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, long count, unsigned long max)
  27{
  28        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  29        long res = 0;
  30
  31        /*
  32         * Truncate 'max' to the user-specified limit, so that
  33         * we only have one limit we need to check in the loop
  34         */
  35        if (max > count)
  36                max = count;
  37
  38        if (IS_UNALIGNED(src, dst))
  39                goto byte_at_a_time;
  40
  41        while (max >= sizeof(unsigned long)) {
  42                unsigned long c, data;
  43
  44                /* Fall back to byte-at-a-time if we get a page fault */
  45                unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  46
  47                *(unsigned long *)(dst+res) = c;
  48                if (has_zero(c, &data, &constants)) {
  49                        data = prep_zero_mask(c, data, &constants);
  50                        data = create_zero_mask(data);
  51                        return res + find_zero(data);
  52                }
  53                res += sizeof(unsigned long);
  54                max -= sizeof(unsigned long);
  55        }
  56
  57byte_at_a_time:
  58        while (max) {
  59                char c;
  60
  61                unsafe_get_user(c,src+res, efault);
  62                dst[res] = c;
  63                if (!c)
  64                        return res;
  65                res++;
  66                max--;
  67        }
  68
  69        /*
  70         * Uhhuh. We hit 'max'. But was that the user-specified maximum
  71         * too? If so, that's ok - we got as much as the user asked for.
  72         */
  73        if (res >= count)
  74                return res;
  75
  76        /*
  77         * Nope: we hit the address space limit, and we still had more
  78         * characters the caller would have wanted. That's an EFAULT.
  79         */
  80efault:
  81        return -EFAULT;
  82}
  83
  84/**
  85 * strncpy_from_user: - Copy a NUL terminated string from userspace.
  86 * @dst:   Destination address, in kernel space.  This buffer must be at
  87 *         least @count bytes long.
  88 * @src:   Source address, in user space.
  89 * @count: Maximum number of bytes to copy, including the trailing NUL.
  90 *
  91 * Copies a NUL-terminated string from userspace to kernel space.
  92 *
  93 * On success, returns the length of the string (not including the trailing
  94 * NUL).
  95 *
  96 * If access to userspace fails, returns -EFAULT (some data may have been
  97 * copied).
  98 *
  99 * If @count is smaller than the length of the string, copies @count bytes
 100 * and returns @count.
 101 */
 102long strncpy_from_user(char *dst, const char __user *src, long count)
 103{
 104        unsigned long max_addr, src_addr;
 105
 106        if (unlikely(count <= 0))
 107                return 0;
 108
 109        max_addr = user_addr_max();
 110        src_addr = (unsigned long)src;
 111        if (likely(src_addr < max_addr)) {
 112                unsigned long max = max_addr - src_addr;
 113                long retval;
 114
 115                kasan_check_write(dst, count);
 116                check_object_size(dst, count, false);
 117                user_access_begin();
 118                retval = do_strncpy_from_user(dst, src, count, max);
 119                user_access_end();
 120                return retval;
 121        }
 122        return -EFAULT;
 123}
 124EXPORT_SYMBOL(strncpy_from_user);
 125