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         if (unlikely(unsafe_get_user(c,(unsigned long __user *)src)))
  49                 return 0;
  50         c |= aligned_byte_mask(align);
  51
  52         for (;;) {
  53                 unsigned long data;
  54                 if (has_zero(c, &data, &constants)) {
  55                         data = prep_zero_mask(c, data, &constants);
  56                         data = create_zero_mask(data);
  57                         return res + find_zero(data) + 1 - align;
  58                 }
  59                 res += sizeof(unsigned long);
  60                 /* We already handled 'unsigned long' bytes. Did we do it all ? */
  61                 if (unlikely(max <= sizeof(unsigned long)))
  62                         break;
  63                 max -= sizeof(unsigned long);
  64                 if (unlikely(unsafe_get_user(c,(unsigned long __user *)(src+res))))
  65                         return 0;
  66         }
  67         res -= align;
  68
  69         /*
  70          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  71          * too? If so, return the marker for "too long".
  72          */
  73         if (res >= count)
  74                 return count+1;
  75
  76         /*
  77          * Nope: we hit the address space limit, and we still had more
  78          * characters the caller would have wanted. That's 0.
  79          */
  80         return 0;
  81 }
  82
  83 /**
  84  * strnlen_user: - Get the size of a user string INCLUDING final NUL.
  85  * @str: The string to measure.
  86  * @count: Maximum count (including NUL character)
  87  *
  88  * Context: User context only. This function may sleep if pagefaults are
  89  *          enabled.
  90  *
  91  * Get the size of a NUL-terminated string in user space.
  92  *
  93  * Returns the size of the string INCLUDING the terminating NUL.
  94  * If the string is too long, returns a number larger than @count. User
  95  * has to check the return value against "> count".
  96  * On exception (or invalid count), returns 0.
  97  *
  98  * NOTE! You should basically never use this function. There is
  99  * almost never any valid case for using the length of a user space
 100  * string, since the string can be changed at any time by other
 101  * threads. Use "strncpy_from_user()" instead to get a stable copy
 102  * of the string.
 103  */
 104 long strnlen_user(const char __user *str, long count)
 105 {
 106         unsigned long max_addr, src_addr;
 107
 108         if (unlikely(count <= 0))
 109                 return 0;
 110
 111         max_addr = user_addr_max();
 112         src_addr = (unsigned long)str;
 113         if (likely(src_addr < max_addr)) {
 114                 unsigned long max = max_addr - src_addr;
 115                 long retval;
 116
 117                 user_access_begin();
 118                 retval = do_strnlen_user(str, count, max);
 119                 user_access_end();
 120                 return retval;
 121         }
 122         return 0;
 123 }
 124 EXPORT_SYMBOL(strnlen_user);
 125
 126 /**
 127  * strlen_user: - Get the size of a user string INCLUDING final NUL.
 128  * @str: The string to measure.
 129  *
 130  * Context: User context only. This function may sleep if pagefaults are
 131  *          enabled.
 132  *
 133  * Get the size of a NUL-terminated string in user space.
 134  *
 135  * Returns the size of the string INCLUDING the terminating NUL.
 136  * On exception, returns 0.
 137  *
 138  * If there is a limit on the length of a valid string, you may wish to
 139  * consider using strnlen_user() instead.
 140  */
 141 long strlen_user(const char __user *str)
 142 {
 143         unsigned long max_addr, src_addr;
 144
 145         max_addr = user_addr_max();
 146         src_addr = (unsigned long)str;
 147         if (likely(src_addr < max_addr)) {
 148                 unsigned long max = max_addr - src_addr;
 149                 long retval;
 150
 151                 user_access_begin();
 152                 retval = do_strnlen_user(str, ~0ul, max);
 153                 user_access_end();
 154                 return retval;
 155         }
 156         return 0;
 157 }
 158 EXPORT_SYMBOL(strlen_user);