Monday 3 March 2014

Be strict with me, dear CLI

There is an old adage in code:

Be liberal in what you accept, and conservative in what you send

And right now, this adage can [obscenity]. The week before last, my main gripe was with broken browser implementations (not behind proxy servers) that clearly didn’t read RFC 6455 (aka WebSocket) – or at least, they read it enough to know to tend a Sec-WebSocket-Key, but not enough to understand client-to-server masking, or even simply to add a Connection or Upgrade header. But that’s not what I’m moaning about today; last week, my gripe was .NET. Specifically, IL emit.

(wibbly-wobbly screen-fade going to backstory)

A good number of the tools I work on involve metaprogramming, some of them to quite a scary degree. Which means lots of keeping track of the state of the stack as you bounce around the IL you are writing. And to be fair, yes it is my responsibility to make sure that the IL I emit is meaningful.. I just wish that the CLI was more consistent in terms of what it will allow.

You see, there’s an interesting rule in the CLI specification:

In particular, if that single-pass analysis arrives at an instruction, call it location X, that immediately follows an unconditional branch, and where X is not the target of an earlier branch instruction, then the state of the evaluation stack at X, clearly, cannot be derived from existing information. In this case, the CLI demands that the evaluation stack at X be empty.

The scenario this describes is actually very common – for example in a while loop (which is also the core of foreach), the most common way of setting those out is:

  • (preceding code)
  • unconditional-branch: {condition test}
  • label: {the actual code}
  • (the actual code)
  • label: {condition test}
  • (condition test)
  • branch-if-true: {the actual code}
  • (subsequent code)

The important point is that “{the actual code}” meets this special case; it is precisely the X mentioned in the specification: it immediately follows an unconditional branch, and is not the target of an earlier branch instruction (it is, however, the target of a later branch instruction). This means that to be valid, the stack (relative to that method) must be empty.

This would actually be easy enough to smoke-test… just setup some simple IL that forces this condition, press the button, and wait for the CLI to complain about it. Here’s the C# for that, on The only problem is that it runs without complaining. Well, maybe DynamicMethod is a special case… we’ll try a full assembly instead. And again: it works without the slightest complaint. To get it to notice, we need to write it to disk (assembly.Save("Broken.dll");) and then run peverify Broken.dll, which finally gives us the complaint we wanted:

[IL]: Error: [Broken.dll : BrokenType::BrokenMethod][offset 0x00000002] Stack height at all points must be determinable in a single forward scan of IL.
1 Error(s) Verifying Broken.dll

You might think I’m being fussy… I mean, if the CLI runs it anyway then what is the problem? A fair question, but the truth is more complicated. When the CLI is loading an assembly from disk it is often more strict. This depends on a lot of things, including the framework version, the framework platform, and the trust settings.

Oh for a simple “strict mode” for running in-memory dynamic assemblies: that would make it so much easier for us long-suffering metaprogrammers. And I’m not alone here: a quick google search shows this issue has bitten mono, ikvm, mono-cecil, and many others.

A silver lining…

The good news is that the excellent Sigil utility (by my colleague Kevin Montrose) now has support for this, via the strictBranchVerification flag. It also makes the IL emit a lot easier to grok – here’s the same example via Sigil. Sadly, I can’t use it in my particular scenario, unless I create a custom Sigil build that uses IKVM for cross-platform emit, but for most people this should help a lot.