/*
   * Access kernel memory without faulting.
   */

  #include <linux/export.h>

  #include <linux/mm.h>

  #include <linux/uaccess.h>

  
  /**
   * probe_kernel_read(): safely attempt to read from a location
   * @dst: pointer to the buffer that shall take the data
   * @src: address to read from
   * @size: size of the data chunk
   *
   * Safely read from address @src to the buffer at @dst.  If a kernel fault
   * happens, handle that and return -EFAULT.

   *
   * We ensure that the copy_from_user is executed in atomic context so that
   * do_page_fault() doesn't attempt to take mmap_sem.  This makes
   * probe_kernel_read() suitable for use within regions where the caller
   * already holds mmap_sem, or other locks which nest inside mmap_sem.

   */

  long __weak probe_kernel_read(void *dst, const void *src, size_t size)

      __attribute__((alias("__probe_kernel_read")));

  long __probe_kernel_read(void *dst, const void *src, size_t size)

  {
  	long ret;

  	mm_segment_t old_fs = get_fs();

  	set_fs(KERNEL_DS);

  	pagefault_disable();
  	ret = __copy_from_user_inatomic(dst,
  			(__force const void __user *)src, size);
  	pagefault_enable();

  	set_fs(old_fs);

  
  	return ret ? -EFAULT : 0;
  }
  EXPORT_SYMBOL_GPL(probe_kernel_read);
  
  /**
   * probe_kernel_write(): safely attempt to write to a location
   * @dst: address to write to
   * @src: pointer to the data that shall be written
   * @size: size of the data chunk
   *
   * Safely write to address @dst from the buffer at @src.  If a kernel fault
   * happens, handle that and return -EFAULT.
   */

  long __weak probe_kernel_write(void *dst, const void *src, size_t size)

      __attribute__((alias("__probe_kernel_write")));

  long __probe_kernel_write(void *dst, const void *src, size_t size)

  {
  	long ret;

  	mm_segment_t old_fs = get_fs();

  	set_fs(KERNEL_DS);

  	pagefault_disable();
  	ret = __copy_to_user_inatomic((__force void __user *)dst, src, size);
  	pagefault_enable();

  	set_fs(old_fs);

  
  	return ret ? -EFAULT : 0;
  }
  EXPORT_SYMBOL_GPL(probe_kernel_write);

  
  /**
   * strncpy_from_unsafe: - Copy a NUL terminated string from unsafe address.
   * @dst:   Destination address, in kernel space.  This buffer must be at
   *         least @count bytes long.
   * @src:   Unsafe address.
   * @count: Maximum number of bytes to copy, including the trailing NUL.
   *
   * Copies a NUL-terminated string from unsafe address to kernel buffer.
   *
   * On success, returns the length of the string INCLUDING the trailing NUL.
   *
   * If access fails, returns -EFAULT (some data may have been copied
   * and the trailing NUL added).
   *
   * If @count is smaller than the length of the string, copies @count-1 bytes,
   * sets the last byte of @dst buffer to NUL and returns @count.
   */
  long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
  {
  	mm_segment_t old_fs = get_fs();
  	const void *src = unsafe_addr;
  	long ret;
  
  	if (unlikely(count <= 0))
  		return 0;
  
  	set_fs(KERNEL_DS);
  	pagefault_disable();
  
  	do {

  		ret = __get_user(*dst++, (const char __user __force *)src++);

  	} while (dst[-1] && ret == 0 && src - unsafe_addr < count);
  
  	dst[-1] = '\0';
  	pagefault_enable();
  	set_fs(old_fs);

  	return ret ? -EFAULT : src - unsafe_addr;

  }