到目前为止，我们有两种方法可以从内核模块生成输出：我们可以注册一个设备驱动程序并使用 mknod 创建一个设备文件，或者我们可以创建一个/proc文件。这允许内核模块告诉我们任何它想说的内容。唯一的问题是我们无法与它对话。我们将向内核模块发送输入的第一种方式是通过写回到/proc文件。
因为 proc 文件系统主要是为了允许内核向进程报告其状态而编写的，所以没有针对输入的特殊规定。因此，struct proc_dir_entry不包含指向输入函数的指针，就像它包含指向输出函数的指针一样。相反，要写入一个/proc文件，我们需要使用标准的 文件系统机制。
在 Linux 中，有一个标准的文件系统注册机制。由于每个文件系统都必须有自己的函数来处理 inode 和文件操作[1]，因此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，它包含一个指向struct file_operations。在 /proc 中，每当我们注册一个新文件时，我们被允许指定哪个struct inode_operations将被用于访问它。这就是我们使用的机制，一个struct inode_operations它包含一个指向一个struct file_operations它包含指向我们的module_input和module_output函数。
重要的是要注意，在内核中，读和写的标准角色是相反的。读取函数用于输出，而写入函数用于输入。原因是读取和写入指的是用户的角度 --- 如果一个进程从内核读取内容，那么内核需要输出它；如果一个进程向内核写入内容，那么内核会将其作为输入接收。
另一个有趣的方面是module_permission函数。每当一个进程尝试对/proc文件执行某些操作时，都会调用此函数，它可以决定是否允许访问。目前，它仅基于操作和当前用户的 uid（在current中可用，它是一个指向包含当前正在运行的进程信息的结构的指针），但它可以基于我们想要的任何内容，例如其他进程对同一文件执行的操作、一天中的时间或我们收到的最后一个输入。
原因是put_user和get_user是因为 Linux 内存（在 Intel 架构下，在其他处理器下可能有所不同）是分段的。这意味着指针本身并不引用内存中的唯一位置，而只是内存段中的一个位置，您需要知道它是哪个内存段才能使用它。内核有一个内存段，每个进程也都有一个内存段。
进程唯一可访问的内存段是它自己的，因此在编写作为进程运行的常规程序时，无需担心段的问题。当您编写内核模块时，通常您希望访问内核内存段，这由系统自动处理。但是，当需要在当前运行的进程和内核之间传递内存缓冲区的内容时，内核函数接收一个指向进程段中内存缓冲区的指针。这些put_user和get_user宏允许您访问该内存。
示例 6-1. procfs.c
/* procfs.c - create a "file" in /proc, which allows both input and output.
*/
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* 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
#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);
} |
| [1] | 两者之间的区别在于文件操作处理文件本身，而 inode 操作处理引用文件的方式，例如创建指向它的链接。 |