I've always wondered how these are made. Does it run in some kind of virtualised container?
I did a few things to get this running:
- The application server and build server are separate, with access control to the build server controlled by an API key that I can revoke should anyone gain access to it.
- The process that compiles and runs your code is an unprivileged user.
- All import statements are disabled in user submitted code
- Foundation is imported selectively with only a few classes allowed to be used by submitted code (namely, the NSString, NSArray, NSDictionary, etc APIs)
- Code is checked for a list of blacklisted APIs before hitting the compiler
- The execution of your code has a timeout of a few seconds, and the process will be killed should it take any longer.
I looked into more advanced techniques for sandboxing but couldn't find anything on OS X that was feasible. Happy to be schooled on security though, if you know better than I do :)
That said, the best, most foolproof way of doing this really comes down to using OS X to virtualize OS X.
Or you could try cross-compiling. It seems to work here: https://github.com/swift-x/tools (which looks interesting...)
It's possible that by the end of this week, maybe next, it will be very easy indeed to compile and/or run Swift on Linux from Linux, at which point you could try LXC or a million other ways to virtualize.