This was a great read! Really cool to see someone dive deep into WebAssembly by building an interpreter from scratch.
Asd
Test 6
I Wrote a WebAssembly VM in C
91–100 of 104 posts
Re: I Wrote a WebAssembly VM in C
#92Re: I Wrote a WebAssembly VM in C
#93Re: I Wrote a WebAssembly VM in C
#94Re: I Wrote a WebAssembly VM in C
#95Earlier quoted context omitted.
A JIT should be able to translate most arithmetic and binary instructions to single-opcodes, however anything involving memory and functions calls needs safety checks that becomes multi-instruction. branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops (if it also wants to be crash-safe even if it's exploit safe).
> anything involving memory [..] needs safety checks that becomes multi-instruction Not necessarily; on AMD64 you can do memory accesses in a single instruction relatively easily by using the CPU's paging machinery for safety checks plus some clever use of address space. > branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops Even with metering the branches wo…
Can't remember exactly what one but I remember reading an article about some VM that added interruption checks not at block boundaries, but rather only at _backwards_ branches and call-sites, so "safe" forward jumps (if/else/break) wouldn't cost anything extra but anything that could go on "forever" had the checks.
Reserving 4gb address space oughta work on any 64bit machine with a decent OS/paging system though? I was looking into it but couldn't use it in my case however since it needs to cooperate with another VM that already hooks the PF handler (although maybe I should take another stab if there is a way to put in a hierarhcy).
Re: I Wrote a WebAssembly VM in C
#96Earlier quoted context omitted.
Metering also doesn't require a branch if you implement it with page faults. See "Implicit suspend checks" in https://android-developers.googleblog.com/2023/11/the-secret...
Yep. That's a nice trick; unfortunately it's non-deterministic.
How's it nondeterministic?
Re: I Wrote a WebAssembly VM in C
#97Re: I Wrote a WebAssembly VM in C
#98This is great! The WebAssembly Core Specification is actually quite readable, although some of the language can be a bit intimidating if you're not used to reading programming language papers. If anyone is looking for a slightly more accessible way to learn WebAssembly, you might enjoy WebAssembly from the Ground Up: https://wasmgroundup.com (Disclaimer: I'm one of the authors)
I know one of WebAssembly's biggest features by design is security / "sandbox". But I've always gotten confused with... it is secure because by default it can't do much. I don't quite understand how to view WebAssembly. You write in one language, it compiles things like basic math (nothing with network or filesystem) to another and it runs in an interpreter. I feel like I have a severe lack/misunderstanding. There's…
It's like JavaScript, Python or PHP. There are no pointers, only arrays, that cause the app to crash if it attempts to read out of the bounds.
Re: I Wrote a WebAssembly VM in C
#99[deleted]
Re: I Wrote a WebAssembly VM in C
#100This was a great read! Really cool to see someone dive deep into WebAssembly by building an interpreter from scratch.
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