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(__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                 if (unlikely(max < sizeof(unsigned long)))
  61                         break;
  62                 max -= sizeof(unsigned long);
  63                 if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
  64                         return 0;
  65         }
  66         res -= align;
  67
  68         /*
  69          * Uhhuh. We hit 'max'. But was that the user-specified maximum
  70          * too? If so, return the marker for "too long".
  71          */
  72         if (res >= count)
  73                 return count+1;
  74
  75         /*
  76          * Nope: we hit the address space limit, and we still had more
  77          * characters the caller would have wanted. That's 0.
  78          */
  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.
  88  *
  89  * Get the size of a NUL-terminated string in user space.
  90  *
  91  * Returns the size of the string INCLUDING the terminating NUL.
  92  * If the string is too long, returns 'count+1'.
  93  * On exception (or invalid count), returns 0.
  94  */
  95 long strnlen_user(const char __user *str, long count)
  96 {
  97         unsigned long max_addr, src_addr;
  98
  99         if (unlikely(count <= 0))
 100                 return 0;
 101
 102         max_addr = user_addr_max();
 103         src_addr = (unsigned long)str;
 104         if (likely(src_addr < max_addr)) {
 105                 unsigned long max = max_addr - src_addr;
 106                 return do_strnlen_user(str, count, max);
 107         }
 108         return 0;
 109 }
 110 EXPORT_SYMBOL(strnlen_user);
 111
 112 /**
 113  * strlen_user: - Get the size of a user string INCLUDING final NUL.
 114  * @str: The string to measure.
 115  *
 116  * Context: User context only.  This function may sleep.
 117  *
 118  * Get the size of a NUL-terminated string in user space.
 119  *
 120  * Returns the size of the string INCLUDING the terminating NUL.
 121  * On exception, returns 0.
 122  *
 123  * If there is a limit on the length of a valid string, you may wish to
 124  * consider using strnlen_user() instead.
 125  */
 126 long strlen_user(const char __user *str)
 127 {
 128         unsigned long max_addr, src_addr;
 129
 130         max_addr = user_addr_max();
 131         src_addr = (unsigned long)str;
 132         if (likely(src_addr < max_addr)) {
 133                 unsigned long max = max_addr - src_addr;
 134                 return do_strnlen_user(str, ~0ul, max);
 135         }
 136         return 0;
 137 }
 138 EXPORT_SYMBOL(strlen_user);