LDD3源码分析之ioctl操作

作者：刘昊昱 

博客：http://blog.csdn.net/liuhaoyutz

编译环境：Ubuntu 10.10

内核版本：2.6.32-38-generic-pae

LDD3源码路径：examples/scull/main.c

 

本文分析LDD3第六章中关于ioctl操作的代码，并编写测试程序对ioctl功能进行测试。

 

一、ioctl操作

驱动程序中ioctl函数的函数原型如下：

int (*ioctl)(struct inode *inode, struct file *filp,

             unsigned int cmd, unsigned long arg);

其中cmd和arg参数是ioctl与其它驱动程序函数不同的地方。cmd是预先定义好的一些命令编号，对应要求ioctl执行的命令。arg是与cmd配合使用的参数。

ioctl函数的功能比较繁琐，从函数名可以看出，它一般是实现对设备的各种控制操作。可以这样理解，通过常规的read，write，lseek等等函数实现不合理的功能，就交给ioctl来实现。例如：要求设备锁门，弹出介质，改变波特率，甚至执行自我破坏，等等。

ioctl的实现一般是通过一个大的switch语句，根据cmd参数执行不同的操作。所以，在实现ioctl函数之前，要先定义好cmd对应的命令编号。为了防止发生混淆，命令编号应该在系统范围内是唯一的。为此，Linux内核将命令编号分为4个部分，即4个位段，分别是：

type: 幻数(magic number)，它占8位。个人理解幻数就是一个标志，代表一个(类)对象。后面我们会看到，scull使用字符’k’作为幻数。

number：序数，即顺序编号，它也占8位。

direction：如果相关命令涉及到数据的传输，则这个位段表示数据传输的方向，可用的值包括_IOC_NONE(没有数据传输)，_IOC_READ(读)、_IOC_WRITE(写)、_IOC_READ | _IOC_WRITE(双向传输数据)。注意，数据传输方向是从应用程序的角度看的，也就是说_IOC_READ意味着从设备中读数据，所以驱动程序必须向用户空间写数据。

size：所涉及的用户数据大小。这个位段的宽度与体系结构有关，通常是13或14位。

<linux/ioctl.h>中包含的<asm/ioctl.h>头文件定义了一些构造命令编号的宏：

_IO(type, nr)，用于构造无数据传输的命令编号。

_IOR(type, nr, datatype)，用于构造从驱动程序中读取数据的命令编号。

_IOW(type, nr, datatype)，用于构造向设备写入数据的命令编号。

_IOWR(type, nr, datatype)，用于双向传输命令编号。

其中，type和number位段从以上宏的参数中传入，size位段通过对datatype参数取sizeof获得。

另外，<asm/ioctl.h>头文件中还定义了一些用于解析命令编号的宏，如_IOC_DIR(cmd)，_IOC_TYPE(cmd)，_IOC_NR(cmd)，_IOC_SIZE(cmd)。

首先我们来看一下scull是如何定义命令编号的，理解scull的ioctl函数的实现，关键是理解这些命令是什么含义，即要求完成什么工作。在scull.h中有如下定义：

