Port PHP7 CSPRNG functions random_int() and random_bytes()

Summary:
Hello! This PR is a port of the [CSPRNG functions](http://php.net/csprng) `random_*()` which were [added in PHP7](https://wiki.php.net/rfc/easy_userland_csprng).

This PR is very much a work in progress and I'll need a lot of help from you smarter kids to get this implemented properly. But it works well on my machine thanks to some one-on-one help from jmikola.

The things that I know need to be addressed:

1. The PHP version [checks for sources of random at compile time](https://github.com/php/php-src/blob/97f159d70288ae94f9a05a370121bcbea760ed9a/Zend/Zend.m4#L406-L412). Not sure how to do this in HHVM.
2. PHP [supports Windows with `arc4random_buf()`](https://github.com/php/php-src/blob/cbcacbb2dad4c97ba5653f42729f7956193d9112/ext/standard/random.c#L86). Not sure if we want to support this as well as HHVM gets more Windows friendly... is that a thing?
3. There is [an open PR](php/php-src#1397) to change the behavior of handling the cases when a reliable source of random cann

Closes #5498
Closes #5925

Reviewed By: sgolemon

Differential Revision: D2342229

Pulled By: jwatzman

fb-gh-sync-id: 419d949e3dd06fb1c13ecf86223c1b28eb57bb84

hphp/runtime/ext/random/config.cmake

@@ -0,0 +1,2 @@
+HHVM_EXTENSION(random ext_random.cpp)
+HHVM_SYSTEMLIB(random ext_random.php)

hphp/runtime/ext/random/ext_random.cpp

@@ -0,0 +1,105 @@
+/*
+   +----------------------------------------------------------------------+
+   | HipHop for PHP                                                       |
+   +----------------------------------------------------------------------+
+   | Copyright (c) 2010-2015 Facebook, Inc. (http://www.facebook.com)     |
+   | Copyright (c) 1997-2010 The PHP Group                                |
+   +----------------------------------------------------------------------+
+   | This source file is subject to version 3.01 of the PHP license,      |
+   | that is bundled with this package in the file LICENSE, and is        |
+   | available through the world-wide-web at the following url:           |
+   | http://www.php.net/license/3_01.txt                                  |
+   | If you did not receive a copy of the PHP license and are unable to   |
+   | obtain it through the world-wide-web, please send a note to          |
+   | license@php.net so we can mail you a copy immediately.               |
+   +----------------------------------------------------------------------+
+*/
+
+#include "hphp/runtime/ext/extension.h"
+#include "hphp/runtime/base/execution-context.h"
+#include "hphp/runtime/vm/runtime.h"
+
+#include <folly/Random.h>
+
+namespace HPHP {
+///////////////////////////////////////////////////////////////////////////////
+
+static bool getRandomBytes(void *bytes, size_t length) {
+  // TODO fix folly to better detect error cases (currently I think it just
+  // aborts) and catch them here.
+  folly::Random::secureRandom(bytes, length);
+  return true;
+}
+
+String HHVM_FUNCTION(random_bytes, int64_t length) {
+  if (length < 1) {
+    // TODO(https://github.com/facebook/hhvm/issues/6012)
+    // Make this throw PHP 7 `Error` when it's implemented
+    SystemLib::throwExceptionObject("Length must be greater than 0");
+  }
+
+  String ret(length, ReserveString);
+  if (!getRandomBytes(ret.mutableData(), ret.capacity())) {
+    SystemLib::throwExceptionObject("Could not gather sufficient random data");
+  }
+
+  return ret.setSize(length);
+}
+
+int64_t HHVM_FUNCTION(random_int, int64_t min, int64_t max) {
+  if (min > max) {
+    // TODO(https://github.com/facebook/hhvm/issues/6012)
+    // Make this throw PHP 7 `Error` when it's implemented
+    SystemLib::throwExceptionObject(
+      "Minimum value must be less than or equal to the maximum value");
+  }
+
+  if (min == max) {
+    return min;
+  }
+
+  uint64_t umax = max - min;
+  uint64_t result;
+  if (!getRandomBytes(&result, sizeof(result))) {
+    SystemLib::throwExceptionObject("Could not gather sufficient random data");
+  }
+
+  // Special case where no modulus is required
+  if (umax == std::numeric_limits<uint64_t>::max()) {
+    return result;
+  }
+
+  // Increment the max so the range is inclusive of max
+  umax++;
+
+  // Powers of two are not biased
+  if ((umax & (umax - 1)) != 0) {
+    // Ceiling under which std::numeric_limits<uint64_t>::max() % max == 0
+    int64_t limit = std::numeric_limits<uint64_t>::max() -
+      (std::numeric_limits<uint64_t>::max() % umax) - 1;
+
+    // Discard numbers over the limit to avoid modulo bias
+    while (result > limit) {
+      if (!getRandomBytes(&result, sizeof(result))) {
+        SystemLib::throwExceptionObject(
+          "Could not gather sufficient random data");
+      }
+    }
+  }
+
+  return (int64_t)((result % umax) + min);
+}
+
+static struct RandomExtension final : public Extension {
+  RandomExtension() : Extension("random") {}
+  void moduleInit() override {
+    HHVM_FE(random_bytes);
+    HHVM_FE(random_int);
+    loadSystemlib();
+  }
+} s_random_extension;
+
+HHVM_GET_MODULE(random);
+
+///////////////////////////////////////////////////////////////////////////////
+}

hphp/runtime/ext/random/ext_random.php

@@ -0,0 +1,31 @@
+<?hh
+
+/**
+ * Generates cryptographically secure pseudo-random bytes that are suitable for
+ *   use in cryptography when generating salts, keys and initialization vectors.
+ *
+ * @param int $length - The length of the random string in bytes.
+ *
+ * @return string - The crypto-secure random bytes in binary format.
+ *
+ * @throws Exception - If generating sufficiently random data fails.
+ *
+ */
+<<__Native>>
+function random_bytes(int $length): string;
+
+/**
+ * Generates cryptographic random integers that are suitable for use where
+ *   unbiased results are critical (e.g. shuffling a Poker deck).
+ *
+ * @param int $min - The lowest value to be returned down to PHP_INT_MIN.
+ * @param int $max - The highest value to be returned up to PHP_INT_MAX.
+ *
+ * @return int - The crypto-secure random integer.
+ *
+ * @throws Exception - If generating sufficiently random data fails.
+ * @throws Error - If $min > $max.
+ *
+ */
+<<__Native>>
+function random_int(int $min, int $max): int;

hphp/test/slow/ext_random/random.php

@@ -0,0 +1,11 @@
+<?php
+
+var_dump(strlen(random_bytes(7)));
+var_dump(random_int(1, 10));
+
+// Astronomically low chance of a false positive here, and making sure we don't
+// accidentally return a constant value is worth it.
+var_dump(random_bytes(16) === random_bytes(16));
+var_dump(
+  random_int(PHP_INT_MIN, PHP_INT_MAX) === random_int(PHP_INT_MIN, PHP_INT_MAX)
+);

hphp/test/slow/ext_random/random.php.expectf

@@ -0,0 +1,4 @@
+int(7)
+int(%d)
+bool(false)
+bool(false)