lib/strncpy_from_user.c

   1 #include <linux/module.h>
   2 #include <linux/uaccess.h>
   3 #include <linux/kernel.h>
   4 #include <linux/errno.h>
   5
   6 #include <asm/byteorder.h>
   7 #include <asm/word-at-a-time.h>
   8
   9 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  10 #define IS_UNALIGNED(src, dst)  0
  11 #else
  12 #define IS_UNALIGNED(src, dst)  \
  13         (((long) dst | (long) src) & (sizeof(long) - 1))
  14 #endif
  15
  16 /*
  17  * Do a strncpy, return length of string without final '\0'.
  18  * 'count' is the user-supplied count (return 'count' if we
  19  * hit it), 'max' is the address space maximum (and we return
  20  * -EFAULT if we hit it).
  21  */
  22 static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
  23 {
  24         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  25         long res = 0;
  26
  27         /*
  28          * Truncate 'max' to the user-specified limit, so that
  29          * we only have one limit we need to check in the loop
  30          */
  31         if (max > count)
  32                 max = count;
  33
  34         if (IS_UNALIGNED(src, dst))
  35                 goto byte_at_a_time;
  36
  37         while (max >= sizeof(unsigned long)) {
  38                 unsigned long c, data;
  39
  40                 /* Fall back to byte-at-a-time if we get a page fault */
  41                 if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
  42                         break;
  43                 *(unsigned long *)(dst+res) = c;
  44                 if (has_zero(c, &data, &constants)) {
  45                         data = prep_zero_mask(c, data, &constants);
  46                         data = create_zero_mask(data);
  47                         return res + find_zero(data);
  48                 }
  49                 res += sizeof(unsigned long);
  50                 max -= sizeof(unsigned long);
  51         }
  52
  53 byte_at_a_time:
  54         while (max) {
  55                 char c;
  56
  57                 if (unlikely(__get_user(c,src+res)))
  58                         return -EFAULT;
  59                 dst[res] = c;
  60                 if (!c)
  61                         return res;
  62                 res++;
  63                 max--;
  64         }
  65
  66         /*
  67          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  68          * too? If so, that's ok - we got as much as the user asked for.
  69          */
  70         if (res >= count)
  71                 return res;
  72
  73         /*
  74          * Nope: we hit the address space limit, and we still had more
  75          * characters the caller would have wanted. That's an EFAULT.
  76          */
  77         return -EFAULT;
  78 }
  79
  80 /**
  81  * strncpy_from_user: - Copy a NUL terminated string from userspace.
  82  * @dst:   Destination address, in kernel space.  This buffer must be at
  83  *         least @count bytes long.
  84  * @src:   Source address, in user space.
  85  * @count: Maximum number of bytes to copy, including the trailing NUL.
  86  *
  87  * Copies a NUL-terminated string from userspace to kernel space.
  88  *
  89  * On success, returns the length of the string (not including the trailing
  90  * NUL).
  91  *
  92  * If access to userspace fails, returns -EFAULT (some data may have been
  93  * copied).
  94  *
  95  * If @count is smaller than the length of the string, copies @count bytes
  96  * and returns @count.
  97  */
  98 long strncpy_from_user(char *dst, const char __user *src, long count)
  99 {
 100         unsigned long max_addr, src_addr;
 101
 102         if (unlikely(count <= 0))
 103                 return 0;
 104
 105         max_addr = user_addr_max();
 106         src_addr = (unsigned long)src;
 107         if (likely(src_addr < max_addr)) {
 108                 unsigned long max = max_addr - src_addr;
 109                 return do_strncpy_from_user(dst, src, count, max);
 110         }
 111         return -EFAULT;
 112 }
 113 EXPORT_SYMBOL(strncpy_from_user);