sleep.c

对linux内核编程做了详细的解析和举例

/* sleep.c - create a /proc file, and if several 
 * processes try to open it at the same time, put all 
 * but one to sleep */


/* Copyright (C) 1998-99 by Ori Pomerantz */


/* The necessary header files */

/* Standard in kernel modules */
#include <linux/kernel.h>   /* We're doing kernel work */
#include <linux/module.h>   /* Specifically, a module */

/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif        

/* Necessary because we use proc fs */
#include <linux/proc_fs.h>

/* For putting processes to sleep and waking them up */
#include <linux/sched.h>
#include <linux/wrapper.h>



/* In 2.2.3 /usr/include/linux/version.h includes a 
 * macro for this, but 2.0.35 doesn't - so I add it 
 * here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif


#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h>  /* for get_user and put_user */
#endif


/* The module's file functions ********************** */


/* Here we keep the last message received, to prove 
 * that we can process our input */
#define MESSAGE_LENGTH 80
static char Message[MESSAGE_LENGTH];


/* Since we use the file operations struct, we can't use 
 * the special proc output provisions - we have to use 
 * a standard read function, which is this function */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_output(
    struct file *file,   /* The file read */
    char *buf, /* The buffer to put data to (in the
                * user segment) */
    size_t len,  /* The length of the buffer */
    loff_t *offset) /* Offset in the file - ignore */
#else
static int module_output(
    struct inode *inode, /* The inode read */
    struct file *file,   /* The file read */
    char *buf, /* The buffer to put data to (in the
                * user segment) */
    int len)  /* The length of the buffer */
#endif
{
  static int finished = 0;
  int i;
  char message[MESSAGE_LENGTH+30];

  /* Return 0 to signify end of file - that we have 
   * nothing more to say at this point. */
  if (finished) {
    finished = 0;
    return 0;
  }

  /* If you don't understand this by now, you're 
   * hopeless as a kernel  programmer. */
  sprintf(message, "Last input:%s\n", Message);
  for(i=0; i<len && message[i]; i++) 
    put_user(message[i], buf+i);

  finished = 1;
  return i;  /* Return the number of bytes "read" */
}


/* This function receives input from the user when 
 * the user writes to the /proc file. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_input(
    struct file *file,   /* The file itself */
    const char *buf,     /* The buffer with input */
    size_t length,       /* The buffer's length */
    loff_t *offset)      /* offset to file - ignore */
#else
static int module_input(
    struct inode *inode, /* The file's inode */
    struct file *file,   /* The file itself */
    const char *buf,     /* The buffer with the input */
    int length)          /* The buffer's length */
#endif
{
  int i;

  /* Put the input into Message, where module_output 
   * will later be able to use it */
  for(i=0; i<MESSAGE_LENGTH-1 && i<length; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
     get_user(Message[i], buf+i);
#else
  Message[i] = get_user(buf+i);
#endif
/* we want a standard, zero terminated string */
  Message[i] = '\0';  
  
  /* We need to return the number of input 
   * characters used */
  return i;
}

/* 1 if the file is currently open by somebody */
int Already_Open = 0;

/* Queue of processes who want our file */
static struct wait_queue *WaitQ = NULL;


/* Called when the /proc file is opened */
static int module_open(struct inode *inode,
                       struct file *file)
{
  /* If the file's flags include O_NONBLOCK, it means 
   * the process doesn't want to wait for the file. 
   * In this case, if the file is already open, we 
   * should fail with -EAGAIN, meaning "you'll have to 
   * try again", instead of blocking a process which 
   * would rather stay awake. */
  if ((file->f_flags & O_NONBLOCK) && Already_Open) 
    return -EAGAIN;

  /* This is the correct place for MOD_INC_USE_COUNT 
   * because if a process is in the loop, which is 
   * within the kernel module, the kernel module must
   * not be removed. */
  MOD_INC_USE_COUNT;

  /* If the file is already open, wait until it isn't */
  while (Already_Open) 
  {
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
    int i, is_sig=0;
#endif

    /* This function puts the current process, 
     * including any system calls, such as us, to sleep. 
     * Execution will be resumed right after the function 
     * call, either because somebody called 
     * wake_up(&WaitQ) (only module_close does that, 
     * when the file is closed) or when a signal, such 
     * as Ctrl-C, is sent to the process */
    module_interruptible_sleep_on(&WaitQ);
 
    /* If we woke up because we got a signal we're not 
     * blocking, return  -EINTR (fail the system call). 
     * This allows processes to be killed or stopped. */



/*
 * Emmanuel Papirakis:
 *
 * This is a little update to work with 2.2.*. Signals 
 * now are contained in two words (64 bits) and are 
 * stored in a structure that contains an array of two 
 * unsigned longs. We now have to make 2 checks in our if.
 *
 * Ori Pomerantz:
 *
 * Nobody promised me they'll never use more than 64 
 * bits, or that this book won't be used for a version 
 * of Linux with a word size of 16 bits. This code
 * would work in any case.
 */	  
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)

    for(i=0; i<_NSIG_WORDS && !is_sig; i++)
      is_sig = current->signal.sig[i] & 
        ~current->blocked.sig[i];
    if (is_sig) {
#else
    if (current->signal & ~current->blocked) {
#endif
      /* It's important to put MOD_DEC_USE_COUNT here, 
       * because for processes where the open is 
       * interrupted there will never be a corresponding
       * close. If we don't decrement the usage count 
       * here, we will be left with a positive usage 
       * count which we'll have no way to bring down to
       * zero, giving us an immortal module, which can 
       * only be killed by rebooting the machine. */
      MOD_DEC_USE_COUNT;
      return -EINTR;
    }
  }