135/*136 * Ioctl definitions137 */138139/* Use 'k' as magic number */140#define SCULL_IOC_MAGIC  'k'141/* Please use a different 8-bit number in your code */142143#define SCULL_IOCRESET    _IO(SCULL_IOC_MAGIC, 0)144145/*146 * S means "Set" through a ptr,147 * T means "Tell" directly with the argument value148 * G means "Get": reply by setting through a pointer149 * Q means "Query": response is on the return value150 * X means "eXchange": switch G and S atomically151 * H means "sHift": switch T and Q atomically152 */153#define SCULL_IOCSQUANTUM _IOW(SCULL_IOC_MAGIC,  1, int)154#define SCULL_IOCSQSET    _IOW(SCULL_IOC_MAGIC,  2, int)155#define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC,   3)156#define SCULL_IOCTQSET    _IO(SCULL_IOC_MAGIC,   4)157#define SCULL_IOCGQUANTUM _IOR(SCULL_IOC_MAGIC,  5, int)158#define SCULL_IOCGQSET    _IOR(SCULL_IOC_MAGIC,  6, int)159#define SCULL_IOCQQUANTUM _IO(SCULL_IOC_MAGIC,   7)160#define SCULL_IOCQQSET    _IO(SCULL_IOC_MAGIC,   8)161#define SCULL_IOCXQUANTUM _IOWR(SCULL_IOC_MAGIC, 9, int)162#define SCULL_IOCXQSET    _IOWR(SCULL_IOC_MAGIC,10, int)163#define SCULL_IOCHQUANTUM _IO(SCULL_IOC_MAGIC,  11)164#define SCULL_IOCHQSET    _IO(SCULL_IOC_MAGIC,  12)165166/*167 * The other entities only have "Tell" and "Query", because they're168 * not printed in the book, and there's no need to have all six.169 * (The previous stuff was only there to show different ways to do it.170 */171#define SCULL_P_IOCTSIZE _IO(SCULL_IOC_MAGIC,   13)172#define SCULL_P_IOCQSIZE _IO(SCULL_IOC_MAGIC,   14)173/* ... more to come */174175#define SCULL_IOC_MAXNR 14

140行，定义scull的幻数是字符'k'

146行，’S’代表通过参数arg指向的内容设置。

147行，’T’代表直接通过参数arg的值设置。

148行，’G’代表通过参数arg指向的地址返回请求的值。

149行，’Q’代表通过ioctl函数的返回值返回请求的值。

150行，’X’代表通过参数arg指向的内容设置，再把原来的值通过arg指向的地址返回。即’S’与’G’两个操作合为一步。

151行，’H’代表通过参数arg的值直接设置，再通过ioctl函数的返回值将原来的值返回。即’T’和’Q’两个操作合为一步。

153行，定义命令SCULL_IOCSQUANTUM，该命令表示通过参数arg指向的内容设置quantum。

154行，定义命令SCULL_IOCSQSET，该命令表示通过参数arg指向的内容设置qset。

155行，定义命令SCULL_IOCTQUANTUM，该命令表示通过参数arg的值直接设置quantum。

156行，定义命令SCULL_IOCTQSET，该命令表示通过参数arg的值直接设置qset。

157行，定义命令SCULL_IOCGQUANTUM，该命令表示通过参数arg指向的地址返回quantum。

158行，定义命令SCULL_IOCGQSET，该命令表示通过参数arg指向的地址返回qset。

159行，定义命令SCULL_IOCQQUANTUM，该命令表示通过ioctl的返回值返回quantum。

160行，定义命令SCULL_IOCQQSET，该命令表示通过ioctl的返回值返回qset。

161行，定义命令SCULL_IOCXQUANTUM，该命令表示通过参数arg指向的内容设置quantum，然后，再把quantum原来的值写入arg指向的地址返回。

162行，定义命令SCULL_IOCXQSET，该命令表示通过参数arg指向的内容设置qset，然后，再把qset原来的值写入arg指向的地址返回。

163行，定义命令SCULL_IOCHQUANTUM，该命令表示通过参数arg的值直接设置quantum，然后，再通过ioctl的返回值返回quantum原来的值。

