是/usr/src/linux-headers-4.4.0-135-generic/

驱动代码的头文件都需要从该目录下进行搜索

M=$(PWD)该参数表示,驱动编译的结果输出在当前目录下

最后通过命令obj-m := hello.o,表示把hello.o编译出hello.ko, 这个ko文件就是内核模块文件

加载驱动到内核

需要使用到的一些系统命令:

  • lsmod: 查看当前已经被加载的内核模块
  • insmod: 加载内核模块,需要root权限
  • rmmod: 移除模块

比如:

# insmod hello.ko        // 把hello.ko模块加载到内核中
# rmmod hello            // 把hello模块从内核中移除

旧版的内核就是使用上面这样的方法进行内核的加载与移除,但是新版的Linux内核增加了对模块的验证,当前实际的情况如下:

# insmod hello.ko
insmod: ERROR: could not insert module hello.ko: Required key not available

从安全的角度考虑,现在的内核都是假设模块为不可信的,需要使用可信的证书对模块进行签名,才能加载模块

解决方法用两种:

  1. 进入BIOS,关闭UEFI的Secure Boot
  2. 向内核添加一个自签名证书,然后使用证书对驱动模块进行签名,参考[3]

查看结果

在/dev下增加设备文件

同样先提供一份代码,然后讲解这份实例代码[4]

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>   /* printk() */
#include <linux/slab.h>     /* kmalloc() */
#include <linux/fs.h>       /* everything... */
#include <linux/errno.h>    /* error codes */
#include <linux/types.h>    /* size_t */
#include <linux/fcntl.h>    /* O_ACCMODE */
#include <linux/cdev.h>
#include <asm/uaccess.h>    /* copy_*_user */


MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Hcamael");

int scull_major =   0;
int scull_minor =   0;
int scull_nr_devs = 4;
int scull_quantum = 4000;
int scull_qset = 1000;

struct scull_qset {
    void **data;
    struct scull_qset *next;
};

struct scull_dev {
    struct scull_qset *data;  /* Pointer to first quantum set. */
    int quantum;              /* The current quantum size. */
    int qset;                 /* The current array size. */
    unsigned long size;       /* Amount of data stored here. */
    unsigned int access_key;  /* Used by sculluid and scullpriv. */
    struct mutex mutex;       /* Mutual exclusion semaphore. */
    struct cdev cdev;     /* Char device structure. */
};

struct scull_dev *scull_devices;    /* allocated in scull_init_module */

/*
 * Follow the list.
 */
struct scull_qset *scull_follow(struct scull_dev *dev, int n)
{
    struct scull_qset *qs = dev->data;

        /* Allocate the first qset explicitly if need be. */
    if (! qs) {
        qs = dev->data = kmalloc(sizeof(struct scull_qset), GFP_KERNEL);
        if (qs == NULL)
            return NULL;
        memset(qs, 0, sizeof(struct scull_qset));
    }

    /* Then follow the list. */
    while (n--) {
        if (!qs->next) {
            qs->next = kmalloc(sizeof(struct scull_qset), GFP_KERNEL);
            if (qs->next == NULL)
                return NULL;
            memset(qs->next, 0, sizeof(struct scull_qset));
        }
        qs = qs->next;
        continue;
    }
    return qs;
}

/*
 * Data management: read and write.
 */

ssize_t scull_read(struct file *filp, char __user *buf, size_t count,
                loff_t *f_pos)
{
    struct scull_dev *dev = filp->private_data;
    struct scull_qset *dptr; /* the first listitem */
    int quantum = dev->quantum, qset = dev->qset;
    int itemsize = quantum * qset; /* how many bytes in the listitem */
    int item, s_pos, q_pos, rest;
    ssize_t retval = 0;

    if (mutex_lock_interruptible(&dev->mutex))
        return -ERESTARTSYS;
    if (*f_pos >= dev->size)
        goto out;
    if (*f_pos + count > dev->size)
        count = dev->size - *f_pos;

    /* Find listitem, qset index, and offset in the quantum */
    item = (long)*f_pos / itemsize;
    rest = (long)*f_pos % itemsize;
    s_pos = rest / quantum; q_pos = rest % quantum;

    /* follow the list up to the right position (defined elsewhere) */
    dptr = scull_follow(dev, item);

    if (dptr == NULL || !dptr->data || ! dptr->data[s_pos])
        goto out; /* don't fill holes */

    /* read only up to the end of this quantum */
    if (count > quantum - q_pos)
        count = quantum - q_pos;

    if (raw_copy_to_user(buf, dptr->data[s_pos] + q_pos, count)) {
        retval = -EFAULT;
        goto out;
    }
    *f_pos += count;
    retval = count;

  out:
    mutex_unlock(&dev->mutex);
    return retval;
}

ssize_t scull_write(struct file *filp, const char __user *buf, size_t count,
                loff_t *f_pos)
{
    struct scull_dev *dev = filp->private_data;
    struct scull_qset *dptr;
    int quantum = dev->quantum, qset = dev->qset;
    int itemsize = quantum * qset;
    int item, s_pos, q_pos, rest;
    ssize_t retval = -ENOMEM; /* Value used in "goto out" statements. */

    if (mutex_lock_interruptible(&dev->mutex))
        return -ERESTARTSYS;

