LXR linux/lib/strnlen

   1// SPDX-License-Identifier: GPL-2.0
   2#include <linux/kernel.h>
   3#include <linux/export.h>
   4#include <linux/uaccess.h>
   5
   6#include <asm/word-at-a-time.h>
   7
   8/* Set bits in the first 'n' bytes when loaded from memory */
   9#ifdef __LITTLE_ENDIAN
  10#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
  11#else
  12#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
  13#endif
  14
  15/*
  16 * Do a strnlen, return length of string *with* final '\0'.
  17 * 'count' is the user-supplied count, while 'max' is the
  18 * address space maximum.
  19 *
  20 * Return 0 for exceptions (which includes hitting the address
  21 * space maximum), or 'count+1' if hitting the user-supplied
  22 * maximum count.
  23 *
  24 * NOTE! We can sometimes overshoot the user-supplied maximum
  25 * if it fits in a aligned 'long'. The caller needs to check
  26 * the return value against "> max".
  27 */
  28static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
  29{
  30        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  31        long align, res = 0;
  32        unsigned long c;
  33
  34        /*
  35         * Truncate 'max' to the user-specified limit, so that
  36         * we only have one limit we need to check in the loop
  37         */
  38        if (max > count)
  39                max = count;
  40
  41        /*
  42         * Do everything aligned. But that means that we
  43         * need to also expand the maximum..
  44         */
  45        align = (sizeof(long) - 1) & (unsigned long)src;
  46        src -= align;
  47        max += align;
  48
  49        unsafe_get_user(c, (unsigned long __user *)src, efault);
  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                unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
  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         */
  79efault:
  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 */
 104long 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}
 124EXPORT_SYMBOL(strnlen_user);
 125