RFE: Expose module custom sections

[FGasper]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Module/customSections

^^ I may just be missing it, but I don't see an interface in wasmtime that corresponds to this one. Would it be possible to add this? (And also in the C API?)

Thank you!

[alexcrichton]

Sorry for the delay in responding here, but thanks for the report!

While wasmtime doesn't currently yet expose an API for this, you can pretty easily build one with the wasmparser crate:

use wasmparser::{Parser, Payload, Result};

fn custom_sections(bytes: &[u8]) -> impl Iterator<Item = Result<(&str, &[u8])>> {
    Parser::new(0).parse_all(bytes).filter_map(|payload| {
        let payload = match payload {
            Ok(s) => s,
            Err(e) => return Some(Err(e)),
        };
        match payload {
            Payload::CustomSection { name, data, .. } => Some(Ok((name, data))),
            _ => None,
        }
    })
}

Would that work for your purposes? Or is there a reason you need an API added to wasmtime itself?

[FGasper]

Is wasmparser available from C? I’m using this from Perl so would need something I can call from C.

Thank you!

FYI, there’s also discussion of adding such an interface to the standard C API:
WebAssembly/wasm-c-api#168 (comment)

[alexcrichton]

There is not currently a C API for wasmparser, so if you don't want to write Rust code then this will need to wait until there's a C API binding for this in Wasmtime's C API.

[ajihyf]

Sorry for the delay in responding here, but thanks for the report!

While wasmtime doesn't currently yet expose an API for this, you can pretty easily build one with the wasmparser crate:

use wasmparser::{Parser, Payload, Result};

fn custom_sections(bytes: &[u8]) -> impl Iterator<Item = Result<(&str, &[u8])>> {
    Parser::new(0).parse_all(bytes).filter_map(|payload| {
        let payload = match payload {
            Ok(s) => s,
            Err(e) => return Some(Err(e)),
        };
        match payload {
            Payload::CustomSection { name, data, .. } => Some(Ok((name, data))),
            _ => None,
        }
    })
}

Would that work for your purposes? Or is there a reason you need an API added to wasmtime itself?

Hi, this approach only accepts raw wasm input. Is it possible to get custom sections from a module which may be deserialized from compiled artifact?

[bjorn3]

I don't think we preserve custom sections in precompiled modules.

[ajihyf]

I'm using dynamic linking to run multiple module instances with the same linear memory. The custom section naming "dylink.0" is required to provide proper imports to instantiate the modules. It seems that wasmer supports custom section query API like JavaScript, which works both in raw and compiled modules. Maybe wasmtime should preserve the custom sections, too?

[alexcrichton]

To confirm, precompiled artifacts don't store custom sections so this API would not be possible. I'd recommend extracting the custom section and saving it adjacent or next to wasmtime's compiled artifact.

[stevefan1999-personal]

This is needed for Den now. For now I have to mask it with not implemented...pity