Example 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 */
#include <linux/proc_fs.h>	/* Necessary because we use proc fs */
#include <asm/uaccess.h>	/* for get_user and put_user */

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

#define PROC_ENTRY_FILENAME "rw_test"

static ssize_t module_output(struct file *filp,	/* see include/linux/fs.h   */
			     char *buffer,	/* buffer to fill with data */
			     size_t length,	/* length of the buffer     */
			     loff_t * offset)
{
	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 < length && message[i]; i++)
		put_user(message[i], buffer + 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" */
}

static ssize_t
module_input(struct file *filp, const char *buff, size_t len, loff_t * off)
{
	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 < len; i++)
		get_user(Message[i], buff + i);

	Message[i] = '\0';	/* we want a standard, zero terminated string */
	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, struct nameidata *foo)
{
	/* 
	 * 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)
{
	try_module_get(THIS_MODULE);
	return 0;
}

/* 
 * The file is closed - again, interesting only because
 * of the reference count. 
 */
int module_close(struct inode *inode, struct file *file)
{
	module_put(THIS_MODULE);
	return 0;		/* success */
}

static struct file_operations File_Ops_4_Our_Proc_File = {
	.read = module_output,
	.write = module_input,
	.open = module_open,
	.release = module_close,
};

/* 
 * 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 = {
	.permission = module_permission,	/* check for permissions */
};

/* 
 * Module initialization and cleanup 
 */
int init_module()
{
	int rv = 0;
	Our_Proc_File = create_proc_entry(PROC_ENTRY_FILENAME, 0644, NULL);
	Our_Proc_File->owner = THIS_MODULE;
	Our_Proc_File->proc_iops = &Inode_Ops_4_Our_Proc_File;
	Our_Proc_File->proc_fops = &File_Ops_4_Our_Proc_File;
	Our_Proc_File->mode = S_IFREG | S_IRUGO | S_IWUSR;
	Our_Proc_File->uid = 0;
	Our_Proc_File->gid = 0;
	Our_Proc_File->size = 80;

	if (Our_Proc_File == NULL) {
		rv = -ENOMEM;
		remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
		printk(KERN_INFO "Error: Could not initialize /proc/test\n");
	}

	return rv;
}

void cleanup_module()
{
	remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
}