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QRandom: retry the use of RDRAND instruction as recommended by manuals

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The Intel whitepaper[1] recommends retrying RDRAND some 10 times even
after it fails, since the hardware has a fairness algorithm and reseeds
itself quite quickly.

[1] https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide

Change-Id: I907a43cd9a714da288a2fffd15baafd88242d8b6
Reviewed-by: André Hartmann <aha_1980@gmx.de>
This commit is contained in:
Thiago Macieira 2019-08-13 21:32:44 -07:00
parent 72a04b132c
commit 5cea83a8a2
1 changed files with 12 additions and 7 deletions
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19
src/corelib/global/qrandom.cpp
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@ -1,6 +1,6 @@
/****************************************************************************
**
** Copyright (C) 2017 Intel Corporation.
** Copyright (C) 2019 Intel Corporation.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the QtCore module of the Qt Toolkit.
@ -103,17 +103,22 @@ static QT_FUNCTION_TARGET(RDRND) qsizetype qt_random_cpu(void *buffer, qsizetype
{
unsigned *ptr = reinterpret_cast<unsigned *>(buffer);
unsigned *end = ptr + count;
int retries = 10;
while (ptr + sizeof(qregisteruint)/sizeof(*ptr) <= end) {
if (_rdrandXX_step(reinterpret_cast<qregisteruint *>(ptr)) == 0)
if (_rdrandXX_step(reinterpret_cast<qregisteruint *>(ptr)))
ptr += sizeof(qregisteruint)/sizeof(*ptr);
else if (--retries == 0)
goto out;
ptr += sizeof(qregisteruint)/sizeof(*ptr);
}
if (sizeof(*ptr) != sizeof(qregisteruint) && ptr != end) {
if (_rdrand32_step(ptr))
goto out;
++ptr;
while (sizeof(*ptr) != sizeof(qregisteruint) && ptr != end) {
bool ok = _rdrand32_step(ptr);
if (!ok && --retries)
continue;
if (ok)
++ptr;
break;
}
out: