C++ _can_ be written safely, as long as the right static and dynamic analysers are used. So while I wouldn't recommend it for _many_ new projects these days, the WasmEdge authors have good reasons to choose C++ and appear to be using it responsibly.
Also, there are so many things that need to be done right for a VM specifically to actually be safe that merely being written in a safer language is lightyears away from being adequate. You _still_ need very careful design, dynamic analysers, and a whole lot of fuzzing if you want to end up with something you can reasonably trust. So if it were written in Rust as Wasmtime is, that alone wouldn't actually give me much more confidence that it's safe to use it on untrusted inputs.
I say all this as a huge advocate for Rust in general. Sometimes the less attractive tool is actually a better choice.
One can do lots of things. Yes all of those things are still issues, but in enumerating all the things as you have, you make a compelling argument not to use C++. If one picks something else like SML, Go or even Rust, it allows them to focus on those common areas of correctness and not also, all that other stuff.
I don't think you are apologizing for C++, but I hear this a lot, whatabout all the other parts that are fixed by a more sound system? My response is that we only have so many decisions we can make in the day, using a system that removes whole classes of problems allows me to focus on the non-accidental (forced) complexity.
> Rust as Wasmtime is, that alone wouldn't actually give me much more confidence that it's safe
I never made that argument. But let's say I did. Given two systems, one made in Rust and one made in C++. How do I audit the C++ code to ensure that it has the same level of safety as basically _anything_ else?
No application of control flow guard [2], either in the build or in the generated LLVM output.
Infact the only sanitization flags they are passing are -fsanitize=fuzzer,address
0 mentions of 'control flow' in the WasmEdge codebase, 65 mentions in Wasmtime. Although it doesn't appear that Wasmtime is itself being compiled with cfguard.
309 mention of spectre mitigations in Wasmtime, 1 link to a cloudflare blogpost in WasmEdge.
In my cursor look at Wasmtime, I see
Much better fuzzing support, 85 uses of unsafe in cranelift itself, 1400+ uses of unsafe in crates/*
The changelog and git messages for WasmEdge talk about type errors (those type errors look impossible in Rust), bus errors, out of bounds, etc.
It does not inspire confidence. C++ is already starting from behind. This software is targeted to be run on bare metal, embedded and widely distributed. It just doesn't have the level of detail around testing and correctness that I would expect for such a critical piece of infrastructure.
I am not saying that Wasmtime does not also have issues, that would be preposterous. But on the face of it, WasmEdge has a lot of work to do to catch up to where Wasmtime already is.
Thank you for your detailed response. You've looked a lot more closely at what WasmEdge is doing than I did, and I find myself agreeing with you more and more as I read your findings. (I don't know how I got the impression that their tooling use was more thorough than it actually is, but long story short: I was wrong.)
I'm left with one difficult question, though. What should we do when trying to target environments that aren't well-served by languages like Rust? The demand is clearly there, so people will keep building new things using C and C++. What do you see as the most realistic path forward for these use cases? Push harder on platform support in Rust?
Also, there are so many things that need to be done right for a VM specifically to actually be safe that merely being written in a safer language is lightyears away from being adequate. You _still_ need very careful design, dynamic analysers, and a whole lot of fuzzing if you want to end up with something you can reasonably trust. So if it were written in Rust as Wasmtime is, that alone wouldn't actually give me much more confidence that it's safe to use it on untrusted inputs.
I say all this as a huge advocate for Rust in general. Sometimes the less attractive tool is actually a better choice.