fs: don't throw in read if buffer too big

lib/fs.js

@@ -599,10 +599,13 @@ fs.read = function(fd, buffer, offset, length, position, callback) {
 
     callback = function(err, bytesRead) {
       if (!cb) return;
+      if (err) return cb(err);
 
-      var str = (bytesRead > 0) ? buffer.toString(encoding, 0, bytesRead) : '';
-
-      (cb)(err, str, bytesRead);
+      if (bytesRead > 0) {
+        tryToStringWithEnd(buffer, encoding, bytesRead, cb);
+      } else {
+        (cb)(err, '', bytesRead);
+      }
     };
   }
 
@@ -617,6 +620,16 @@ fs.read = function(fd, buffer, offset, length, position, callback) {
   binding.read(fd, buffer, offset, length, position, req);
 };
 
+function tryToStringWithEnd(buf, encoding, end, callback) {
+  var e;
+  try {
+    buf = buf.toString(encoding, 0, end);
+  } catch (err) {
+    e = err;
+  }
+  callback(e, buf, end);
+}
+
 fs.readSync = function(fd, buffer, offset, length, position) {
   var legacy = false;
   var encoding;

test/parallel/test-fs-read-buffer-tostring-fail.js

@@ -0,0 +1,58 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+const Buffer = require('buffer').Buffer;
+const kStringMaxLength = process.binding('buffer').kStringMaxLength;
+const kMaxLength = process.binding('buffer').kMaxLength;
+
+var fd;
+
+common.refreshTmpDir();
+
+const file = path.join(common.tmpDir, 'toobig2.txt');
+const stream = fs.createWriteStream(file, {
+  flags: 'a'
+});
+
+const size = kStringMaxLength / 200;
+const a = new Buffer(size).fill('a');
+
+for (var i = 0; i < 201; i++) {
+  stream.write(a);
+}
+
+stream.end();
+stream.on('finish', common.mustCall(function() {
+  fd = fs.openSync(file, 'r');
+  fs.read(fd, kStringMaxLength + 1, 0, 'utf8', common.mustCall(function(err) {
+    assert.ok(err instanceof Error);
+    assert.strictEqual('toString failed', err.message);
+  }));
+}));
+
+function destroy() {
+  try {
+    // Make sure we close fd and unlink the file
+    fs.closeSync(fd);
+    fs.unlinkSync(file);
+  } catch (err) {
+    // it may not exist
+  }
+}
+
+process.on('exit', destroy);
+
+process.on('SIGINT', function() {
+  destroy();
+  process.exit();
+});
+
+// To make sure we don't leave a very large file
+// on test machines in the event this test fails.
+process.on('uncaughtException', function(err) {
+  destroy();
+  throw err;
+});