Prevent Null Pointer Dereference in kernel space code

Prevent Null Pointer Dereference in kernel space code
before passing a kernel pointer to user space syscall / function related make sure use:
set_fs(KERNEL_DS)

the old_fs will be used to save the old value

old_fs=get_fs();
    set_fs(KERNEL_DS);

/**some code to pass a kernel space pointer to user space syscall , etc**/

then we gotta bring it back:

set_fs(old_fs);