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#include <linux/mm.h>
   6#include <linux/bitops.h>
   7
   8#include <asm/word-at-a-time.h>
   9
  10/*
  11 * Do a strnlen, return length of string *with* final '\0'.
  12 * 'count' is the user-supplied count, while 'max' is the
  13 * address space maximum.
  14 *
  15 * Return 0 for exceptions (which includes hitting the address
  16 * space maximum), or 'count+1' if hitting the user-supplied
  17 * maximum count.
  18 *
  19 * NOTE! We can sometimes overshoot the user-supplied maximum
  20 * if it fits in a aligned 'long'. The caller needs to check
  21 * the return value against "> max".
  22 */
  23static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
  24{
  25        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  26        unsigned long align, res = 0;
  27        unsigned long c;
  28
  29        /*
  30         * Do everything aligned. But that means that we
  31         * need to also expand the maximum..
  32         */
  33        align = (sizeof(unsigned long) - 1) & (unsigned long)src;
  34        src -= align;
  35        max += align;
  36
  37        unsafe_get_user(c, (unsigned long __user *)src, efault);
  38        c |= aligned_byte_mask(align);
  39
  40        for (;;) {
  41                unsigned long data;
  42                if (has_zero(c, &data, &constants)) {
  43                        data = prep_zero_mask(c, data, &constants);
  44                        data = create_zero_mask(data);
  45                        return res + find_zero(data) + 1 - align;
  46                }
  47                res += sizeof(unsigned long);
  48                /* We already handled 'unsigned long' bytes. Did we do it all ? */
  49                if (unlikely(max <= sizeof(unsigned long)))
  50                        break;
  51                max -= sizeof(unsigned long);
  52                unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
  53        }
  54        res -= align;
  55
  56        /*
  57         * Uhhuh. We hit 'max'. But was that the user-specified maximum
  58         * too? If so, return the marker for "too long".
  59         */
  60        if (res >= count)
  61                return count+1;
  62
  63        /*
  64         * Nope: we hit the address space limit, and we still had more
  65         * characters the caller would have wanted. That's 0.
  66         */
  67efault:
  68        return 0;
  69}
  70
  71/**
  72 * strnlen_user: - Get the size of a user string INCLUDING final NUL.
  73 * @str: The string to measure.
  74 * @count: Maximum count (including NUL character)
  75 *
  76 * Context: User context only. This function may sleep if pagefaults are
  77 *          enabled.
  78 *
  79 * Get the size of a NUL-terminated string in user space.
  80 *
  81 * Returns the size of the string INCLUDING the terminating NUL.
  82 * If the string is too long, returns a number larger than @count. User
  83 * has to check the return value against "> count".
  84 * On exception (or invalid count), returns 0.
  85 *
  86 * NOTE! You should basically never use this function. There is
  87 * almost never any valid case for using the length of a user space
  88 * string, since the string can be changed at any time by other
  89 * threads. Use "strncpy_from_user()" instead to get a stable copy
  90 * of the string.
  91 */
  92long strnlen_user(const char __user *str, long count)
  93{
  94        unsigned long max_addr, src_addr;
  95
  96        if (unlikely(count <= 0))
  97                return 0;
  98
  99        max_addr = user_addr_max();
 100        src_addr = (unsigned long)untagged_addr(str);
 101        if (likely(src_addr < max_addr)) {
 102                unsigned long max = max_addr - src_addr;
 103                long retval;
 104
 105                /*
 106                 * Truncate 'max' to the user-specified limit, so that
 107                 * we only have one limit we need to check in the loop
 108                 */
 109                if (max > count)
 110                        max = count;
 111
 112                if (user_read_access_begin(str, max)) {
 113                        retval = do_strnlen_user(str, count, max);
 114                        user_read_access_end();
 115                        return retval;
 116                }
 117        }
 118        return 0;
 119}
 120EXPORT_SYMBOL(strnlen_user);
 121