1/*
2 * Copyright (C) 2018 The Android Open Source Project
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 *  * Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 *  * Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in
12 *    the documentation and/or other materials provided with the
13 *    distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
16 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
17 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
18 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
19 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
21 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
22 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
23 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
24 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
25 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28
29#include <errno.h>
30#include <fcntl.h>
31#include <sys/random.h>
32#include <unistd.h>
33
34static int getentropy_urandom(void* buffer, size_t buffer_size, int saved_errno) {
35  int fd = TEMP_FAILURE_RETRY(open("/dev/urandom", O_RDONLY | O_NOFOLLOW | O_CLOEXEC, 0));
36  if (fd == -1) return -1;
37
38  size_t collected = 0;
39  while (collected < buffer_size) {
40    ssize_t count = TEMP_FAILURE_RETRY(read(fd, static_cast<char*>(buffer) + collected,
41                                            buffer_size - collected));
42    if (count == -1) {
43      close(fd);
44      return -1;
45    }
46    collected += count;
47  }
48
49  close(fd);
50  errno = saved_errno;
51  return 0;
52}
53
54int getentropy(void* buffer, size_t buffer_size) {
55  if (buffer_size > 256) {
56    errno = EIO;
57    return -1;
58  }
59
60  int saved_errno = errno;
61
62  size_t collected = 0;
63  while (collected < buffer_size) {
64    long count = TEMP_FAILURE_RETRY(getrandom(static_cast<char*>(buffer) + collected,
65                                              buffer_size - collected, GRND_NONBLOCK));
66    if (count == -1) {
67      // EAGAIN: there isn't enough entropy right now.
68      // ENOSYS/EINVAL: getrandom(2) or GRND_NONBLOCK isn't supported.
69      // EFAULT: `buffer` is invalid.
70      // Try /dev/urandom regardless because it can't hurt,
71      // and we don't need to optimize the EFAULT case.
72      // See http://b/33059407 and http://b/67015565.
73      return getentropy_urandom(buffer, buffer_size, saved_errno);
74    }
75    collected += count;
76  }
77
78  errno = saved_errno;
79  return 0;
80}
81