fopen.cのfopen/freads/fcloseをEXPORT_SYMBOLして、他のLKMからファイルの文字列単位での読み込みを実装したサンプルです。

fopen.c

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/syscalls.h>

struct file     *fopen(char *file, int mode);
int             freads(char *buff, int size, struct file *fp);
int             fclose(struct file *fp);

struct file *fopen(char *fname, int mode)
{
       struct  file    *fp;

       fp = filp_open(fname,  mode, 0);
        if(IS_ERR(fp)) {
               return 0;
       }
       return fp;
}

int     fclose(struct file *fp)
{
       return filp_close(fp, NULL);
}

int     freads(char *buf, int size, struct file *fp)
{
       int     index = 0;
       char    a;
       int     err = 0;

       while (kernel_read(fp, fp->f_pos++, &a, 1)) {
               if (a == 0x0a || index >= size - 1) {
                       buf[index] = 0;
                       err = index;
                       break;
               }
               else {
                       buf[index++] = a;
               }
       }
       return err;
}

static int __init fopen_init( void )
{
       return 0;
}

static void __exit fopen_exit( void )
{
}
module_init(fopen_init);
module_exit(fopen_exit);

EXPORT_SYMBOL(fopen);
EXPORT_SYMBOL(freads);
EXPORT_SYMBOL(fclose);

MODULE_LICENSE( "GPL2" );

fread.c

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>

struct file *fopen(char *file, int mode);
int     freads(char *buff, int size, struct file *fp);
int     fclose(struct file *fp);

static char* file = "";
module_param(file, charp, S_IRUGO);

static int __init fread_init( void )
{
       char    buff[100];
       struct file *fp;

       fp = fopen(fname, 0);
       if (fp) {
               while (freads(buff, 100, fp)) {
                       printk("%s\n", buff);
               }
		fclose(fp);
       }
       else {
               printk("open err\n");
       }
       return -1;
}
module_init(fread_init);

MODULE_LICENSE( "GPL2" );

結果

[root@localhost north]# cat a.txt
a
bb
ccc
dddd
eeeee
ffffff
ggggggg

[root@localhost north]# insmod fopen.ko
[root@localhost north]#
[root@localhost north]# insmod fread.ko file=a.txt
insmod: error inserting 'fread.ko': -1 Operation not permitted
[root@localhost north]# dmesg
 :
[ 1274.667410] a
[ 1274.667416] bb
[ 1274.667420] ccc
[ 1274.667423] dddd
[ 1274.667426] eeeee
[ 1274.667430] ffffff
[ 1274.667435] ggggggg

[root@localhost north]# rmmod fopen.ko
[root@localhost north]# insmod fread.ko file=a.txt
insmod: error inserting 'fread.ko': -1 Unknown symbol in module
[root@localhost north]#
[root@localhost north]# dmesg
 :
[ 4243.067933] fread: Unknown symbol freads (err 0)
[ 4243.068029] fread: Unknown symbol fopen (err 0)
[ 4243.068126] fread: Unknown symbol fclose (err 0)