kernel module access user space file:

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is it possible to create a file from kernel space?
is it possible to create store the data in kernel space?

Yes. its possible 





source code:

 #include <linux/module.h>
  #include <linux/init.h>
  #include <linux/fs.h>
  #include <linux/uaccess.h>
  
  static char buf[] ="access from the kernel \n";
  static char buf1[32];
   
  int __init test_init(void)
  {
      struct file *fp;
      mm_segment_t fs; 
      int result;
      loff_t pos;
      printk("test enter\n");
      printk("KERNEL_DS=0x%llx USER_DS=0x%llx get_fs()=0x%llx\n", KERNEL_DS, USER_DS, get_fs());
      fp =filp_open("/home/kishore/kernel_file",O_RDWR | O_CREAT,0644);
      if (IS_ERR(fp)){
          printk("create file error\n");
          return -1; 
      }   
      fs =get_fs();
      set_fs(KERNEL_DS); //  setting the thread addr_limit to KERNEL_DS 
      pos =0; 
      printk("fp=%lx, buf=%lx get_fs()=0x%llx\n", fp, buf, get_fs());
      result=vfs_write(fp,buf, sizeof(buf), &pos);
      pos =0; 
      printk("fp=%lx, buf1=%lx \n", fp, buf1);
      result=vfs_read(fp,buf1, sizeof(buf), &pos);
      printk("Write contet=%s\n",buf1);
      filp_close(fp,NULL); 
      set_fs(fs);
      return 0;
  }
  void __exit test_exit(void)
  {
      printk("test exit\n");
  }
   
  module_init(test_init);
  module_exit(test_exit);
   
  MODULE_LICENSE("GPL"); 

Make file:

  obj-m :=read_userspace.o  
  KERNELDIR ?= /lib/modules/$(shell uname -r)/build

  all default: modules
  install: modules_install
  
  modules modules_install help clean:
      $(MAKE) -C $(KERNELDIR) M=$(shell  pwd ) $@



source code for debugging purpose:

  #include <linux/module.h>
  #include <linux/init.h>
  #include <linux/fs.h>
  #include <linux/uaccess.h>
  
  static char buf[] ="access from the kernel \n";
  static char buf1[32];
   
  int __init test_init(void)
  {
      struct file *fp;
      mm_segment_t fs; 
      int result;
      loff_t pos;
      printk("test enter\n");
      printk("KERNEL_DS=0x%llx USER_DS=0x%llx get_fs()=0x%llx\n", KERNEL_DS, USER_DS, get_fs());
      fp =filp_open("/home/kishore/kernel_file",O_RDWR | O_CREAT,0644);
      if (IS_ERR(fp)){
          printk("create file error\n");
          return -1; 
      }   
      fs =get_fs();
      //set_fs(KERNEL_DS); //  setting the thread addr_limit to KERNEL_DS 
      pos =0; 
      printk("fp=%lx, buf=%lx get_fs()=0x%llx\n", fp, buf, get_fs());
      result=vfs_write(fp,buf, sizeof(buf), &pos);
      printk("error no=%d",result);
      pos =0; 
      printk("fp=%lx, buf1=%lx \n", fp, buf1);
      result=vfs_read(fp,buf1, sizeof(buf), &pos);
      printk("error no=%d",result);
      printk("Write contet=%s\n",buf1);
      filp_close(fp,NULL); 
      //set_fs(fs);
      return 0;
  }
  void __exit test_exit(void)
  {
      printk("test exit\n");
  }
   
  module_init(test_init);
  module_exit(test_exit);
   
  MODULE_LICENSE("GPL"); 



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