buffer: backport --zero-fill-buffers command line option

This backports the --zero-fill-buffers command line flag introduced
in master. When used, all Buffer and SlowBuffer instances will zero
fill by default.

This does *not* backport any of the other Buffer API or behavior
changes.

doc/api/buffer.markdown

@@ -145,6 +145,23 @@ for (var b of buf)
 Additionally, the [`buf.values()`][], [`buf.keys()`][], and
 [`buf.entries()`][] methods can be used to create iterators.
 
+## The `--zero-fill-buffers` command line option
+
+Node.js can be started using the `--zero-fill-buffers` command line option to
+force all newly allocated `Buffer` and `SlowBuffer` instances created using
+either `new Buffer(size)` and `new SlowBuffer(size)` to be *automatically 
+zero-filled* upon creation. Use of this flag *changes the default behavior* of 
+these methods and *can have a significant impact* on performance. Use of the 
+`--zero-fill-buffers` option is recommended only when absolutely necessary to 
+enforce that newly allocated `Buffer` instances cannot contain potentially 
+sensitive data.
+
+```
+$ node --zero-fill-buffers
+> Buffer(5);
+<Buffer 00 00 00 00 00>
+```
+
 ## Class: Buffer
 
 The Buffer class is a global type for dealing with binary data directly.

doc/node.1

@@ -64,6 +64,9 @@ and servers.
 
   --track-heap-objects   track heap object allocations for heap snapshots
 
+  --zero-fill-buffers    Automatically zero-fills all newly allocated Buffer 
+                         and SlowBuffer instances.
+
   --prof-process         process v8 profiler output generated using --prof
 
   --v8-options           print v8 command line options

src/node.cc

@@ -942,7 +942,9 @@ Local<Value> WinapiErrnoException(Isolate* isolate,
 
 
 void* ArrayBufferAllocator::Allocate(size_t size) {
-  if (env_ == nullptr || !env_->array_buffer_allocator_info()->no_zero_fill())
+  if (env_ == nullptr ||
+      !env_->array_buffer_allocator_info()->no_zero_fill() ||
+      zero_fill_all_buffers)
     return calloc(size, 1);
   env_->array_buffer_allocator_info()->reset_fill_flag();
   return malloc(size);
@@ -3226,6 +3228,8 @@ static void PrintHelp() {
          "snapshots\n"
          "  --prof-process        process v8 profiler output generated\n"
          "                        using --prof\n"
+         "  --zero-fill-buffers   automatically zero-fill all newly allocated\n"
+         "                        Buffer and SlowBuffer instances\n"
          "  --v8-options          print v8 command line options\n"
 #if HAVE_OPENSSL
          "  --tls-cipher-list=val use an alternative default TLS cipher list\n"
@@ -3365,6 +3369,8 @@ static void ParseArgs(int* argc,
     } else if (strcmp(arg, "--prof-process") == 0) {
       prof_process = true;
       short_circuit = true;
+    } else if (strcmp(arg, "--zero-fill-buffers") == 0) {
+      zero_fill_all_buffers = true;
     } else if (strcmp(arg, "--v8-options") == 0) {
       new_v8_argv[new_v8_argc] = "--help";
       new_v8_argc += 1;

src/node_buffer.cc

@@ -48,7 +48,14 @@
   CHECK_NOT_OOB(end <= end_max);                                            \
   size_t length = end - start;
 
+#define BUFFER_MALLOC(length)                                               \
+  zero_fill_all_buffers ? calloc(length, 1) : malloc(length)
+
 namespace node {
+
+// if true, all Buffer and SlowBuffer instances will automatically zero-fill
+bool zero_fill_all_buffers = false;
+
 namespace Buffer {
 
 using v8::ArrayBuffer;
@@ -220,7 +227,7 @@ MaybeLocal<Object> New(Isolate* isolate,
   // nullptr for zero-sized allocation requests.  Normalize by always using
   // a nullptr.
   if (length > 0) {
-    data = static_cast<char*>(malloc(length));
+    data = static_cast<char*>(BUFFER_MALLOC(length));
 
     if (data == nullptr)
       return Local<Object>();
@@ -266,7 +273,7 @@ MaybeLocal<Object> New(Environment* env, size_t length) {
 
   void* data;
   if (length > 0) {
-    data = malloc(length);
+    data = BUFFER_MALLOC(length);
     if (data == nullptr)
       return Local<Object>();
   } else {

src/node_buffer.h

@@ -5,6 +5,9 @@
 #include "v8.h"
 
 namespace node {
+
+extern bool zero_fill_all_buffers;
+
 namespace Buffer {
 
 static const unsigned int kMaxLength =

test/parallel/test-buffer-zero-fill-cli.js

@@ -0,0 +1,30 @@
+'use strict';
+// Flags: --zero-fill-buffers
+
+// when using --zero-fill-buffers, every Buffer and SlowBuffer
+// instance must be zero filled upon creation
+
+require('../common');
+const SlowBuffer = require('buffer').SlowBuffer;
+const assert = require('assert');
+
+function isZeroFilled(buf) {
+  for (let n = 0; n < buf.length; n++)
+    if (buf[n] > 0) return false;
+  return true;
+}
+
+// This can be somewhat unreliable because the
+// allocated memory might just already happen to
+// contain all zeroes. The test is run multiple
+// times to improve the reliability.
+for (let i = 0; i < 50; i++) {
+  const bufs = [
+    SlowBuffer(20),
+    Buffer(20),
+    new SlowBuffer(20)
+  ];
+  for (const buf of bufs) {
+    assert(isZeroFilled(buf));
+  }
+}