lib/strnlen_user.c

   1 #include <linux/kernel.h>
   2 #include <linux/export.h>
   3 #include <linux/uaccess.h>
   4
   5 #include <asm/word-at-a-time.h>
   6
   7 /* Set bits in the first 'n' bytes when loaded from memory */
   8 #ifdef __LITTLE_ENDIAN
   9 #  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
  10 #else
  11 #  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
  12 #endif
  13
  14 /*
  15  * Do a strnlen, return length of string *with* final '\0'.
  16  * 'count' is the user-supplied count, while 'max' is the
  17  * address space maximum.
  18  *
  19  * Return 0 for exceptions (which includes hitting the address
  20  * space maximum), or 'count+1' if hitting the user-supplied
  21  * maximum count.
  22  *
  23  * NOTE! We can sometimes overshoot the user-supplied maximum
  24  * if it fits in a aligned 'long'. The caller needs to check
  25  * the return value against "> max".
  26  */
  27 static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
  28 {
  29         const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  30         long align, res = 0;
  31         unsigned long c;
  32
  33         /*
  34          * Truncate 'max' to the user-specified limit, so that
  35          * we only have one limit we need to check in the loop
  36          */
  37         if (max > count)
  38                 max = count;
  39
  40         /*
  41          * Do everything aligned. But that means that we
  42          * need to also expand the maximum..
  43          */
  44         align = (sizeof(long) - 1) & (unsigned long)src;
  45         src -= align;
  46         max += align;
  47
  48         unsafe_get_user(c, (unsigned long __user *)src, efault);
  49         c |= aligned_byte_mask(align);
  50
  51         for (;;) {
  52                 unsigned long data;
  53                 if (has_zero(c, &data, &constants)) {
  54                         data = prep_zero_mask(c, data, &constants);
  55                         data = create_zero_mask(data);
  56                         return res + find_zero(data) + 1 - align;
  57                 }
  58                 res += sizeof(unsigned long);
  59                 /* We already handled 'unsigned long' bytes. Did we do it all ? */
  60                 if (unlikely(max <= sizeof(unsigned long)))
  61                         break;
  62                 max -= sizeof(unsigned long);
  63                 unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
  64         }
  65         res -= align;
  66
  67         /*
  68          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  69          * too? If so, return the marker for "too long".
  70          */
  71         if (res >= count)
  72                 return count+1;
  73
  74         /*
  75          * Nope: we hit the address space limit, and we still had more
  76          * characters the caller would have wanted. That's 0.
  77          */
  78 efault:
  79         return 0;
  80 }
  81
  82 /**
  83  * strnlen_user: - Get the size of a user string INCLUDING final NUL.
  84  * @str: The string to measure.
  85  * @count: Maximum count (including NUL character)
  86  *
  87  * Context: User context only. This function may sleep if pagefaults are
  88  *          enabled.
  89  *
  90  * Get the size of a NUL-terminated string in user space.
  91  *
  92  * Returns the size of the string INCLUDING the terminating NUL.
  93  * If the string is too long, returns a number larger than @count. User
  94  * has to check the return value against "> count".
  95  * On exception (or invalid count), returns 0.
  96  *
  97  * NOTE! You should basically never use this function. There is
  98  * almost never any valid case for using the length of a user space
  99  * string, since the string can be changed at any time by other
 100  * threads. Use "strncpy_from_user()" instead to get a stable copy
 101  * of the string.
 102  */
 103 long strnlen_user(const char __user *str, long count)
 104 {
 105         unsigned long max_addr, src_addr;
 106
 107         if (unlikely(count <= 0))
 108                 return 0;
 109
 110         max_addr = user_addr_max();
 111         src_addr = (unsigned long)str;
 112         if (likely(src_addr < max_addr)) {
 113                 unsigned long max = max_addr - src_addr;
 114                 long retval;
 115
 116                 user_access_begin();
 117                 retval = do_strnlen_user(str, count, max);
 118                 user_access_end();
 119                 return retval;
 120         }
 121         return 0;
 122 }
 123 EXPORT_SYMBOL(strnlen_user);