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The implementation is based on scalar Chacha20 with per-thread cache. It uses getrandom or /dev/urandom as fallback to get the initial entropy, and reseeds the internal state on every 16MB of consumed buffer. To improve performance and lower memory consumption the per-thread cache is allocated lazily on first arc4random functions call, and if the memory allocation fails getentropy or /dev/urandom is used as fallback. The cache is also cleared on thread exit iff it was initialized (so if arc4random is not called it is not touched). Although it is lock-free, arc4random is still not async-signal-safe (the per thread state is not updated atomically). The ChaCha20 implementation is based on RFC8439 [1], omitting the final XOR of the keystream with the plaintext because the plaintext is a stream of zeros. This strategy is similar to what OpenBSD arc4random does. The arc4random_uniform is based on previous work by Florian Weimer, where the algorithm is based on Jérémie Lumbroso paper Optimal Discrete Uniform Generation from Coin Flips, and Applications (2013) [2], who credits Donald E. Knuth and Andrew C. Yao, The complexity of nonuniform random number generation (1976), for solving the general case. The main advantage of this method is the that the unit of randomness is not the uniform random variable (uint32_t), but a random bit. It optimizes the internal buffer sampling by initially consuming a 32-bit random variable and then sampling byte per byte. Depending of the upper bound requested, it might lead to better CPU utilization. Checked on x86_64-linux-gnu, aarch64-linux, and powerpc64le-linux-gnu. Co-authored-by: Florian Weimer <fweimer@redhat.com> Reviewed-by: Yann Droneaud <ydroneaud@opteya.com> [1] https://datatracker.ietf.org/doc/html/rfc8439 [2] https://arxiv.org/pdf/1304.1916.pdf
52 lines
1.9 KiB
C
52 lines
1.9 KiB
C
/* _Fork implementation. Linux version.
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Copyright (C) 2021-2022 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <arch-fork.h>
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#include <pthreadP.h>
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pid_t
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_Fork (void)
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{
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pid_t pid = arch_fork (&THREAD_SELF->tid);
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if (pid == 0)
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{
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struct pthread *self = THREAD_SELF;
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/* Initialize the robust mutex list setting in the kernel which has
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been reset during the fork. We do not check for errors because if
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it fails here, it must have failed at process startup as well and
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nobody could have used robust mutexes.
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Before we do that, we have to clear the list of robust mutexes
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because we do not inherit ownership of mutexes from the parent.
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We do not have to set self->robust_head.futex_offset since we do
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inherit the correct value from the parent. We do not need to clear
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the pending operation because it must have been zero when fork was
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called. */
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#if __PTHREAD_MUTEX_HAVE_PREV
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self->robust_prev = &self->robust_head;
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#endif
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self->robust_head.list = &self->robust_head;
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INTERNAL_SYSCALL_CALL (set_robust_list, &self->robust_head,
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sizeof (struct robust_list_head));
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call_function_static_weak (__arc4random_fork_subprocess);
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}
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return pid;
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}
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libc_hidden_def (_Fork)
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