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1 /* SPDX-License-Identifier: LGPL-2.1+ */
3 This file is part of systemd.
5 Copyright 2010 Lennart Poettering
7 systemd is free software; you can redistribute it and/or modify it
8 under the terms of the GNU Lesser General Public License as published by
9 the Free Software Foundation; either version 2.1 of the License, or
10 (at your option) any later version.
12 systemd is distributed in the hope that it will be useful, but
13 WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 Lesser General Public License for more details.
17 You should have received a copy of the GNU Lesser General Public License
18 along with systemd; If not, see <http://www.gnu.org/licenses/>.
27 #include <linux/random.h>
31 # include <sys/auxv.h>
35 # include <sys/random.h>
37 # include <linux/random.h>
43 #include "random-util.h"
44 #include "time-util.h"
46 int acquire_random_bytes(void *p
, size_t n
, bool high_quality_required
) {
47 static int have_syscall
= -1;
49 _cleanup_close_
int fd
= -1;
50 unsigned already_done
= 0;
53 /* Gathers some randomness from the kernel. This call will never block. If
54 * high_quality_required, it will always return some data from the kernel,
55 * regardless of whether the random pool is fully initialized or not.
56 * Otherwise, it will return success if at least some random bytes were
57 * successfully acquired, and an error if the kernel has no entropy whatsover
60 /* Use the getrandom() syscall unless we know we don't have it. */
61 if (have_syscall
!= 0) {
62 r
= getrandom(p
, n
, GRND_NONBLOCK
);
67 if (!high_quality_required
) {
68 /* Fill in the remaining bytes using pseudorandom values */
69 pseudorandom_bytes((uint8_t*) p
+ r
, n
- r
);
74 } else if (errno
== ENOSYS
)
75 /* We lack the syscall, continue with reading from /dev/urandom. */
77 else if (errno
== EAGAIN
) {
78 /* The kernel has no entropy whatsoever. Let's remember to
79 * use the syscall the next time again though.
81 * If high_quality_required is false, return an error so that
82 * random_bytes() can produce some pseudorandom
83 * bytes. Otherwise, fall back to /dev/urandom, which we know
84 * is empty, but the kernel will produce some bytes for us on
85 * a best-effort basis. */
88 if (!high_quality_required
)
94 fd
= open("/dev/urandom", O_RDONLY
|O_CLOEXEC
|O_NOCTTY
);
96 return errno
== ENOENT
? -ENOSYS
: -errno
;
98 return loop_read_exact(fd
, (uint8_t*) p
+ already_done
, n
- already_done
, true);
101 void initialize_srand(void) {
102 static bool srand_called
= false;
112 /* The kernel provides us with 16 bytes of entropy in auxv, so let's
113 * try to make use of that to seed the pseudo-random generator. It's
114 * better than nothing... */
116 auxv
= (void*) getauxval(AT_RANDOM
);
118 assert_cc(sizeof(x
) <= 16);
119 memcpy(&x
, auxv
, sizeof(x
));
125 x
^= (unsigned) now(CLOCK_REALTIME
);
126 x
^= (unsigned) gettid();
132 /* INT_MAX gives us only 31 bits, so use 24 out of that. */
133 #if RAND_MAX >= INT_MAX
136 /* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
140 void pseudorandom_bytes(void *p
, size_t n
) {
145 for (q
= p
; q
< (uint8_t*) p
+ n
; q
+= RAND_STEP
) {
148 rr
= (unsigned) rand();
151 if ((size_t) (q
- (uint8_t*) p
+ 2) < n
)
155 if ((size_t) (q
- (uint8_t*) p
+ 1) < n
)
162 void random_bytes(void *p
, size_t n
) {
165 r
= acquire_random_bytes(p
, n
, false);
169 /* If some idiot made /dev/urandom unavailable to us, or the
170 * kernel has no entropy, use a PRNG instead. */
171 return pseudorandom_bytes(p
, n
);