// SPDX-License-Identifier: GPL-2.0

  #include <linux/compiler.h>
  #include <linux/export.h>

  #include <linux/fault-inject-usercopy.h>

  #include <linux/kasan-checks.h>

  #include <linux/thread_info.h>

  #include <linux/uaccess.h>
  #include <linux/kernel.h>
  #include <linux/errno.h>

  #include <linux/mm.h>

  
  #include <asm/byteorder.h>

  #include <asm/word-at-a-time.h>

  
  #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  #define IS_UNALIGNED(src, dst)	0
  #else
  #define IS_UNALIGNED(src, dst)	\
  	(((long) dst | (long) src) & (sizeof(long) - 1))
  #endif
  
  /*
   * Do a strncpy, return length of string without final '\0'.
   * 'count' is the user-supplied count (return 'count' if we
   * hit it), 'max' is the address space maximum (and we return
   * -EFAULT if we hit it).
   */

  static inline long do_strncpy_from_user(char *dst, const char __user *src,
  					unsigned long count, unsigned long max)

  {

  	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;

  	unsigned long res = 0;

  	if (IS_UNALIGNED(src, dst))
  		goto byte_at_a_time;
  
  	while (max >= sizeof(unsigned long)) {

  		unsigned long c, data, mask;

  
  		/* Fall back to byte-at-a-time if we get a page fault */

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

  		/*
  		 * Note that we mask out the bytes following the NUL. This is
  		 * important to do because string oblivious code may read past
  		 * the NUL. For those routines, we don't want to give them
  		 * potentially random bytes after the NUL in `src`.
  		 *
  		 * One example of such code is BPF map keys. BPF treats map keys
  		 * as an opaque set of bytes. Without the post-NUL mask, any BPF
  		 * maps keyed by strings returned from strncpy_from_user() may
  		 * have multiple entries for semantically identical strings.
  		 */

  		if (has_zero(c, &data, &constants)) {
  			data = prep_zero_mask(c, data, &constants);
  			data = create_zero_mask(data);

  			mask = zero_bytemask(data);
  			*(unsigned long *)(dst+res) = c & mask;

  			return res + find_zero(data);

  		}

  
  		*(unsigned long *)(dst+res) = c;

  		res += sizeof(unsigned long);
  		max -= sizeof(unsigned long);
  	}
  
  byte_at_a_time:
  	while (max) {
  		char c;

  		unsafe_get_user(c,src+res, efault);

  		dst[res] = c;
  		if (!c)
  			return res;
  		res++;
  		max--;
  	}
  
  	/*
  	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
  	 * too? If so, that's ok - we got as much as the user asked for.
  	 */
  	if (res >= count)
  		return res;
  
  	/*
  	 * Nope: we hit the address space limit, and we still had more
  	 * characters the caller would have wanted. That's an EFAULT.
  	 */

  efault:

  	return -EFAULT;
  }
  
  /**
   * strncpy_from_user: - Copy a NUL terminated string from userspace.
   * @dst:   Destination address, in kernel space.  This buffer must be at
   *         least @count bytes long.
   * @src:   Source address, in user space.
   * @count: Maximum number of bytes to copy, including the trailing NUL.
   *
   * Copies a NUL-terminated string from userspace to kernel space.
   *
   * On success, returns the length of the string (not including the trailing
   * NUL).
   *
   * If access to userspace fails, returns -EFAULT (some data may have been
   * copied).
   *
   * If @count is smaller than the length of the string, copies @count bytes
   * and returns @count.
   */
  long strncpy_from_user(char *dst, const char __user *src, long count)
  {
  	unsigned long max_addr, src_addr;

  	might_fault();

  	if (should_fail_usercopy())
  		return -EFAULT;

  	if (unlikely(count <= 0))
  		return 0;
  
  	max_addr = user_addr_max();

  	src_addr = (unsigned long)untagged_addr(src);

  	if (likely(src_addr < max_addr)) {
  		unsigned long max = max_addr - src_addr;

  		long retval;

  		/*
  		 * 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;

  		kasan_check_write(dst, count);

  		check_object_size(dst, count, false);

  		if (user_read_access_begin(src, max)) {

  			retval = do_strncpy_from_user(dst, src, count, max);

  			user_read_access_end();

  			return retval;
  		}

  	}
  	return -EFAULT;
  }
  EXPORT_SYMBOL(strncpy_from_user);