  /* If we got here, Already_Open must be zero */

  /* Open the file */
  Already_Open = 1;
  return 0;  /* Allow the access */
}



/* Called when the /proc file is closed */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int module_close(struct inode *inode, struct file *file)
#else
void module_close(struct inode *inode, struct file *file)
#endif
{
  /* Set Already_Open to zero, so one of the processes 
   * in the WaitQ will be able to set Already_Open back 
   * to one and to open the file. All the other processes 
   * will be called when Already_Open is back to one, so
   * they'll go back to sleep. */
  Already_Open = 0;

  /* Wake up all the processes in WaitQ, so if anybody 
   * is waiting for the file, they can have it. */
  module_wake_up(&WaitQ);

  MOD_DEC_USE_COUNT;

#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
  return 0;  /* success */
#endif
}




/* This function decides whether to allow an operation 
 * (return zero) or not allow it (return a non-zero 
 * which indicates why it is not allowed).
 *
 * The operation can be one of the following values:
 * 0 - Execute (run the "file" - meaningless in our case)
 * 2 - Write (input to the kernel module)
 * 4 - Read (output from the kernel module)
 *
 * This is the real function that checks file 
 * permissions. The permissions returned by ls -l are 
 * for referece only, and can be overridden here. 
 */
static int module_permission(struct inode *inode, int op)
{
  /* We allow everybody to read from our module, but 
   * only root (uid 0) may write to it */ 
  if (op == 4 || (op == 2 && current->euid == 0))
    return 0; 

  /* If it's anything else, access is denied */
  return -EACCES;
}


/* Structures to register as the /proc file, with 
 * pointers to all the relevant functions. *********** */


/* File operations for our proc file. This is where 
 * we place pointers to all the functions called when 
 * somebody tries to do something to our file. NULL 
 * means we don't want to deal with something. */
static struct file_operations File_Ops_4_Our_Proc_File =
  {
    NULL,  /* lseek */
    module_output,  /* "read" from the file */
    module_input,   /* "write" to the file */
    NULL,  /* readdir */
    NULL,  /* select */
    NULL,  /* ioctl */
    NULL,  /* mmap */
    module_open,/* called when the /proc file is opened */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
    NULL,   /* flush */
#endif
    module_close      /* called when it's classed */
  };



/* Inode operations for our proc file. We need it so 
 * we'll have somewhere to specify the file operations 
 * structure we want to use, and the function we use for 
 * permissions. It's also possible to specify functions 
 * to be called for anything else which could be done to an
 * inode (although we don't bother, we just put NULL). */
static struct inode_operations Inode_Ops_4_Our_Proc_File =
  {
    &File_Ops_4_Our_Proc_File,
    NULL, /* create */
    NULL, /* lookup */
    NULL, /* link */
    NULL, /* unlink */
    NULL, /* symlink */
    NULL, /* mkdir */
    NULL, /* rmdir */
    NULL, /* mknod */
    NULL, /* rename */
    NULL, /* readlink */
    NULL, /* follow_link */
    NULL, /* readpage */
    NULL, /* writepage */
    NULL, /* bmap */
    NULL, /* truncate */
    module_permission /* check for permissions */
  };

/* Directory entry */
static struct proc_dir_entry Our_Proc_File = 
  {
    0, /* Inode number - ignore, it will be filled by 
        * proc_register[_dynamic] */
    5, /* Length of the file name */
    "sleep", /* The file name */
    S_IFREG | S_IRUGO | S_IWUSR, 
    /* File mode - this is a regular file which 
     * can be read by its owner, its group, and everybody
     * else. Also, its owner can write to it.
     *
     * Actually, this field is just for reference, it's
     * module_permission that does the actual check. It 
     * could use this field, but in our implementation it
     * doesn't, for simplicity. */
    1,  /* Number of links (directories where the 
         * file is referenced) */
    0, 0,  /* The uid and gid for the file - we give 
            * it to root */
    80, /* The size of the file reported by ls. */
    &Inode_Ops_4_Our_Proc_File, 
    /* A pointer to the inode structure for
     * the file, if we need it. In our case we
     * do, because we need a write function. */
    NULL  /* The read function for the file. 
           * Irrelevant, because we put it
           * in the inode structure above */
  }; 



/* Module initialization and cleanup **************** */


/* Initialize the module - register the proc file */
int init_module()
{
  /* Success if proc_register_dynamic is a success, 
   * failure otherwise */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
  return proc_register(&proc_root, &Our_Proc_File);
#else
  return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif 

  /* proc_root is the root directory for the proc 
   * fs (/proc). This is where we want our file to be 
   * located. 
   */
}


/* Cleanup - unregister our file from /proc. This could 
 * get dangerous if there are still processes waiting in 
 * WaitQ, because they are inside our open function, 
 * which will get unloaded. I'll explain how to avoid 
 * removal of a kernel module in such a case in 
 * chapter 10. */
void cleanup_module()
{
  proc_unregister(&proc_root, Our_Proc_File.low_ino);
}