mirror of
https://github.com/zephyrproject-rtos/zephyr
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fcc56e315f
Not only removes a branch during normal operation, but also ensures that, by initializing at the PRE_KERNEL_2 stage, and granting privileges to all threads to the semaphore, this code will work in early boot situations and in user mode. This assumes that entropy drivers will all initialize during PRE_KERNEL_1 stage. All in-tree drivers do that. Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>
108 lines
3.0 KiB
C
108 lines
3.0 KiB
C
/*
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* Copyright (c) 2017 Intel Corporation
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*
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* SPDX-License-Identifier: CC0-1.0
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*
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* Based on code written in 2016 by David Blackman and Sebastiano Vigna
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* (vigna@acm.org)
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*
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* To the extent possible under law, the author has dedicated all copyright
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* and related and neighboring rights to this software to the public domain
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* worldwide. This software is distributed without any warranty.
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*
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* See <http://creativecommons.org/publicdomain/zero/1.0/>.
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*/
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/* This is the successor to xorshift128+. It is the fastest full-period
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* generator passing BigCrush without systematic failures, but due to the
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* relatively short period it is acceptable only for applications with a
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* mild amount of parallelism; otherwise, use a xorshift1024* generator.
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*
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* Beside passing BigCrush, this generator passes the PractRand test suite
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* up to (and included) 16TB, with the exception of binary rank tests, as
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* the lowest bit of this generator is an LSFR. The next bit is not an
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* LFSR, but in the long run it will fail binary rank tests, too. The
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* other bits have no LFSR artifacts.
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*
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* We suggest to use a sign test to extract a random Boolean value, and
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* right shifts to extract subsets of bits.
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*
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* Note that the generator uses a simulated rotate operation, which most C
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* compilers will turn into a single instruction. In Java, you can use
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* Long.rotateLeft(). In languages that do not make low-level rotation
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* instructions accessible xorshift128+ could be faster.
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*
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* The state must be seeded so that it is not everywhere zero. If you have
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* a 64-bit seed, we suggest to seed a splitmix64 generator and use its
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* output to fill s.
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*/
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#include <init.h>
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#include <device.h>
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#include <entropy.h>
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#include <kernel.h>
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static u64_t state[2];
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K_SEM_DEFINE(state_sem, 1, 1);
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static inline u64_t rotl(const u64_t x, int k)
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{
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return (x << k) | (x >> (64 - k));
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}
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static int xoroshiro128_initialize(struct device *dev)
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{
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dev = device_get_binding(CONFIG_ENTROPY_NAME);
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if (!dev) {
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return -EINVAL;
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}
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if (entropy_get_entropy(dev, (uint8_t *)&state, sizeof(state)) < 0) {
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return -EINVAL;
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}
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k_object_access_all_grant(&state_sem);
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return 0;
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}
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static u32_t xoroshiro128_next(void)
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{
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const u64_t s0 = state[0];
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u64_t s1 = state[1];
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const u64_t result = s0 + s1;
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s1 ^= s0;
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state[0] = rotl(s0, 55) ^ s1 ^ (s1 << 14);
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state[1] = rotl(s1, 36);
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return (u32_t)result;
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}
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u32_t sys_rand32_get(void)
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{
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u32_t ret;
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if (k_sem_take(&state_sem, K_FOREVER) < 0) {
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/* FIXME: with all threads having access to this semaphore,
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* it's possible that they can corrupt state_sem in a way
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* that k_sem_take will fail. This can be abused to
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* generate numbers without using the xoroshiro128+ RNG.
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*/
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return k_cycle_get_32();
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}
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ret = xoroshiro128_next();
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k_sem_give(&state_sem);
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return ret;
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}
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/* In-tree entropy drivers will initialize in PRE_KERNEL_1; ensure that they're
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* initialized properly before initializing ourselves.
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*/
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SYS_INIT(xoroshiro128_initialize, PRE_KERNEL_2,
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CONFIG_KERNEL_INIT_PRIORITY_DEFAULT);
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