Bytecode VMs

I came to be looking at different language VMs recently- Erlang BEAM and the Smalltalk VM. Smalltalk VM was a bytecode based stack VM (just like Java VM), and likely influenced the VM of Erlang. 

Java bytecode verification checks (and their vulnerabilities in bytecode loaders) are well known, but the security characteristics of other VMs and even languages running on JVM is less known (clojure/scala ?).

BEAM is the Erlang VM (Bogdan/Björn’s Erlang Abstract Machine). It is the successor of JAM (Joe’s Abstract Machine) which was inspired by Prolog WAM. Details on it are found in the Hitchhiker’s Tour of BEAM, with ways to crash the BEAM VM such as creating too many atoms (which never get deleted). BEAM can run on bare-metal on XEN – http://erlangonxen.org . The format is described as based on EA IFF 85 – Standard for Interchange Format Files with a FOR1 starting 4 bytes. Here’s the full 6k lines of beam_load.c .

The code-to-BEAM-bytecode processing pipeline is described here as including a preprocessing step (see -E, -S, -P options to erlc). An interesting problem of peeking and pattern-matching inflight messages is discussed here. I find it interesting to think what would happen if one froze an erlang VM to see all inflight messages – like putting a breakpoint in the kernel. The way to get a stacktrace is erlang:get_stacktrace()

The hot-swapping capability is worrying, coming from the objective-c world that had code-signing to lock down changes. Erlang does have a strong process isolation model.

This article on Dart makes the case for non-bytecode VMs with the following  line “When your VM’s input is program source code, it can rely on the grammar of the language itself to enforce important invariants”. Sounds relevant to byte-code VMs as well.

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