164行，定义命令SCULL_IOCHQSET，该命令表示通过参数arg的值直接设置qset，然后，再通过ioctl的返回值返回qset原来的值。

171行，定义命令SCULL_P_IOCTSIZE，该命令表示通过参数arg的值直接设置scull_p_buffer。

172行，定义命令SCULL_P_IOCQSIZE，该命令表示通过ioctl的返回值返回scull_p_buffer。

175定义SCULL_IOC_MAXNR为14，代表一共有14个命令。

理解了scull的ioctl命令的含义，我们就可以看ioctl的代码了，下面列出scull的ioctl函数代码如下：

389/*390 * The ioctl() implementation391 */392393int scull_ioctl(struct inode *inode, struct file *filp,394                 unsigned int cmd, unsigned long arg)395{396397    int err = 0, tmp;398    int retval = 0;399400    /*401     * extract the type and number bitfields, and don't decode402     * wrong cmds: return ENOTTY (inappropriate ioctl) before access_ok()403     */404    if (_IOC_TYPE(cmd) != SCULL_IOC_MAGIC) return -ENOTTY;405    if (_IOC_NR(cmd) > SCULL_IOC_MAXNR) return -ENOTTY;406407    /*408     * the direction is a bitmask, and VERIFY_WRITE catches R/W409     * transfers. `Type' is user-oriented, while410     * access_ok is kernel-oriented, so the concept of "read" and411     * "write" is reversed412     */413    if (_IOC_DIR(cmd) & _IOC_READ)414        err = !access_ok(VERIFY_WRITE, (void __user *)arg, _IOC_SIZE(cmd));415    else if (_IOC_DIR(cmd) & _IOC_WRITE)416        err =  !access_ok(VERIFY_READ, (void __user *)arg, _IOC_SIZE(cmd));417    if (err) return -EFAULT;418419    switch(cmd) {420421      case SCULL_IOCRESET:422        scull_quantum = SCULL_QUANTUM;423        scull_qset = SCULL_QSET;424        break;425426      case SCULL_IOCSQUANTUM: /* Set: arg points to the value */427        if (! capable (CAP_SYS_ADMIN))428            return -EPERM;429        retval = __get_user(scull_quantum, (int __user *)arg);430        break;431432      case SCULL_IOCTQUANTUM: /* Tell: arg is the value */433        if (! capable (CAP_SYS_ADMIN))434            return -EPERM;435        scull_quantum = arg;436        break;437438      case SCULL_IOCGQUANTUM: /* Get: arg is pointer to result */439        retval = __put_user(scull_quantum, (int __user *)arg);440        break;441442      case SCULL_IOCQQUANTUM: /* Query: return it (it's positive) */443        return scull_quantum;444445      case SCULL_IOCXQUANTUM: /* eXchange: use arg as pointer */446        if (! capable (CAP_SYS_ADMIN))447            return -EPERM;448        tmp = scull_quantum;449        retval = __get_user(scull_quantum, (int __user *)arg);450        if (retval == 0)451            retval = __put_user(tmp, (int __user *)arg);452        break;453454      case SCULL_IOCHQUANTUM: /* sHift: like Tell + Query */455        if (! capable (CAP_SYS_ADMIN))456            return -EPERM;457        tmp = scull_quantum;458        scull_quantum = arg;459        return tmp;460461      case SCULL_IOCSQSET:462        if (! capable (CAP_SYS_ADMIN))463            return -EPERM;464        retval = __get_user(scull_qset, (int __user *)arg);465        break;466467      case SCULL_IOCTQSET:468        if (! capable (CAP_SYS_ADMIN))469            return -EPERM;470        scull_qset = arg;471        break;472473      case SCULL_IOCGQSET:474        retval = __put_user(scull_qset, (int __user *)arg);475        break;476477      case SCULL_IOCQQSET:478        return scull_qset;479480      case SCULL_IOCXQSET:481        if (! capable (CAP_SYS_ADMIN))482            return -EPERM;483        tmp = scull_qset;484        retval = __get_user(scull_qset, (int __user *)arg);485        if (retval == 0)486            retval = put_user(tmp, (int __user *)arg);487        break;488489      case SCULL_IOCHQSET:490        if (! capable (CAP_SYS_ADMIN))491            return -EPERM;492        tmp = scull_qset;493        scull_qset = arg;494        return tmp;495496        /*497         * The following two change the buffer size for scullpipe.498         * The scullpipe device uses this same ioctl method, just to499         * write less code. Actually, it's the same driver, isn't it?500         */501502      case SCULL_P_IOCTSIZE:503        scull_p_buffer = arg;504        break;505506      case SCULL_P_IOCQSIZE:507        return scull_p_buffer;508509510      default:  /* redundant, as cmd was checked against MAXNR */511        return -ENOTTY;512    }513    return retval;514515}

