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#include <linux/mm.h>
  10
  11#include <asm/byteorder.h>
  12#include <asm/word-at-a-time.h>
  13
  14#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
  15#define IS_UNALIGNED(src, dst)  0
  16#else
  17#define IS_UNALIGNED(src, dst)  \
  18        (((long) dst | (long) src) & (sizeof(long) - 1))
  19#endif
  20
  21/*
  22 * Do a strncpy, return length of string without final '\0'.
  23 * 'count' is the user-supplied count (return 'count' if we
  24 * hit it), 'max' is the address space maximum (and we return
  25 * -EFAULT if we hit it).
  26 */
  27static inline long do_strncpy_from_user(char *dst, const char __user *src,
  28                                        unsigned long count, unsigned long max)
  29{
  30        const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
  31        unsigned long res = 0;
  32
  33        /*
  34         * Truncate 'max' to the user-specified limit, so that
  35         * we only have one limit we need to check in the loop
  36         */
  37        if (max > count)
  38                max = count;
  39
  40        if (IS_UNALIGNED(src, dst))
  41                goto byte_at_a_time;
  42
  43        while (max >= sizeof(unsigned long)) {
  44                unsigned long c, data;
  45
  46                /* Fall back to byte-at-a-time if we get a page fault */
  47                unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
  48
  49                *(unsigned long *)(dst+res) = c;
  50                if (has_zero(c, &data, &constants)) {
  51                        data = prep_zero_mask(c, data, &constants);
  52                        data = create_zero_mask(data);
  53                        return res + find_zero(data);
  54                }
  55                res += sizeof(unsigned long);
  56                max -= sizeof(unsigned long);
  57        }
  58
  59byte_at_a_time:
  60        while (max) {
  61                char c;
  62
  63                unsafe_get_user(c,src+res, efault);
  64                dst[res] = c;
  65                if (!c)
  66                        return res;
  67                res++;
  68                max--;
  69        }
  70
  71        /*
  72         * Uhhuh. We hit 'max'. But was that the user-specified maximum
  73         * too? If so, that's ok - we got as much as the user asked for.
  74         */
  75        if (res >= count)
  76                return res;
  77
  78        /*
  79         * Nope: we hit the address space limit, and we still had more
  80         * characters the caller would have wanted. That's an EFAULT.
  81         */
  82efault:
  83        return -EFAULT;
  84}
  85
  86/**
  87 * strncpy_from_user: - Copy a NUL terminated string from userspace.
  88 * @dst:   Destination address, in kernel space.  This buffer must be at
  89 *         least @count bytes long.
  90 * @src:   Source address, in user space.
  91 * @count: Maximum number of bytes to copy, including the trailing NUL.
  92 *
  93 * Copies a NUL-terminated string from userspace to kernel space.
  94 *
  95 * On success, returns the length of the string (not including the trailing
  96 * NUL).
  97 *
  98 * If access to userspace fails, returns -EFAULT (some data may have been
  99 * copied).
 100 *
 101 * If @count is smaller than the length of the string, copies @count bytes
 102 * and returns @count.
 103 */
 104long strncpy_from_user(char *dst, const char __user *src, 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)untagged_addr(src);
 113        if (likely(src_addr < max_addr)) {
 114                unsigned long max = max_addr - src_addr;
 115                long retval;
 116
 117                kasan_check_write(dst, count);
 118                check_object_size(dst, count, false);
 119                if (user_access_begin(src, max)) {
 120                        retval = do_strncpy_from_user(dst, src, count, max);
 121                        user_access_end();
 122                        return retval;
 123                }
 124        }
 125        return -EFAULT;
 126}
 127EXPORT_SYMBOL(strncpy_from_user);
 128