#include <linux/kernel.h>
  #include <linux/export.h>
  #include <linux/uaccess.h>
  
  #include <asm/word-at-a-time.h>
  
  /* Set bits in the first 'n' bytes when loaded from memory */
  #ifdef __LITTLE_ENDIAN
  #  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
  #else

  #  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))

  #endif
  
  /*
   * Do a strnlen, return length of string *with* final '\0'.
   * 'count' is the user-supplied count, while 'max' is the
   * address space maximum.
   *
   * Return 0 for exceptions (which includes hitting the address
   * space maximum), or 'count+1' if hitting the user-supplied
   * maximum count.
   *
   * NOTE! We can sometimes overshoot the user-supplied maximum
   * if it fits in a aligned 'long'. The caller needs to check
   * the return value against "> max".
   */
  static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
  {
  	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  	long align, res = 0;
  	unsigned long c;
  
  	/*
  	 * Truncate 'max' to the user-specified limit, so that
  	 * we only have one limit we need to check in the loop
  	 */
  	if (max > count)
  		max = count;
  
  	/*
  	 * Do everything aligned. But that means that we
  	 * need to also expand the maximum..
  	 */
  	align = (sizeof(long) - 1) & (unsigned long)src;
  	src -= align;
  	max += align;

  	unsafe_get_user(c, (unsigned long __user *)src, efault);

  	c |= aligned_byte_mask(align);
  
  	for (;;) {
  		unsigned long data;
  		if (has_zero(c, &data, &constants)) {
  			data = prep_zero_mask(c, data, &constants);
  			data = create_zero_mask(data);
  			return res + find_zero(data) + 1 - align;
  		}
  		res += sizeof(unsigned long);

  		/* We already handled 'unsigned long' bytes. Did we do it all ? */
  		if (unlikely(max <= sizeof(unsigned long)))

  			break;
  		max -= sizeof(unsigned long);

  		unsafe_get_user(c, (unsigned long __user *)(src+res), efault);

  	}
  	res -= align;
  
  	/*
  	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
  	 * too? If so, return the marker for "too long".
  	 */
  	if (res >= count)
  		return count+1;
  
  	/*
  	 * Nope: we hit the address space limit, and we still had more
  	 * characters the caller would have wanted. That's 0.
  	 */

  efault:

  	return 0;
  }
  
  /**
   * strnlen_user: - Get the size of a user string INCLUDING final NUL.
   * @str: The string to measure.
   * @count: Maximum count (including NUL character)
   *

   * Context: User context only. This function may sleep if pagefaults are
   *          enabled.

   *
   * Get the size of a NUL-terminated string in user space.
   *
   * Returns the size of the string INCLUDING the terminating NUL.

   * If the string is too long, returns a number larger than @count. User
   * has to check the return value against "> count".

   * On exception (or invalid count), returns 0.

   *
   * NOTE! You should basically never use this function. There is
   * almost never any valid case for using the length of a user space
   * string, since the string can be changed at any time by other
   * threads. Use "strncpy_from_user()" instead to get a stable copy
   * of the string.

   */
  long strnlen_user(const char __user *str, long count)
  {
  	unsigned long max_addr, src_addr;
  
  	if (unlikely(count <= 0))
  		return 0;
  
  	max_addr = user_addr_max();
  	src_addr = (unsigned long)str;
  	if (likely(src_addr < max_addr)) {
  		unsigned long max = max_addr - src_addr;

  		long retval;
  
  		user_access_begin();
  		retval = do_strnlen_user(str, count, max);
  		user_access_end();
  		return retval;

  	}
  	return 0;
  }
  EXPORT_SYMBOL(strnlen_user);
  
  /**
   * strlen_user: - Get the size of a user string INCLUDING final NUL.
   * @str: The string to measure.
   *

   * Context: User context only. This function may sleep if pagefaults are
   *          enabled.

   *
   * Get the size of a NUL-terminated string in user space.
   *
   * Returns the size of the string INCLUDING the terminating NUL.
   * On exception, returns 0.
   *
   * If there is a limit on the length of a valid string, you may wish to
   * consider using strnlen_user() instead.
   */
  long strlen_user(const char __user *str)
  {
  	unsigned long max_addr, src_addr;
  
  	max_addr = user_addr_max();
  	src_addr = (unsigned long)str;
  	if (likely(src_addr < max_addr)) {
  		unsigned long max = max_addr - src_addr;

  		long retval;
  
  		user_access_begin();
  		retval = do_strnlen_user(str, ~0ul, max);
  		user_access_end();
  		return retval;

  	}
  	return 0;
  }
  EXPORT_SYMBOL(strlen_user);