    /* Find the list item, qset index, and offset in the quantum. */
    item = (long)*f_pos / itemsize;
    rest = (long)*f_pos % itemsize;
    s_pos = rest / quantum;
    q_pos = rest % quantum;

    /* Follow the list up to the right position. */
    dptr = scull_follow(dev, item);
    if (dptr == NULL)
        goto out;
    if (!dptr->data) {
        dptr->data = kmalloc(qset * sizeof(char *), GFP_KERNEL);
        if (!dptr->data)
            goto out;
        memset(dptr->data, 0, qset * sizeof(char *));
    }
    if (!dptr->data[s_pos]) {
        dptr->data[s_pos] = kmalloc(quantum, GFP_KERNEL);
        if (!dptr->data[s_pos])
            goto out;
    }
    /* Write only up to the end of this quantum. */
    if (count > quantum - q_pos)
        count = quantum - q_pos;

    if (raw_copy_from_user(dptr->data[s_pos]+q_pos, buf, count)) {
        retval = -EFAULT;
        goto out;
    }
    *f_pos += count;
    retval = count;

        /* Update the size. */
    if (dev->size < *f_pos)
        dev->size = *f_pos;

  out:
    mutex_unlock(&dev->mutex);
    return retval;
}

/* Beginning of the scull device implementation. */

/*
 * Empty out the scull device; must be called with the device
 * mutex held.
 */
int scull_trim(struct scull_dev *dev)
{
    struct scull_qset *next, *dptr;
    int qset = dev->qset;   /* "dev" is not-null */
    int i;

    for (dptr = dev->data; dptr; dptr = next) { /* all the list items */
        if (dptr->data) {
            for (i = 0; i < qset; i++)
                kfree(dptr->data[i]);
            kfree(dptr->data);
            dptr->data = NULL;
        }
        next = dptr->next;
        kfree(dptr);
    }
    dev->size = 0;
    dev->quantum = scull_quantum;
    dev->qset = scull_qset;
    dev->data = NULL;
    return 0;
}

int scull_release(struct inode *inode, struct file *filp)
{
    printk(KERN_DEBUG "process %i (%s) success release minor(%u) file\n", current->pid, current->comm, iminor(inode));
    return 0;
}

/*
 * Open and close
 */

int scull_open(struct inode *inode, struct file *filp)
{
    struct scull_dev *dev; /* device information */

    dev = container_of(inode->i_cdev, struct scull_dev, cdev);
    filp->private_data = dev; /* for other methods */

    /* If the device was opened write-only, trim it to a length of 0. */
    if ( (filp->f_flags & O_ACCMODE) == O_WRONLY) {
        if (mutex_lock_interruptible(&dev->mutex))
            return -ERESTARTSYS;
        scull_trim(dev); /* Ignore errors. */
        mutex_unlock(&dev->mutex);
    }
    printk(KERN_DEBUG "process %i (%s) success open minor(%u) file\n", current->pid, current->comm, iminor(inode));
    return 0;
}

/*
 * The "extended" operations -- only seek.
 */

loff_t scull_llseek(struct file *filp, loff_t off, int whence)
{
    struct scull_dev *dev = filp->private_data;
    loff_t newpos;

    switch(whence) {
      case 0: /* SEEK_SET */
        newpos = off;
        break;

      case 1: /* SEEK_CUR */
        newpos = filp->f_pos + off;
        break;

      case 2: /* SEEK_END */
        newpos = dev->size + off;
        break;

      default: /* can't happen */
        return -EINVAL;
    }
    if (newpos < 0)
        return -EINVAL;
    filp->f_pos = newpos;
    return newpos;
}

struct file_operations scull_fops = {
    .owner =    THIS_MODULE,
    .llseek =   scull_llseek,
    .read =     scull_read,
    .write =    scull_write,
    // .unlocked_ioctl = scull_ioctl,
    .open =     scull_open,
    .release =  scull_release,
};

/*
 * Set up the char_dev structure for this device.
 */
static void scull_setup_cdev(struct scull_dev *dev, int index)
{
    int err, devno = MKDEV(scull_major, scull_minor + index);

    cdev_init(&dev->cdev, &scull_fops);
    dev->cdev.owner = THIS_MODULE;
    dev->cdev.ops = &scull_fops;
    err = cdev_add (&dev->cdev, devno, 1);
    /* Fail gracefully if need be. */
    if (err)
        printk(KERN_NOTICE "Error %d adding scull%d", err, index);
    else
        printk(KERN_INFO "scull: %d add success\n", index);
}


void scull_cleanup_module(void)
{
    int i;
    dev_t devno = MKDEV(scull_major, scull_minor);

