From a2d196e75cef95c2b70734ad02e94f9da0e769fe Mon Sep 17 00:00:00 2001 From: Jonathan Wakely Date: Tue, 19 May 2020 16:49:21 +0100 Subject: [PATCH] libstdc++: Use RDRAND as fallback if RDSEED keeps failing (PR 94087) It's not difficult for multiple threads to drain the entropy available to the RDSEED instruction, at which point we throw an exception. This change will try to use RDRAND after RDSEED fails repeatedly, and only throw if RDRAND also fails repeatedly. This doesn't guarantee a random value can always be read, but reduces the likelihood of failure when using the RDSEED instruction. PR libstdc++/94087 * src/c++11/random.cc (__x86_rdseed): Allow fallback function to be passed in. (__x86_rdseed_rdrand): New function that uses rdseed with rdrand fallback. (random_device::_M_init): Use __x86_rdseed_rdrand when both instructions are available. * testsuite/26_numerics/random/random_device/94087.cc: New test. --- libstdc++-v3/ChangeLog | 11 ++++ libstdc++-v3/src/c++11/random.cc | 26 +++++++- .../26_numerics/random/random_device/94087.cc | 63 +++++++++++++++++++ 3 files changed, 98 insertions(+), 2 deletions(-) create mode 100644 libstdc++-v3/testsuite/26_numerics/random/random_device/94087.cc diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog index ae1ec87589c..67ca5b76d21 100644 --- a/libstdc++-v3/ChangeLog +++ b/libstdc++-v3/ChangeLog @@ -1,3 +1,14 @@ +2020-05-19 Jonathan Wakely + + PR libstdc++/94087 + * src/c++11/random.cc (__x86_rdseed): Allow fallback function to be + passed in. + (__x86_rdseed_rdrand): New function that uses rdseed with rdrand + fallback. + (random_device::_M_init): Use __x86_rdseed_rdrand when both + instructions are available. + * testsuite/26_numerics/random/random_device/94087.cc: New test. + 2020-05-19 Patrick Palka PR c++/66439 diff --git a/libstdc++-v3/src/c++11/random.cc b/libstdc++-v3/src/c++11/random.cc index 236eccfc177..62ed274479a 100644 --- a/libstdc++-v3/src/c++11/random.cc +++ b/libstdc++-v3/src/c++11/random.cc @@ -97,7 +97,7 @@ namespace std _GLIBCXX_VISIBILITY(default) #if USE_RDSEED unsigned int __attribute__ ((target("rdseed"))) - __x86_rdseed(void*) + __x86_rdseed(void* fallback) { unsigned int retries = 100; unsigned int val; @@ -105,12 +105,25 @@ namespace std _GLIBCXX_VISIBILITY(default) while (__builtin_ia32_rdseed_si_step(&val) == 0) { if (--retries == 0) - std::__throw_runtime_error(__N("random_device: rdseed failed")); + { + if (auto f = reinterpret_cast(fallback)) + return f(nullptr); + std::__throw_runtime_error(__N("random_device: rdseed failed")); + } __builtin_ia32_pause(); } return val; } + +#if USE_RDRAND + unsigned int + __attribute__ ((target("rdseed,rdrnd"))) + __x86_rdseed_rdrand(void*) + { + return __x86_rdseed(reinterpret_cast(&__x86_rdrand)); + } +#endif #endif #ifdef _GLIBCXX_USE_CRT_RAND_S @@ -205,6 +218,15 @@ namespace std _GLIBCXX_VISIBILITY(default) __cpuid_count(7, 0, eax, ebx, ecx, edx); if (ebx & bit_RDSEED) { +#ifdef USE_RDRAND + // CPUID.01H:ECX.RDRAND[bit 30] + __cpuid(1, eax, ebx, ecx, edx); + if (ecx & bit_RDRND) + { + _M_func = &__x86_rdseed_rdrand; + return; + } +#endif _M_func = &__x86_rdseed; return; } diff --git a/libstdc++-v3/testsuite/26_numerics/random/random_device/94087.cc b/libstdc++-v3/testsuite/26_numerics/random/random_device/94087.cc new file mode 100644 index 00000000000..cfcc261906e --- /dev/null +++ b/libstdc++-v3/testsuite/26_numerics/random/random_device/94087.cc @@ -0,0 +1,63 @@ +// Copyright (C) 2020 Free Software Foundation, Inc. +// +// This file is part of the GNU ISO C++ Library. This library is free +// software; you can redistribute it and/or modify it under the +// terms of the GNU General Public License as published by the +// Free Software Foundation; either version 3, or (at your option) +// any later version. + +// This library 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 General Public License for more details. + +// You should have received a copy of the GNU General Public License along +// with this library; see the file COPYING3. If not see +// . + +// { dg-do run } +// { dg-options "-pthread" } +// { dg-require-effective-target c++11 } +// { dg-require-effective-target pthread } +// { dg-require-gthreads "" } + +#include +#include +#include +#include + +bool +random_device_available(const char* token) noexcept +{ + try { + std::random_device dev(token); + return true; + } catch (...) { + std::printf("random_device(\"%s\") not available\n", token); + return false; + } +} + +void read_random_device(const char* token, int iterations) +{ + std::random_device dev(token); + for (int i = 0; i != iterations; ++i) + (void) dev(); +} + +int main() { + std::thread workers[8]; + + // N.B. don't test /dev/random as it might block, and /dev/urandom + // "can incur an appreciable delay when requesting large amounts of data". + for (const char* dev : { "default", "rdrand", "rdseed", "rand_s" }) + { + if (random_device_available(dev)) + { + for (auto& w : workers) + w = std::thread{read_random_device, dev, 1000}; + for (auto& w : workers) + w.join(); + } + } +} -- 2.30.2