404行，如果_IOC_TYPE(cmd) != SCULL_IOC_MAGIC，即cmd的幻数不是’k’，则退出。

405行，如果_IOC_NR(cmd) > SCULL_IOC_MAXNR，即cmd的序数大于14，则退出。

413 - 417行，如果要使用arg指向的地址进行数据的读或写，必须保证对该地址的访问是合法的，这可通过access_ok函数来验证，如果访问不合法，则退出。

419行，进入switch语句块。根据传入的cmd值，进入不同的分支执行。

420 - 512行，是个各cmd的处理分支，只要我们理解了各个cmd的含义，就很容易实现这些命令要求完成的工作。如果有不理解的地方，回到前面的各个cmd的定义处再研究一下。值得一提的是，驱动程序与用户空间传递数据，采用的是__put_user和__get_user函数，相比copy_to_user和copy_from_user来说，这些函数在处理1、2、4、8个字节的数据传输时，效率更高。另外，scull允许任何用户查询quantum和qset的大小，但只允许被授权的用户修改quantum和qset的值。这种权能的检查是通过capable()函数实现的。

 

二、测试ioctl

要测试scull驱动中ioctl函数是否实现了我们要求的功能，需要编写用户空间程序对scull模块进行测试。下面是我写的一个比较简单的测试程序：

首先是头文件scull_ioctl.h：

#ifndef _SCULL_IOCTL_H_#define _SCULL_IOCTL_H_#include <linux/ioctl.h> /* needed for the _IOW etc stuff used later *//* * Ioctl definitions *//* Use 'k' as magic number */#define SCULL_IOC_MAGIC  'k'/* Please use a different 8-bit number in your code */#define SCULL_IOCRESET    _IO(SCULL_IOC_MAGIC, 0)/* * S means "Set" through a ptr, * T means "Tell" directly with the argument value * G means "Get": reply by setting through a pointer * Q means "Query": response is on the return value * X means "eXchange": switch G and S atomically * H means "sHift": switch T and Q atomically */#define SCULL_IOCSQUANTUM _IOW(SCULL_IOC_MAGIC,  1, int)#define SCULL_IOCSQSET    _IOW(SCULL_IOC_MAGIC,  2, int)#define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC,   3)#define SCULL_IOCTQSET    _IO(SCULL_IOC_MAGIC,   4)#define SCULL_IOCGQUANTUM _IOR(SCULL_IOC_MAGIC,  5, int)#define SCULL_IOCGQSET    _IOR(SCULL_IOC_MAGIC,  6, int)#define SCULL_IOCQQUANTUM _IO(SCULL_IOC_MAGIC,   7)#define SCULL_IOCQQSET    _IO(SCULL_IOC_MAGIC,   8)#define SCULL_IOCXQUANTUM _IOWR(SCULL_IOC_MAGIC, 9, int)#define SCULL_IOCXQSET    _IOWR(SCULL_IOC_MAGIC,10, int)#define SCULL_IOCHQUANTUM _IO(SCULL_IOC_MAGIC,  11)#define SCULL_IOCHQSET    _IO(SCULL_IOC_MAGIC,  12)/* * The other entities only have "Tell" and "Query", because they're * not printed in the book, and there's no need to have all six. * (The previous stuff was only there to show different ways to do it. */#define SCULL_P_IOCTSIZE _IO(SCULL_IOC_MAGIC,   13)#define SCULL_P_IOCQSIZE _IO(SCULL_IOC_MAGIC,   14)/* ... more to come */#define SCULL_IOC_MAXNR 14#endif /* _SCULL_IOCTL_H_ */

下面是测试程序scull_ioctl_test.c的代码：

#include <sys/types.h>#include <sys/stat.h>#include <sys/ioctl.h>#include <fcntl.h>#include <stdio.h>#include "scull_ioctl.h"#define SCULL_DEVICE "/dev/scull0"int main(int argc, char *argv[]){    int fd = 0;    int quantum = 8000;    int quantum_old = 0;    int qset = 2000;    int qset_old = 0;        fd = open(SCULL_DEVICE, O_RDWR);    if(fd < 0)    {        printf("open scull device error!\n");        return 0;    }    printf("SCULL_IOCSQUANTUM: quantum = %d\n", quantum);    ioctl(fd, SCULL_IOCSQUANTUM, &quantum);    quantum -= 500;    printf("SCULL_IOCTQUANTUM: quantum = %d\n", quantum);        ioctl(fd, SCULL_IOCTQUANTUM, quantum);        ioctl(fd, SCULL_IOCGQUANTUM, &quantum);    printf("SCULL_IOCGQUANTUM: quantum = %d\n", quantum);        quantum = ioctl(fd, SCULL_IOCQQUANTUM);    printf("SCULL_IOCQQUANTUM: quantum = %d\n", quantum);        quantum -= 500;    quantum_old = ioctl(fd, SCULL_IOCHQUANTUM, quantum);    printf("SCULL_IOCHQUANTUM: quantum = %d, quantum_old = %d\n", quantum, quantum_old);        quantum -= 500;    printf("SCULL_IOCXQUANTUM: quantum = %d\n", quantum);    ioctl(fd, SCULL_IOCXQUANTUM, &quantum);    printf("SCULL_IOCXQUANTUM: old quantum = %d\n", quantum);    printf("SCULL_IOCSQSET: qset = %d\n", qset);    ioctl(fd, SCULL_IOCSQSET, &qset);    qset += 500;    printf("SCULL_IOCTQSET: qset = %d\n", qset);    ioctl(fd, SCULL_IOCTQSET, qset);    ioctl(fd, SCULL_IOCGQSET, &qset);    printf("SCULL_IOCGQSET: qset = %d\n", qset);    qset = ioctl(fd, SCULL_IOCQQSET);    printf("SCULL_IOCQQSET: qset = %d\n", qset);    qset += 500;    qset_old = ioctl(fd, SCULL_IOCHQSET, qset);    printf("SCULL_IOCHQSET: qset = %d, qset_old = %d\n", qset, qset_old);        qset += 500;    printf("SCULL_IOCXQSET: qset = %d\n", qset);            ioctl(fd, SCULL_IOCXQSET, &qset);    printf("SCULL_IOCHQSET: old qset = %d\n", qset);    return 0;}

