Thursday, 19 June 2014

SNK, we need to talk…

Because the world needs another rant about SNK and NuGet

In .NET assemblies, strong names are an optional lightweight signing mechanism that provides identity, including versioning support. Part of the idea here is that a calling assembly can have a pretty good idea that what it asks for is what it gets – it asks for Foo.Bar with key {blah}, version “1.2.3”, and it gets exactly that from the GAC, and the world is rosy. If somebody installs a new additional version of Foo.Bar, or a Foo.Bar from a different author, the old code still gets the dll it wanted, happy in the knowledge that the identity is reliable.
There are only a few problems with this:
  • Most applications don’t use the GAC; the only times people generally “choose” to use the GAC is when their list of options had exactly one option: “use the GAC”. Sharepoint and COM+, I’m looking at you and judging you harshly
  • Actually, there’s a second category of this: people who use strong names because that is what their corporate policy says they must do, with some some well-meaning but completely misguided and incorrect notion that this provides some kind of security. Strong naming is not a security feature. You are just making work and issues for yourself; seriously
  • It doesn’t actually guarantee the version: binding redirect configuration options (just in an xml file in your application) allow for a different version (with the same key) to be provided
  • It doesn’t actually guarantee the integrity of the dll: if somebody has enough access to your computer that they have access to the GAC, they also have enough access to configure .NET to skip assembly identity checking for that dll (just a “snk –Vr {assembly}” away)
And of course, it introduces a range of problems:
  • Versioning becomes a huge pain the backside for all downstream callers, who now need to manage the binding redirect configuration every time a dll gets upgraded anywhere (there are some tools that can help with this, but it isn’t perfect)
  • A strong-named assembly can only reference other strong-named assemblies
  • You now have all sorts of key management issues over your key file (despite the fact that it is pointless and can be bypassed, as already mentioned)


Assembly management versus package management

Now enter package management, i.e. NuGet and kin (note: I’m only using NuGet as the example here because it is particularly convenient and readily available to .NET developers; other package management tools exist, each with strengths and weaknesses). Here we have a tool that clearly targets the way 95% of the .NET world actually works:
  • no strong name; we could not care less (or for the Americans: we could care less)
  • no GAC: libraries deployed alongside the application for per-application isolation and deployment convenience (this is especially useful for web-farms, where we just want to robocopy the files out)
  • versioning managed by the package management tool
So, as a library author, it is hugely tempting to simply ignore strong naming, and simply put assemblies without a strong name onto NuGet. And that is exactly what I have done with StackExchange.Redis: it has no strong name. And for most people, that is fine. But now I get repeated calls and emails from people saying “please can you strong name it”.
The argument for and against strong-naming in NuGet is very verbose; there are threads with hundreds of messages for and against – both with valid points. There is no simple answer here.
  • if it is strong named, I introduce the problems already mentioned – when for 95% (number totally invented, note) of the people using it, this is simply not an issue
  • if it isn’t strong named, people doing Sharepoint development, or COM+ development, or just with awkward local policies cannot use it – at least not conveniently
They can of course self-sign locally, and there are tools to help with that – including Nivot.StrongNaming. But this only helps for immediate references: any such change will be fatal to indirect references. You can’t use binding redirects to change the identity of an assembly – except for the version.
I could start signing before deployment, but that would be a breaking change. So I’d have to at a minimum do a major version release. Again, direct references will be fine – just update the package and it works – but indirect references are still completely toast, with no way of fixing them except to recompile the intermediate assembly against the new identity. Not ideal.

I’m torn

In some ways, it is tempting to say “screw it, I need to add a strong name so that the tiny number of people bound by strong naming can use it”, but that is also saying “I need to totally and irreconcilably break all indirect references, to add zero functionality and despite the fact that it was working fine”, and also “I actively want to introduce binding redirect problems for users who currently don’t have any issues whatsoever”.
This is not an easy place to be. Frankly, at this stage I’m also not sure I want to be adding implicit support to the problems that SNK introduce by adding a strong name.

But what if…

Imagineering is fun. Let’s be realistic and suppose that there is nothing we can do to get those systems away from strong names, and that we can’t change the strong-named-can-only-reference-strong-named infection. But let’s also assume we don’t want to keep complicating package management by adding them by default. Why can’t we let this all just work. Or at least, that maybe our package management tools could fix it all. It seems to me that we would only need two things:
  • assembly binding redirects that allow unsigned assemblies to be forwarded to signed assemblies
  • some inbuilt well-known publicly available key that the package management tools could use to self-sign assemblies
For example, imagine that you have your signed application. and you use NuGet to add a package reference to Microsoft.Web.RedisSessionStateProvider, which in turn references StackExchange.Redis. In my imaginary world, NuGet would detect that the local project is signed, and these aren’t – so it would self-sign them with the well-known key, and add an assembly-binding redirect from “StackExchange.Redis” to “StackExchange.Redis with key hash and version”. Important note: the well-known key here is not being used to assert any particular authorship etc – it is simply “this is what we got; make it work”. There’s no need to protect the private key of that.
The major wrinkle in this, of course, is that it would require .NET changes to the fusion loader, in order to allow a binding redirect that doesn’t currently exist. But seriously: haven’t we been having this debate for long enough now? Isn’t it time the .NET framework started helping us with this? If I could request a single vNext CLR feature: this would be it.
Because I am so very tired of having this whole conversation, after a decade of it.
There are probably huge holes in my reasoning here, and reasons why it isn’t a simple thing to change. But: this change, or something like it, is so very very overdue.

And for now…

Do I add a strong name to StackExchange.Redis? Local experiments have shown me that whatever I do: somebody gets screwed, and no matter what I do: I’m going to get yelled at by someone. But I’m open to people’s thoughts and suggestions.