VM: weird behaviour in runInContext/runInThisContext

[princejwesley]

While working on my electron based REPL application, I noticed a weird behaviour in vm.runInContext.

Program

Desktop 🙈 ₹ cat repl-vm-strict.js
'use strict';
const vm = require('vm');
const ctx = vm.createContext();

try {
  const result = vm.runInContext('"use strict"; x = 1', ctx);
} catch(e) {
  console.log(e.stack);
  console.log('x is', ctx.x)
}

Output

Desktop 🙈 ₹ node repl-vm-strict.js
ReferenceError: x is not defined
    at evalmachine.<anonymous>:1:17
    at Object.exports.runInContext (vm.js:44:17)
    at Object.<anonymous> (/Users/princejohnwesley/Desktop/repl-vm-strict.js:7:21)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:140:18)
    at node.js:1001:3
x is 1

Behaviour

• vm.runInThisContext/vm.runInContext throws error. ✅
• context is also updated as if variable is created with var or let binding ❌

Versions

Tested versions: v5.6.0 & v5.1.1

Platform

Desktop 🙈 ₹ uname -a
Darwin Princes-MacBook-Pro.local 15.3.0 Darwin Kernel Version 15.3.0: Thu Dec 10 18:40:58 PST 2015; root:xnu-3248.30.4~1/RELEASE_X86_64 x86_64

Subsystem

vm

repl with strict is affected by this issue.

[bnoordhuis]

I can confirm the bug but I suspect it's a V8 issue - it calls the global object's named property handlers (specifically the GlobalPropertySetterCallback) when it shouldn't. I'll have to investigate if it still happens with V8 HEAD.

[bnoordhuis]

Still happens with V8 4.10.253 and 5.0.48. I'll see if I can put a standalone test case together for upstream to take a look at.

[hashseed]

I talked to my colleagues. V8 seems to be working as intended in this case. I assume that there is an interceptor on the global object installed by node. The interceptor probably stores the value onto the global object, but returns false, signalling that it did not intercept. V8 then attempts to store and throws due to strict mode.

So the interceptor should either not store and return false, or throw if v8::PropertyCallbackInfo::ShouldThrowOnError() returns true.

[bnoordhuis]

@hashseed Yang, thanks for chiming in. Is there anything we can do with 4.6 and 4.8? They don't have the ShouldThrowOnError() method.

The best I've been able to come up with is to sniff the script for a 'use strict' string and omit the call to v8::PropertyCallback<T>::Set() in the setter for strict mode scripts if there is no corresponding property on the global object. Basically this:

Local<Object> sandbox = /* ... */;
if (!strict() || sandbox->Has(property)) {
  sandbox->Set(property, value);
  info.GetReturnValue().Set(value);
}

[hashseed]

Would this work for you? Whenever the interceptor is going to do a normal store, it does not store and returns false. V8 will then do the store, unless strict mode causes an exception before that.

[thefourtheye]

@bnoordhuis If the property is newly created, neither the sandbox object nor the global object will have the property defined on them when GlobalPropertySetterCallback is called. So, sandbox->Has(property) will fail and the new property will not be added to sandbox, right?