linux/lib/strnlen_user.c
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   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 */
  27static 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 */
  95long 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}
 110EXPORT_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 */
 126long 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}
 138EXPORT_SYMBOL(strlen_user);
 139
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