QRandomGenerator: use getentropy on Linux & OpenBSD

The getentropy function, first found in OpenBSD, is present in glibc
since version 2.25 and Bionic since Android 6.0 and NDK r11. It uses the
Linux 3.17 getrandom system call. Unlike glibc's getrandom() wrapper,
the glibc implementation of getentropy() function is not a POSIX thread
cancellation point, so we prefer to use that even though we have to
break the reading into 256-byte blocks.

The big advantage is that these functions work even in the absence of a
/dev/urandom device node, in addition to a few cycles shaved off by not
having to open a file descriptor and close it at exit. What's more, the
glibc implementation blocks until entropy is available on early boot, so
we don't have to worry about a failure mode. The Bionic implementation
will fall back by itself to /dev/urandom and, failing that, gathering
entropy from elsewhere in the system in a way it cannot fail either.

uClibc has a wrapper to getrandom(2) but no getentropy(3). MUSL has
neither.

Change-Id: Ia53158e207a94bf49489fffd14c8cee1b968a619
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

2 files changed, 36 insertions, 0 deletions

diff --git a/config.tests/unix/getentropy/getentropy.cpp b/config.tests/unix/getentropy/getentropy.cpp
new file mode 100644
index 0000000000..6cb4dc3a95
--- /dev/null
+++ b/config.tests/unix/getentropy/getentropy.cpp
@@ -0,0 +1,35 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 Intel Corporation.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <unistd.h>
+
+int main()
+{
+    char buf[32];
+    return getentropy(buf, sizeof(buf));
+}
diff --git a/config.tests/unix/getentropy/getentropy.pro b/config.tests/unix/getentropy/getentropy.pro
new file mode 100644
index 0000000000..bdd626b513
--- /dev/null
+++ b/config.tests/unix/getentropy/getentropy.pro
@@ -0,0 +1 @@
+SOURCES = getentropy.cpp