为了能看到测试效果，在修改驱动程序中的ioctl函数，打印一些语句。下面直接列出修改后的ioctl函数的实现：

/* * The ioctl() implementation */int scull_ioctl(struct inode *inode, struct file *filp,                 unsigned int cmd, unsigned long arg){int err = 0, tmp;int retval = 0;    /* * extract the type and number bitfields, and don't decode * wrong cmds: return ENOTTY (inappropriate ioctl) before access_ok() */if (_IOC_TYPE(cmd) != SCULL_IOC_MAGIC) return -ENOTTY;if (_IOC_NR(cmd) > SCULL_IOC_MAXNR) return -ENOTTY;/* * the direction is a bitmask, and VERIFY_WRITE catches R/W * transfers. `Type' is user-oriented, while * access_ok is kernel-oriented, so the concept of "read" and * "write" is reversed */if (_IOC_DIR(cmd) & _IOC_READ)err = !access_ok(VERIFY_WRITE, (void __user *)arg, _IOC_SIZE(cmd));else if (_IOC_DIR(cmd) & _IOC_WRITE)err =  !access_ok(VERIFY_READ, (void __user *)arg, _IOC_SIZE(cmd));if (err) return -EFAULT;switch(cmd) {  case SCULL_IOCRESET:scull_quantum = SCULL_QUANTUM;scull_qset = SCULL_QSET;        printk("SCULL_IOCRESET: scull_quantum = %d, scull_qset = %d\n", scull_quantum, scull_qset);break;          case SCULL_IOCSQUANTUM: /* Set: arg points to the value */if (! capable (CAP_SYS_ADMIN))return -EPERM;retval = __get_user(scull_quantum, (int __user *)arg);        printk("SCULL_IOCSQUANTUM: scull_quantum = %d\n", scull_quantum);break;  case SCULL_IOCTQUANTUM: /* Tell: arg is the value */if (! capable (CAP_SYS_ADMIN))return -EPERM;scull_quantum = arg;        printk("SCULL_IOCTQUANTUM: scull_quantum = %d\n", scull_quantum);break;  case SCULL_IOCGQUANTUM: /* Get: arg is pointer to result */retval = __put_user(scull_quantum, (int __user *)arg);        printk("SCULL_IOCGQUANTUM: use arg return scull_quantum = %d\n", scull_quantum);break;  case SCULL_IOCQQUANTUM: /* Query: return it (it's positive) */        printk("SCULL_IOCQQUANTUM: return scull_quantum = %d\n", scull_quantum);return scull_quantum;  case SCULL_IOCXQUANTUM: /* eXchange: use arg as pointer */if (! capable (CAP_SYS_ADMIN))return -EPERM;tmp = scull_quantum;retval = __get_user(scull_quantum, (int __user *)arg);if (retval == 0)retval = __put_user(tmp, (int __user *)arg);        printk("SCULL_IOCXQUANTUM: scull_quantum = %d, and use arg return old scull_quantum = %d\n", scull_quantum, tmp);        break;  case SCULL_IOCHQUANTUM: /* sHift: like Tell + Query */if (! capable (CAP_SYS_ADMIN))return -EPERM;tmp = scull_quantum;scull_quantum = arg;        printk("SCULL_IOCHQUANTUM: scull_quantum = %d, and return old scull_quantum = %d\n", scull_quantum, tmp);        return tmp;          case SCULL_IOCSQSET:if (! capable (CAP_SYS_ADMIN))return -EPERM;retval = __get_user(scull_qset, (int __user *)arg);        printk("SCULL_IOCSQSET: scull_qset = %d\n", scull_qset);break;  case SCULL_IOCTQSET:if (! capable (CAP_SYS_ADMIN))return -EPERM;scull_qset = arg;        printk("SCULL_IOCTQSET: scull_qset = %d\n", scull_qset);break;  case SCULL_IOCGQSET:retval = __put_user(scull_qset, (int __user *)arg);        printk("SCULL_IOCGQSET: use arg return scull_qset = %d\n", scull_qset);        break;  case SCULL_IOCQQSET:        printk("SCULL_IOCQQSET: return scull_qset = %d\n", scull_qset);                  return scull_qset;  case SCULL_IOCXQSET:if (! capable (CAP_SYS_ADMIN))return -EPERM;tmp = scull_qset;retval = __get_user(scull_qset, (int __user *)arg);if (retval == 0)retval = put_user(tmp, (int __user *)arg);        printk("SCULL_IOCXQSET: scull_qset = %d, and use arg return old scull_qset = %d\n", scull_qset, tmp);        break;  case SCULL_IOCHQSET:if (! capable (CAP_SYS_ADMIN))return -EPERM;tmp = scull_qset;scull_qset = arg;        printk("SCULL_IOCHQSET: scull_qet = %d, and return old scull_qset = %d\n", scull_qset, tmp);                return tmp;        /*         * The following two change the buffer size for scullpipe.         * The scullpipe device uses this same ioctl method, just to         * write less code. Actually, it's the same driver, isn't it?         */  case SCULL_P_IOCTSIZE:scull_p_buffer = arg;break;  case SCULL_P_IOCQSIZE:return scull_p_buffer;  default:  /* redundant, as cmd was checked against MAXNR */return -ENOTTY;}return retval;}

在我的系统上，测试过程如图所示。需要注意的是测试程序必须以root权限运行，因为普通用户只能读quantum和qset的值，只有root用户才能修改。