[lucet usage example] error with lucet-runtime

[lucifer-cpu]

error with lucet runtime

let module = DlModule::load("output.so").unwrap();
let region = MmapRegion::create(1, &Limits::default()).unwrap();

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: DlError(Custom { kind: Other, error: "output.so: undefined symbol: __wasi_fd_prestat_get" })'

[pchickey]

It looks like the lucet-wasi host functions didn't get linked into the executable you made. There are two things required to ensure that:

1. Call the runtime and lucet-wasi ensure_linked functions somewhere in your executable. The calls don't actually do anything, so putting them at the beginning of main is fine. Just has to be in reachable code.
   https://github.com/fastly/lucet/blob/6ae5a401bfd2f3f9d9160623fd7c1312ef312de9/lucet-wasi/src/main.rs#L37-L40

2. You need to configure the linker to make all symbols available to the dynamic linker. The usual way to do this is with a .cargo/config file with the following flags:

[build]
rustflags = ["-C", "link-args=-rdynamic"]

[lucifer-cpu]

Do you have any examples of any complete project written in c that runs with this runtime?

[jedisct1]

Can you run it with lucet-wasi?

[lucifer-cpu]

[awortman-fastly]

are output.so and qjs.so above both the same file?

[awortman-fastly]

The next thing I'd suggest trying is seeing if you can run lucet-wasi qjs.so to double check that both work the same. For the lucet_module_data error, you can verify that the symbol does or doesn't exist with objdump - you can do something like this:

> objdump -x a.out | grep lucet_module_data
0000000000201000 l     O .data	0000000000001177 lucet_module_data

lucet_module_data will be present in artifacts from lucetc, and must be present for for DlModule to understand how to load the file. The undefined symbol: lucet_module_data error you saw is usually an indication that the file that it tried loading didn't come from lucetc.

[lucifer-cpu]

objdump -x a.out | grep lucet_module_data 

00000000005eceb0 l     O .data	0000000000024ed2 lucet_module_data

[jedisct1]

Here's an example of how to run a compiled WebAssembly module using Lucet:

use lucet_runtime::*;
use lucet_wasi::*;

fn main() {
    lucet_wasi::hostcalls::ensure_linked();
    let dl_module = DlModule::load("a.out").unwrap();
    let region = MmapRegion::create(1, &Limits::default()).unwrap();
    let mut instance = region.new_instance(dl_module).unwrap();
    let wasi_ctx = WasiCtx::new(&[]);
    instance.insert_embed_ctx(wasi_ctx);
    instance.run("entrypoint", &[]).unwrap();
}

[lucifer-cpu]

thanks but I always get the same result

IncorrectModule("error loading required symbol `lucet_module_data`: a.out: undefined symbol: lucet_module_data"))', src/libcore/result.rs:999:5

[lucifer-cpu]

https://github.com/lucifer-cpu/lucet-demo

[awortman-fastly]

okay, I think we have an answer about the lucet_module_data issues - we don't have stability guarantees about artifacts from lucetc at the moment. They'll often work with mismatched lucetc and lucet-runtime versions, but the only thing we can always guarantee will work right now is that a lucetc and lucet-runtime built together will behave nicely.

This is important because your a.out is built from a newer lucetc than the 0.1.1 version of lucet-runtime that's available on crates.io. If you got lucetc by cloning the repository and building the latest version, you'll want to use lucet-runtime and lucet-wasi from git as well with something like this in your Cargo.toml:

lucet-runtime = { version = "*", path = "/path/to/lucet-runtime/" }
lucet-wasi = { version = "*", path = "/path/to/lucet-wasi/" }

The other, maybe easier, option is to get lucetc from crates.io just like the lucet-runtime and lucet-wasi crates: cargo install lucetc. Then you should be able to build a wasm module with lucetc and see it work correctly.

The fact that we're not detecting a version mismatch and giving a clear error is good to know, and we'll have to do something about that!

[lucifer-cpu]

@awortman-fastly @jedisct1 It works, thanks for your patience and help.