2 This file is part of systemd.
4 Copyright 2010 Lennart Poettering
6 systemd is free software; you can redistribute it and/or modify it
7 under the terms of the GNU Lesser General Public License as published by
8 the Free Software Foundation; either version 2.1 of the License, or
9 (at your option) any later version.
11 systemd is distributed in the hope that it will be useful, but
12 WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
16 You should have received a copy of the GNU Lesser General Public License
17 along with systemd; If not, see <http://www.gnu.org/licenses/>.
26 #include <linux/random.h>
29 #ifdef HAVE_SYS_AUXV_H
30 # include <sys/auxv.h>
33 #ifdef USE_SYS_RANDOM_H
34 # include <sys/random.h>
36 # include <linux/random.h>
42 #include "random-util.h"
43 #include "time-util.h"
45 int acquire_random_bytes(void *p, size_t n, bool high_quality_required) {
46 static int have_syscall = -1;
48 _cleanup_close_ int fd = -1;
49 unsigned already_done = 0;
52 /* Gathers some randomness from the kernel. This call will never block. If
53 * high_quality_required, it will always return some data from the kernel,
54 * regardless of whether the random pool is fully initialized or not.
55 * Otherwise, it will return success if at least some random bytes were
56 * successfully acquired, and an error if the kernel has no entropy whatsover
59 /* Use the getrandom() syscall unless we know we don't have it. */
60 if (have_syscall != 0) {
61 r = getrandom(p, n, GRND_NONBLOCK);
66 if (!high_quality_required) {
67 /* Fill in the remaing bytes using pseudorandom values */
68 pseudorandom_bytes((uint8_t*) p + r, n - r);
73 } else if (errno == ENOSYS)
74 /* We lack the syscall, continue with reading from /dev/urandom. */
76 else if (errno == EAGAIN) {
77 /* The kernel has no entropy whatsoever. Let's remember to
78 * use the syscall the next time again though.
80 * If high_quality_required is false, return an error so that
81 * random_bytes() can produce some pseudorandom
82 * bytes. Otherwise, fall back to /dev/urandom, which we know
83 * is empty, but the kernel will produce some bytes for us on
84 * a best-effort basis. */
87 if (!high_quality_required)
93 fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
95 return errno == ENOENT ? -ENOSYS : -errno;
97 return loop_read_exact(fd, (uint8_t*) p + already_done, n - already_done, true);
100 void initialize_srand(void) {
101 static bool srand_called = false;
103 #ifdef HAVE_SYS_AUXV_H
110 #ifdef HAVE_SYS_AUXV_H
111 /* The kernel provides us with 16 bytes of entropy in auxv, so let's
112 * try to make use of that to seed the pseudo-random generator. It's
113 * better than nothing... */
115 auxv = (void*) getauxval(AT_RANDOM);
117 assert_cc(sizeof(x) <= 16);
118 memcpy(&x, auxv, sizeof(x));
124 x ^= (unsigned) now(CLOCK_REALTIME);
125 x ^= (unsigned) gettid();
131 /* INT_MAX gives us only 31 bits, so use 24 out of that. */
132 #if RAND_MAX >= INT_MAX
135 /* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
139 void pseudorandom_bytes(void *p, size_t n) {
144 for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
147 rr = (unsigned) rand();
150 if ((size_t) (q - (uint8_t*) p + 2) < n)
154 if ((size_t) (q - (uint8_t*) p + 1) < n)
161 void random_bytes(void *p, size_t n) {
164 r = acquire_random_bytes(p, n, false);
168 /* If some idiot made /dev/urandom unavailable to us, or the
169 * kernel has no entropy, use a PRNG instead. */
170 return pseudorandom_bytes(p, n);