LXR linux/lib/strnlen

   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        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         */
  78efault:
  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 */
 103long 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}
 123EXPORT_SYMBOL(strnlen_user);
 124