lib/strncpy_from_user.c

   1 #include <linux/compiler.h>
   2 #include <linux/export.h>
   3 #include <linux/uaccess.h>
   4 #include <linux/kernel.h>
   5 #include <linux/errno.h>
   6
   7 #include <asm/byteorder.h>
   8 #include <asm/word-at-a-time.h>
   9
  10 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  11 #define IS_UNALIGNED(src, dst)  0
  12 #else
  13 #define IS_UNALIGNED(src, dst)  \
  14         (((long) dst | (long) src) & (sizeof(long) - 1))
  15 #endif
  16
  17 /*
  18  * Do a strncpy, return length of string without final '\0'.
  19  * 'count' is the user-supplied count (return 'count' if we
  20  * hit it), 'max' is the address space maximum (and we return
  21  * -EFAULT if we hit it).
  22  */
  23 static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
  24 {
  25         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  26         long res = 0;
  27
  28         /*
  29          * Truncate 'max' to the user-specified limit, so that
  30          * we only have one limit we need to check in the loop
  31          */
  32         if (max > count)
  33                 max = count;
  34
  35         if (IS_UNALIGNED(src, dst))
  36                 goto byte_at_a_time;
  37
  38         while (max >= sizeof(unsigned long)) {
  39                 unsigned long c, data;
  40
  41                 /* Fall back to byte-at-a-time if we get a page fault */
  42                 if (unlikely(unsafe_get_user(c,(unsigned long __user *)(src+res))))
  43                         break;
  44                 *(unsigned long *)(dst+res) = c;
  45                 if (has_zero(c, &data, &constants)) {
  46                         data = prep_zero_mask(c, data, &constants);
  47                         data = create_zero_mask(data);
  48                         return res + find_zero(data);
  49                 }
  50                 res += sizeof(unsigned long);
  51                 max -= sizeof(unsigned long);
  52         }
  53
  54 byte_at_a_time:
  55         while (max) {
  56                 char c;
  57
  58                 if (unlikely(unsafe_get_user(c,src+res)))
  59                         return -EFAULT;
  60                 dst[res] = c;
  61                 if (!c)
  62                         return res;
  63                 res++;
  64                 max--;
  65         }
  66
  67         /*
  68          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  69          * too? If so, that's ok - we got as much as the user asked for.
  70          */
  71         if (res >= count)
  72                 return res;
  73
  74         /*
  75          * Nope: we hit the address space limit, and we still had more
  76          * characters the caller would have wanted. That's an EFAULT.
  77          */
  78         return -EFAULT;
  79 }
  80
  81 /**
  82  * strncpy_from_user: - Copy a NUL terminated string from userspace.
  83  * @dst:   Destination address, in kernel space.  This buffer must be at
  84  *         least @count bytes long.
  85  * @src:   Source address, in user space.
  86  * @count: Maximum number of bytes to copy, including the trailing NUL.
  87  *
  88  * Copies a NUL-terminated string from userspace to kernel space.
  89  *
  90  * On success, returns the length of the string (not including the trailing
  91  * NUL).
  92  *
  93  * If access to userspace fails, returns -EFAULT (some data may have been
  94  * copied).
  95  *
  96  * If @count is smaller than the length of the string, copies @count bytes
  97  * and returns @count.
  98  */
  99 long strncpy_from_user(char *dst, const char __user *src, long count)
 100 {
 101         unsigned long max_addr, src_addr;
 102
 103         if (unlikely(count <= 0))
 104                 return 0;
 105
 106         max_addr = user_addr_max();
 107         src_addr = (unsigned long)src;
 108         if (likely(src_addr < max_addr)) {
 109                 unsigned long max = max_addr - src_addr;
 110                 long retval;
 111
 112                 user_access_begin();
 113                 retval = do_strncpy_from_user(dst, src, count, max);
 114                 user_access_end();
 115                 return retval;
 116         }
 117         return -EFAULT;
 118 }
 119 EXPORT_SYMBOL(strncpy_from_user);