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/* SPDX-License-Identifier: LGPL-2.1+ */
/***
This file is part of systemd.
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/
#include <elf.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/random.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#if HAVE_SYS_AUXV_H
# include <sys/auxv.h>
#endif
#if USE_SYS_RANDOM_H
# include <sys/random.h>
#else
# include <linux/random.h>
#endif
#include "fd-util.h"
#include "io-util.h"
#include "missing.h"
#include "random-util.h"
#include "time-util.h"
int acquire_random_bytes(void *p, size_t n, bool high_quality_required) {
static int have_syscall = -1;
_cleanup_close_ int fd = -1;
unsigned already_done = 0;
int r;
/* Gathers some randomness from the kernel. This call will never block. If
* high_quality_required, it will always return some data from the kernel,
* regardless of whether the random pool is fully initialized or not.
* Otherwise, it will return success if at least some random bytes were
* successfully acquired, and an error if the kernel has no entropy whatsover
* for us. */
/* Use the getrandom() syscall unless we know we don't have it. */
if (have_syscall != 0) {
r = getrandom(p, n, GRND_NONBLOCK);
if (r > 0) {
have_syscall = true;
if ((size_t) r == n)
return 0;
if (!high_quality_required) {
/* Fill in the remaining bytes using pseudorandom values */
pseudorandom_bytes((uint8_t*) p + r, n - r);
return 0;
}
already_done = r;
} else if (errno == ENOSYS)
/* We lack the syscall, continue with reading from /dev/urandom. */
have_syscall = false;
else if (errno == EAGAIN) {
/* The kernel has no entropy whatsoever. Let's remember to
* use the syscall the next time again though.
*
* If high_quality_required is false, return an error so that
* random_bytes() can produce some pseudorandom
* bytes. Otherwise, fall back to /dev/urandom, which we know
* is empty, but the kernel will produce some bytes for us on
* a best-effort basis. */
have_syscall = true;
if (!high_quality_required)
return -ENODATA;
} else
return -errno;
}
fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
if (fd < 0)
return errno == ENOENT ? -ENOSYS : -errno;
return loop_read_exact(fd, (uint8_t*) p + already_done, n - already_done, true);
}
void initialize_srand(void) {
static bool srand_called = false;
unsigned x;
#if HAVE_SYS_AUXV_H
void *auxv;
#endif
if (srand_called)
return;
#if HAVE_SYS_AUXV_H
/* The kernel provides us with 16 bytes of entropy in auxv, so let's
* try to make use of that to seed the pseudo-random generator. It's
* better than nothing... */
auxv = (void*) getauxval(AT_RANDOM);
if (auxv) {
assert_cc(sizeof(x) <= 16);
memcpy(&x, auxv, sizeof(x));
} else
#endif
x = 0;
x ^= (unsigned) now(CLOCK_REALTIME);
x ^= (unsigned) gettid();
srand(x);
srand_called = true;
}
/* INT_MAX gives us only 31 bits, so use 24 out of that. */
#if RAND_MAX >= INT_MAX
# define RAND_STEP 3
#else
/* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
# define RAND_STEP 1
#endif
void pseudorandom_bytes(void *p, size_t n) {
uint8_t *q;
initialize_srand();
for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
unsigned rr;
rr = (unsigned) rand();
#if RAND_STEP >= 3
if ((size_t) (q - (uint8_t*) p + 2) < n)
q[2] = rr >> 16;
#endif
#if RAND_STEP >= 2
if ((size_t) (q - (uint8_t*) p + 1) < n)
q[1] = rr >> 8;
#endif
q[0] = rr;
}
}
void random_bytes(void *p, size_t n) {
int r;
r = acquire_random_bytes(p, n, false);
if (r >= 0)
return;
/* If some idiot made /dev/urandom unavailable to us, or the
* kernel has no entropy, use a PRNG instead. */
return pseudorandom_bytes(p, n);
}
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