LXR linux/lib/strncpy_from

   1// SPDX-License-Identifier: GPL-2.0
   2#include <linux/compiler.h>
   3#include <linux/export.h>
   4#include <linux/kasan-checks.h>
   5#include <linux/thread_info.h>
   6#include <linux/uaccess.h>
   7#include <linux/kernel.h>
   8#include <linux/errno.h>
   9
  10#include <asm/byteorder.h>
  11#include <asm/word-at-a-time.h>
  12
  13#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  14#define IS_UNALIGNED(src, dst)  0
  15#else
  16#define IS_UNALIGNED(src, dst)  \
  17        (((long) dst | (long) src) & (sizeof(long) - 1))
  18#endif
  19
  20/*
  21 * Do a strncpy, return length of string without final '\0'.
  22 * 'count' is the user-supplied count (return 'count' if we
  23 * hit it), 'max' is the address space maximum (and we return
  24 * -EFAULT if we hit it).
  25 */
  26static inline long do_strncpy_from_user(char *dst, const char __user *src,
  27                                        unsigned long count, unsigned long max)
  28{
  29        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  30        unsigned long res = 0;
  31
  32        /*
  33         * Truncate 'max' to the user-specified limit, so that
  34         * we only have one limit we need to check in the loop
  35         */
  36        if (max > count)
  37                max = count;
  38
  39        if (IS_UNALIGNED(src, dst))
  40                goto byte_at_a_time;
  41
  42        while (max >= sizeof(unsigned long)) {
  43                unsigned long c, data;
  44
  45                /* Fall back to byte-at-a-time if we get a page fault */
  46                unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  47
  48                *(unsigned long *)(dst+res) = c;
  49                if (has_zero(c, &data, &constants)) {
  50                        data = prep_zero_mask(c, data, &constants);
  51                        data = create_zero_mask(data);
  52                        return res + find_zero(data);
  53                }
  54                res += sizeof(unsigned long);
  55                max -= sizeof(unsigned long);
  56        }
  57
  58byte_at_a_time:
  59        while (max) {
  60                char c;
  61
  62                unsafe_get_user(c,src+res, efault);
  63                dst[res] = c;
  64                if (!c)
  65                        return res;
  66                res++;
  67                max--;
  68        }
  69
  70        /*
  71         * Uhhuh. We hit 'max'. But was that the user-specified maximum
  72         * too? If so, that's ok - we got as much as the user asked for.
  73         */
  74        if (res >= count)
  75                return res;
  76
  77        /*
  78         * Nope: we hit the address space limit, and we still had more
  79         * characters the caller would have wanted. That's an EFAULT.
  80         */
  81efault:
  82        return -EFAULT;
  83}
  84
  85/**
  86 * strncpy_from_user: - Copy a NUL terminated string from userspace.
  87 * @dst:   Destination address, in kernel space.  This buffer must be at
  88 *         least @count bytes long.
  89 * @src:   Source address, in user space.
  90 * @count: Maximum number of bytes to copy, including the trailing NUL.
  91 *
  92 * Copies a NUL-terminated string from userspace to kernel space.
  93 *
  94 * On success, returns the length of the string (not including the trailing
  95 * NUL).
  96 *
  97 * If access to userspace fails, returns -EFAULT (some data may have been
  98 * copied).
  99 *
 100 * If @count is smaller than the length of the string, copies @count bytes
 101 * and returns @count.
 102 */
 103long strncpy_from_user(char *dst, const char __user *src, 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)src;
 112        if (likely(src_addr < max_addr)) {
 113                unsigned long max = max_addr - src_addr;
 114                long retval;
 115
 116                kasan_check_write(dst, count);
 117                check_object_size(dst, count, false);
 118                if (user_access_begin(src, max)) {
 119                        retval = do_strncpy_from_user(dst, src, count, max);
 120                        user_access_end();
 121                        return retval;
 122                }
 123        }
 124        return -EFAULT;
 125}
 126EXPORT_SYMBOL(strncpy_from_user);
 127