linux/lib/strncpy_from_user.c
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   1#include <linux/compiler.h>
   2#include <linux/export.h>
   3#include <linux/kasan-checks.h>
   4#include <linux/thread_info.h>
   5#include <linux/uaccess.h>
   6#include <linux/kernel.h>
   7#include <linux/errno.h>
   8
   9#include <asm/byteorder.h>
  10#include <asm/word-at-a-time.h>
  11
  12#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  13#define IS_UNALIGNED(src, dst)  0
  14#else
  15#define IS_UNALIGNED(src, dst)  \
  16        (((long) dst | (long) src) & (sizeof(long) - 1))
  17#endif
  18
  19/*
  20 * Do a strncpy, return length of string without final '\0'.
  21 * 'count' is the user-supplied count (return 'count' if we
  22 * hit it), 'max' is the address space maximum (and we return
  23 * -EFAULT if we hit it).
  24 */
  25static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
  26{
  27        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  28        long res = 0;
  29
  30        /*
  31         * Truncate 'max' to the user-specified limit, so that
  32         * we only have one limit we need to check in the loop
  33         */
  34        if (max > count)
  35                max = count;
  36
  37        if (IS_UNALIGNED(src, dst))
  38                goto byte_at_a_time;
  39
  40        while (max >= sizeof(unsigned long)) {
  41                unsigned long c, data;
  42
  43                /* Fall back to byte-at-a-time if we get a page fault */
  44                unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  45
  46                *(unsigned long *)(dst+res) = c;
  47                if (has_zero(c, &data, &constants)) {
  48                        data = prep_zero_mask(c, data, &constants);
  49                        data = create_zero_mask(data);
  50                        return res + find_zero(data);
  51                }
  52                res += sizeof(unsigned long);
  53                max -= sizeof(unsigned long);
  54        }
  55
  56byte_at_a_time:
  57        while (max) {
  58                char c;
  59
  60                unsafe_get_user(c,src+res, efault);
  61                dst[res] = c;
  62                if (!c)
  63                        return res;
  64                res++;
  65                max--;
  66        }
  67
  68        /*
  69         * Uhhuh. We hit 'max'. But was that the user-specified maximum
  70         * too? If so, that's ok - we got as much as the user asked for.
  71         */
  72        if (res >= count)
  73                return res;
  74
  75        /*
  76         * Nope: we hit the address space limit, and we still had more
  77         * characters the caller would have wanted. That's an EFAULT.
  78         */
  79efault:
  80        return -EFAULT;
  81}
  82
  83/**
  84 * strncpy_from_user: - Copy a NUL terminated string from userspace.
  85 * @dst:   Destination address, in kernel space.  This buffer must be at
  86 *         least @count bytes long.
  87 * @src:   Source address, in user space.
  88 * @count: Maximum number of bytes to copy, including the trailing NUL.
  89 *
  90 * Copies a NUL-terminated string from userspace to kernel space.
  91 *
  92 * On success, returns the length of the string (not including the trailing
  93 * NUL).
  94 *
  95 * If access to userspace fails, returns -EFAULT (some data may have been
  96 * copied).
  97 *
  98 * If @count is smaller than the length of the string, copies @count bytes
  99 * and returns @count.
 100 */
 101long strncpy_from_user(char *dst, const char __user *src, long count)
 102{
 103        unsigned long max_addr, src_addr;
 104
 105        if (unlikely(count <= 0))
 106                return 0;
 107
 108        max_addr = user_addr_max();
 109        src_addr = (unsigned long)src;
 110        if (likely(src_addr < max_addr)) {
 111                unsigned long max = max_addr - src_addr;
 112                long retval;
 113
 114                kasan_check_write(dst, count);
 115                check_object_size(dst, count, false);
 116                user_access_begin();
 117                retval = do_strncpy_from_user(dst, src, count, max);
 118                user_access_end();
 119                return retval;
 120        }
 121        return -EFAULT;
 122}
 123EXPORT_SYMBOL(strncpy_from_user);
 124