c - 如何检查 ioctl 上的现有参数

Question

我想尝试如何使用可加载内核模块的 i/o 控件，这里是字符设备。问题是:如何检查用户空间端的 ioctl 调用是否有参数。我发现在 ioctl 上，参数是可选的。

用户空间函数内部:

// set a parameter - this is a proper call
if( ioctl(fd, IOCTL_SET_PARAM1, 5)<0 )
{
  fprintf(stderr,"Error while ioctl: %s\n", strerror(errno));
}

// call a setter without argument - this should cause an error
if( ioctl(fd, IOCTL_SET_PARAM1)<0 )
{
  fprintf(stderr,"Error while ioctl: %s\n", strerror(errno));
}

对应的内核模块处理程序:

long
fops_unlocked_ioctl (struct file    *p_file,
                     unsigned int    cmd,
                     unsigned long   arg)
{
  switch(cmd)
  {
  case IOCTL_SET_PARAM1:
    printk(KERN_INFO "IOCTL called with IOCTL_SET_PARAM1\n");
    if( /* how to check for the argument here? */ )
    {
      printk(KERN_WARNING "Missing argument\n");
      return -EINVAL;
    }
    param1 = (unsigned short) arg;
    printk(KERN_INFO "param1 set to %d\n",param1);
    break;
  default:
    printk(KERN_WARNING "IOCTL called with wrong request code.\n");
    return -EINVAL;
  }
  return 0;
}

问候，亚历克斯

Answer 1

Jonathan Leffler 说得完全正确，用户应该知道该做什么、不该做什么。

顺便说一下。我提出了一个解决方案，用带有计数和原始参数的数组替换参数。在用户空间方面，用户没有任何变化。在内核空间方面，有两个宏用于获取原始参数和参数数量。

代码如下:

expdev.h

#ifndef EXPDEV_H_INCLUDED
#define EXPDEV_H_INCLUDED

#ifndef __KERNEL__
  #include <stdint.h>
#endif // __KERNEL__

#include "pp_narg.h"

enum {
  IOCTL_SET_PARAM1,
};

/* ioctl - wrapper *************************************************** */

#ifndef __KERNEL__
  long int argW[2]={0};

  long int PP_IOCTL_WRAPARG( long int narg, long int arg )
  {
    argW[0] = narg;
    argW[1] = arg;
    return (long int)(argW);
  }

  #define PP_IOCTL_NARG(n,fd,cmd,arg,...) \
    ( (n==3) ? \
      ( ioctl(fd,cmd,(long int)(PP_IOCTL_WRAPARG(1,(long int)arg))) ) : \
      ( ioctl(fd,cmd,(long int)(PP_IOCTL_WRAPARG(0,(long int)arg))) ) )

  #define ioctl(...) \
    (PP_IOCTL_NARG(PP_NARG(__VA_ARGS__),         \
                   __VA_ARGS__, (long int)(0),(long int)(0)))

#else // __KERNEL__

  #define IOCTL_ARGC(argW) (((long int*)argW)[0])
  #define IOCTL_ARG(argW)  (((long int*)argW)[1])

#endif // __KERNEL__

#endif // EXPDEV_H_INCLUDED

为了计算参数，我遵循这个 post .

pp_narg.h

/*
 Source: https://groups.google.com/forum/#!topic/comp.std.c/d-6Mj5Lko_s
 */

#ifndef PP_NARG_INCLUDED
#define PP_NARG_INCLUDED

#define PP_NARG(...) \
         PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
#define PP_NARG_(...) \
         PP_ARG_N(__VA_ARGS__)
#define PP_ARG_N( \
          _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
         _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
         _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
         _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
         _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
         _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
         _61,_62,_63,N,...) N
#define PP_RSEQ_N() \
         63,62,61,60,                   \
         59,58,57,56,55,54,53,52,51,50, \
         49,48,47,46,45,44,43,42,41,40, \
         39,38,37,36,35,34,33,32,31,30, \
         29,28,27,26,25,24,23,22,21,20, \
         19,18,17,16,15,14,13,12,11,10, \
         9,8,7,6,5,4,3,2,1,0

#endif // PP_NARG_INCLUDED

现在在用户空间函数内进行调用(与平常相同):

#include "expdev.h"

// set a parameter
if( ioctl(fd, IOCTL_SET_PARAM1, 5)<0 )
{
  fprintf(stderr,"Error while ioctl: %s\n", strerror(errno));
}

// call a setter without argument - this should cause an error
if( ioctl(fd, IOCTL_SET_PARAM1)<0 )
{
  fprintf(stderr,"Error while ioctl: %s\n", strerror(errno));
}

和内核空间端(注意IOCTL_ARGC(arg)和IOCTL_ARG(arg)):

#include "expdev.h"

long
fops_unlocked_ioctl (struct file    *p_file,
                     unsigned int    cmd,
                     unsigned long   arg)
{
  switch(cmd)
  {
  case IOCTL_SET_PARAM1:
    printk(KERN_INFO "IOCTL called with IOCTL_SET_PARAM1\n");
    if( IOCTL_ARGC(arg)!=1 )
    {
      printk(KERN_ERR "Missing argument.\n");
      return -EINVAL;
    }
    param1 = (unsigned short) IOCTL_ARG(arg);
    printk(KERN_INFO "param1 set to %d\n",param1);
    break;
  default:
    printk(KERN_WARNING "IOCTL called with wrong request code.\n");
    return -EINVAL;
  }
  return 0;
}

这会导致 kern.log 上出现以下输出:

kernel: [18577.042438] IOCTL called with IOCTL_SET_PARAM1
kernel: [18577.042439] param1 set to 5
kernel: [18577.042442] IOCTL called with IOCTL_SET_PARAM1
kernel: [18577.042443] Missing argument.

在用户空间端:

Error while ioctl: Invalid argument

注意:

宏重新定义了ioctl。这意味着在同一源内的不同设备上使用 ioctl 将导致不支持此宏的设备上出现错误(例如，如果您使用标准 tty)。