    /* Get rid of our char dev entries. */
    if (scull_devices) {
        for (i = 0; i < scull_nr_devs; i++) {
            scull_trim(scull_devices + i);
            cdev_del(&scull_devices[i].cdev);
        }
        kfree(scull_devices);
    }

    /* cleanup_module is never called if registering failed. */
    unregister_chrdev_region(devno, scull_nr_devs);
    printk(KERN_INFO "scull: cleanup success\n");
}


int scull_init_module(void)
{
    int result, i;
    dev_t dev = 0;

    /*
     * Get a range of minor numbers to work with, asking for a dynamic major
     * unless directed otherwise at load time.
     */
    if (scull_major) {
        dev = MKDEV(scull_major, scull_minor);
        result = register_chrdev_region(dev, scull_nr_devs, "scull");
    } else {
        result = alloc_chrdev_region(&dev, scull_minor, scull_nr_devs, "scull");
        scull_major = MAJOR(dev);
    }
    if (result < 0) {
        printk(KERN_WARNING "scull: can't get major %d\n", scull_major);
        return result;
    } else {
        printk(KERN_INFO "scull: get major %d success\n", scull_major);
    }

        /*
     * Allocate the devices. This must be dynamic as the device number can
     * be specified at load time.
     */
    scull_devices = kmalloc(scull_nr_devs * sizeof(struct scull_dev), GFP_KERNEL);
    if (!scull_devices) {
        result = -ENOMEM;
        goto fail;
    }
    memset(scull_devices, 0, scull_nr_devs * sizeof(struct scull_dev));

        /* Initialize each device. */
    for (i = 0; i < scull_nr_devs; i++) {
        scull_devices[i].quantum = scull_quantum;
        scull_devices[i].qset = scull_qset;
        mutex_init(&scull_devices[i].mutex);
        scull_setup_cdev(&scull_devices[i], i);
    }

    return 0; /* succeed */

  fail:
    scull_cleanup_module();
    return result;
}

module_init(scull_init_module);
module_exit(scull_cleanup_module);

知识点1 -- 驱动分类

驱动分为3类,字符设备、块设备和网口接口,上面代码举例的是字符设备,其他两种,之后再说。

如上图所示,brw-rw----权限栏,b开头的表示块设备(block),c开头的表示字符设备(char)

知识点2 -- 主次编号

主编号用来区分驱动,一般主编号相同的表示由同一个驱动程序控制。

一个驱动中能创建多个设备,用次编号来区分。

主编号和次编号一起,决定了一个驱动设备。

如上图所示,

brw-rw----  1 root disk      8,   0 Dec 17 13:02 sda
brw-rw----  1 root disk      8,   1 Dec 17 13:02 sda1

设备sdasda1的主编号为8,一个此编号为0一个此编号为1

知识点3 -- 驱动是如何提供API的

在我的概念中,驱动提供的接口是/dev/xxx,在Linux下Everything is File,所以对驱动设备的操作其实就是对文件的操作,所以一个驱动就是用来定义,打开/读/写/......一个/dev/xxx将会发生啥,驱动提供的API也就是一系列的文件操作。

有哪些文件操作?都被定义在内核<linux/fs.h>[5]头文件中,file_operations结构体

上面我举例的代码中:

struct file_operations scull_fops = {
    .owner =    THIS_MODULE,
    .llseek =   scull_llseek,
    .read =     scull_read,
    .write =    scull_write,
    .open =     scull_open,
    .release =  scull_release,
};

我声明了一个该结构体,并赋值,除了owner,其他成员的值都为函数指针

之后我在scull_setup_cdev函数中,使用cdev_add向每个驱动设备,注册该文件操作结构体

比如我对该驱动设备执行open操作,则会去执行scull_open函数,相当于hook了系统调用中的open函数

知识点4 -- 在/dev下生成相应的设备

对上面的代码进行编译,得到scull.ko,然后对其进行签名,最后使用insmod加载进内核中

查看是否成功加载:

虽然驱动已经加载成功了,但是并不会在/dev目录下创建设备文件,需要我们手动使用mknod进行设备链接:

总结

在该实例中,并没有涉及到对实际物理设备的操作,只是简单的使用kmalloc在内核空间申请一块内存。代码细节上的就不做具体讲解了,都可以通过查头文件或者用Google搜出来。

再这里分享一个我学习驱动开发的方法,首先看书把基础概念给弄懂,细节到需要用到的时候再去查。

比如,我不需要知道驱动一共能提供有哪些API(也就是file_operations结构都有啥),我只要知道一个概念,驱动提供的API都是一些文件操作,而文件操作,目前我只需要open, close, read, write,其他的等有需求,要用到的时候再去查。

参考

  1. https://github.com/jesstess/ldd4
  2. https://raw.githubusercontent.com/Hcamael/Linux_Driver_Study/master/hello.c
  3. https://jin-yang.github.io/post/kernel-modules.html
  4. https://raw.githubusercontent.com/Hcamael/Linux_Driver_Study/master/scull.c
  5. https://raw.githubusercontent.com/torvalds/linux/master/include/linux/fs.h

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