通过/proc输入内容¶
到目前为止，我们有两种方法从内核模块生成输出：
我们可以注册一个设备驱动程序并管理一个设备文件
我们可以创建一个/proc文件。 至目前为止，我们已经可以得到内核模块的输出，但是还没办法向内核模块中传递数据。接下来我们就介绍向内核中传入数据的第一种方法——写
/proc文件
因为编写进程文件系统主要是为了允许内核向进程报告它的情况，所以对于输入没有特殊的规定。proc_dir_entry结构体不像包含指向输出函数的指针那样包含指向输入函数的指针。相反，要写入/proc文件，我们需要使用标准的文件系统机制。
在Linux中，有一个文件系统注册的标准机制。因为每个文件系统都必须有自己的函数来处理inode和文件操作，所以有一个特殊的结构来保存指向所有这些函数的指针，struct inode_operations，其中包括指向struct file_operations的指针。在/proc中，每当我们注册一个新文件时，我们都可以指定将使用哪个结构体inode_operations来访问它。这就是我们使用的机制，一个结构体inode_operations包含一个指向结构体file_operations的指针，而file_operations包含指向module_input和module_output函数的指针。
需要注意的是，读写的标准角色在内核中是颠倒的。读函数用于输出，而写函数用于输入。这样做的原因是，读和写指的是用户的观点——如果一个进程从内核读取一些东西，那么内核需要输出它，如果一个进程向内核写入一些东西，那么内核接收它作为输入。
这里另一个有趣的地方是module_permission函数。每当一个进程试图对/proc文件做一些事情时，这个函数就会被调用，它可以决定是否允许访问。目前，它仅基于操作和当前使用的uid(在current中可用，指向包含当前运行进程信息的结构的指针)，但它可以基于我们喜欢的任何内容，例如其他进程正在对同一文件执行什么操作，一天中的时间，或者我们接收到的最后一个输入。
使用put_user和get_user的原因是Linux内存是分段的(在Intel架构下，在其他一些处理器下可能有所不同)。这意味着指针本身并不引用内存中的唯一位置，而只是引用内存段中的位置，并且您需要知道能够使用它的内存段。内核和每个进程各有一个内存段。
进程可以访问的唯一内存段是它自己的内存段，所以当编写常规程序作为进程运行时，不需要担心内存段。当您编写内核模块时，通常希望访问内核内存段，这是由系统自动处理的。然而，当内存缓冲区的内容需要在当前运行的进程和内核之间传递时，内核函数接收到一个指向进程段中的内存缓冲区的指针。put_user和get_user宏允许您访问该内存。
procfs.c
/* procfs.c - create a "file" in /proc, which allows
* both input and output. */
/* Copyright (C) 1998-1999 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>
/* 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
// 注意 uaccess.h
#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];
/* We return 0 to indicate end of file, that we have
* no more information. Otherwise, processes will
* continue to read from us in an endless loop. */
if (finished) {
finished = 0;
return 0;
}
/* We use put_user to copy the string from the kernel's
* memory segment to the memory segment of the process
* that called us. get_user, BTW, is
* used for the reverse. */
sprintf(message, "Last input:%s", Message);
for(i=0; i<len && message[i]; i++)
put_user(message[i], buf+i);
/* Notice, we assume here that the size of the message
* is below len, or it will be received cut. In a real
* life situation, if the size of the message is less
* than len then we'd return len and on the second call
* start filling the buffer with the len+1'th byte of
* the message. */
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);
/* In version 2.2 the semantics of get_user changed,
* it not longer returns a character, but expects a
* variable to fill up as its first argument and a
* user segment pointer to fill it from as the its
* second.
*
* The reason for this change is that the version 2.2
* get_user can also read an short or an int. The way
* it knows the type of the variable it should read
* is by using sizeof, and for that it needs the
* variable itself.
*/
#else
Message[i] = get_user(buf+i);
#endif
Message[i] = '\0'; /* we want a standard, zero
* terminated string */
/* We need to return the number of input characters
* used */
return i;
}
/* 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;
}
/* The file is opened - we don't really care about
* that, but it does mean we need to increment the
* module's reference count. */
int module_open(struct inode *inode, struct file *file)
{
MOD_INC_USE_COUNT;
return 0;
}
/* The file is closed - again, interesting only because
* of the reference count. */
#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
{
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0; /* success */
#endif
}
/* 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, /* Somebody opened the file */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush, added here in version 2.2 */
#endif
module_close, /* Somebody closed the file */
/* etc. etc. etc. (they are all given in
* /usr/include/linux/fs.h). Since we don't put
* anything here, the system will keep the default
* data, which in Unix is zeros (NULLs when taken as
* pointers). */
};
/* Inode operations for our proc file. We need it so
* we'll have some place 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] */
7, /* Length of the file name */
"rw_test", /* 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)
/* In version 2.2, proc_register assign a dynamic
* inode number automatically if it is zero in the
* structure , so there's no more need for
* proc_register_dynamic
*/
return proc_register(&proc_root, &Our_Proc_File);
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
}
/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
proc_unregister(&proc_root, Our_Proc_File.low_ino);
}