Why when I use copy_from_user, some vague character added to original buffer?

Question

I've created WRITE_IOCTL in kernel module and I call it in user mode:

ioctl(fd, WRITE_IOCTL, "Hello, Kernel!");

In kernel mode I have:

static int device_ioctl(struct file *filp,
    unsigned int cmd, unsigned long args) {
  char buff[14];

  switch (cmd) {
  case WRITE_IOCTL:
    copy_from_user( buff,(char *)args, 14);
    printk("This message received from User Space: %s\n", buff);
    break;
  }
  return 0;
}

When I run this ioctl, I have some thing like theses in /var/log/kern.log :

This message received from User Space: Hello, Kernel!vE�
This message received from User Space: Hello, Kernel!M�
This message received from User Space: Hello, Kernel!M�

How can I solve this problem??

The Mask · Accepted Answer · 2014-05-23T19:56:14.420

5

Probably copy_from_user() isn't putting the null-byte-terminattor because args is greater-or-equal than your n and printk() is expecting a null-terminatted one, so you're accessing garbage values. For solve that, initialize yourself buf to zeros:

  char buff[14 + 1] = {0}; // +1 for make room to 0-byte-terminattor.

It will fill all bytes of buf with zeros.

EDIT:

As @caf mentioned in comments, you need to left some space to null-byte-terminattor. So, instead of give exactly the buffer size to function, pass it n-1 so the function will loop untl n and then put the null-byte.

edited May 23 '14 at 19:56

answered Nov 02 '13 at 18:04

The Mask

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2

@Khajavi: Note that you also need to expand the buffer to size `[15]`, to make room for the null terminator. – caf Nov 03 '13 at 03:46
@caf: Well, I fixed it. – The Mask May 23 '14 at 19:56

Why when I use copy_from_user, some vague character added to original buffer?

1 Answers1