cc83d5197c
We only use this when we don't require the best randomness. The primary usecase for this is UUID generation, as this means we don't drain randomness from the kernel pool for them. Since UUIDs are usually not secrets RDRAND should be goot enough for them to avoid real-life collisions.
259 lines
8.7 KiB
C
259 lines
8.7 KiB
C
/* SPDX-License-Identifier: LGPL-2.1+ */
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#ifdef __x86_64__
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#include <cpuid.h>
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#endif
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#include <elf.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <linux/random.h>
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#include <stdbool.h>
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#include <stdint.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/time.h>
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#if HAVE_SYS_AUXV_H
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# include <sys/auxv.h>
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#endif
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#if USE_SYS_RANDOM_H
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# include <sys/random.h>
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#else
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# include <linux/random.h>
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#endif
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#include "fd-util.h"
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#include "io-util.h"
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#include "missing.h"
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#include "random-util.h"
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#include "time-util.h"
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int rdrand64(uint64_t *ret) {
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#ifdef __x86_64__
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static int have_rdrand = -1;
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unsigned char err;
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if (have_rdrand < 0) {
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uint32_t eax, ebx, ecx, edx;
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/* Check if RDRAND is supported by the CPU */
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if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) == 0) {
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have_rdrand = false;
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return -EOPNOTSUPP;
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}
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have_rdrand = !!(ecx & (1U << 30));
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}
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if (have_rdrand == 0)
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return -EOPNOTSUPP;
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asm volatile("rdrand %0;"
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"setc %1"
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: "=r" (*ret),
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"=qm" (err));
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if (!err)
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return -EAGAIN;
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return 0;
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#else
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return -EOPNOTSUPP;
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#endif
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}
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int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
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static int have_syscall = -1;
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_cleanup_close_ int fd = -1;
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bool got_some = false;
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int r;
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/* Gathers some randomness from the kernel (or the CPU if the RANDOM_ALLOW_RDRAND flag is set). This call won't
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* block, unless the RANDOM_BLOCK flag is set. If RANDOM_DONT_DRAIN is set, an error is returned if the random
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* pool is not initialized. Otherwise it will always return some data from the kernel, regardless of whether
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* the random pool is fully initialized or not. */
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if (n == 0)
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return 0;
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if (FLAGS_SET(flags, RANDOM_ALLOW_RDRAND))
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/* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is not
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* required, as we don't trust it (who does?). Note that we only do a single iteration of RDRAND here,
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* even though the Intel docs suggest calling this in a tight loop of 10 invocations or so. That's
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* because we don't really care about the quality here. We generally prefer using RDRAND if the caller
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* allows us too, since this way we won't drain the kernel randomness pool if we don't need it, as the
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* pool's entropy is scarce. */
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for (;;) {
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uint64_t u;
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size_t m;
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if (rdrand64(&u) < 0) {
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if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
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/* Fill in the remaining bytes using pseudo-random values */
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pseudo_random_bytes(p, n);
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return 0;
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}
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/* OK, this didn't work, let's go to getrandom() + /dev/urandom instead */
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break;
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}
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m = MIN(sizeof(u), n);
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memcpy(p, &u, m);
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p = (uint8_t*) p + m;
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n -= m;
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if (n == 0)
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return 0; /* Yay, success! */
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got_some = true;
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}
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/* Use the getrandom() syscall unless we know we don't have it. */
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if (have_syscall != 0 && !HAS_FEATURE_MEMORY_SANITIZER) {
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for (;;) {
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r = getrandom(p, n, FLAGS_SET(flags, RANDOM_BLOCK) ? 0 : GRND_NONBLOCK);
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if (r > 0) {
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have_syscall = true;
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if ((size_t) r == n)
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return 0; /* Yay, success! */
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assert((size_t) r < n);
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p = (uint8_t*) p + r;
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n -= r;
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if (FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
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/* Fill in the remaining bytes using pseudo-random values */
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pseudo_random_bytes(p, n);
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return 0;
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}
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got_some = true;
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/* Hmm, we didn't get enough good data but the caller insists on good data? Then try again */
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if (FLAGS_SET(flags, RANDOM_BLOCK))
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continue;
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/* Fill in the rest with /dev/urandom */
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break;
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} else if (r == 0) {
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have_syscall = true;
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return -EIO;
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} else if (errno == ENOSYS) {
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/* We lack the syscall, continue with reading from /dev/urandom. */
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have_syscall = false;
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break;
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} else if (errno == EAGAIN) {
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/* The kernel has no entropy whatsoever. Let's remember to use the syscall the next
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* time again though.
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*
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* If RANDOM_DONT_DRAIN is set, return an error so that random_bytes() can produce some
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* pseudo-random bytes instead. Otherwise, fall back to /dev/urandom, which we know is empty,
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* but the kernel will produce some bytes for us on a best-effort basis. */
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have_syscall = true;
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if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
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/* Fill in the remaining bytes using pseudorandom values */
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pseudo_random_bytes(p, n);
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return 0;
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}
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if (FLAGS_SET(flags, RANDOM_DONT_DRAIN))
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return -ENODATA;
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/* Use /dev/urandom instead */
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break;
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} else
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return -errno;
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}
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}
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fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
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if (fd < 0)
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return errno == ENOENT ? -ENOSYS : -errno;
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return loop_read_exact(fd, p, n, true);
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}
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void initialize_srand(void) {
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static bool srand_called = false;
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unsigned x;
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#if HAVE_SYS_AUXV_H
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const void *auxv;
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#endif
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uint64_t k;
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if (srand_called)
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return;
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#if HAVE_SYS_AUXV_H
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/* The kernel provides us with 16 bytes of entropy in auxv, so let's
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* try to make use of that to seed the pseudo-random generator. It's
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* better than nothing... */
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auxv = (const void*) getauxval(AT_RANDOM);
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if (auxv) {
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assert_cc(sizeof(x) <= 16);
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memcpy(&x, auxv, sizeof(x));
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} else
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#endif
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x = 0;
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x ^= (unsigned) now(CLOCK_REALTIME);
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x ^= (unsigned) gettid();
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if (rdrand64(&k) >= 0)
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x ^= (unsigned) k;
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srand(x);
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srand_called = true;
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}
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/* INT_MAX gives us only 31 bits, so use 24 out of that. */
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#if RAND_MAX >= INT_MAX
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# define RAND_STEP 3
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#else
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/* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
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# define RAND_STEP 1
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#endif
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void pseudo_random_bytes(void *p, size_t n) {
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uint8_t *q;
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initialize_srand();
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for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
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unsigned rr;
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rr = (unsigned) rand();
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#if RAND_STEP >= 3
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if ((size_t) (q - (uint8_t*) p + 2) < n)
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q[2] = rr >> 16;
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#endif
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#if RAND_STEP >= 2
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if ((size_t) (q - (uint8_t*) p + 1) < n)
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q[1] = rr >> 8;
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#endif
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q[0] = rr;
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}
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}
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void random_bytes(void *p, size_t n) {
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if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_DONT_DRAIN|RANDOM_ALLOW_RDRAND) >= 0)
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return;
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/* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */
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pseudo_random_bytes(p, n);
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}
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