wadec is a decoder for WebAssembly modules, focusing on developer experience, informative errors and helpful diagnostics.

It can be used as a library or a command-line tool and is fully conforming to the specification.

This project is currently in early development stage of development. The API is unstable and subject to change.

wadec aims to serve as an educational tool for those looking to understand the Wasm binary format, or for developers debugging Wasm modules.

wadec does not aim to be a highly-efficient decoder for performance-critical tasks. For such use-cases, consider using a streaming decoder.

wadec is 100% conforming to the official WebAssembly specification, version 2. (Version 3 is on the roadmap.)

This is ensured by running the official specification test suite (wg-2.0 tag) against our decoder. Specifically, we go through all the test scripts (.wast) and ensure that:

✔️ all modules marked with assert_malformed are rejected by our decoder

✔️ all modules marked with assert_invalid are accepted (note: that 'invalid' modules are those rejected during the Validation phase, which implies that the Decoding phase has succeeded first.)

✔️ all modules marked with module are accepted

✔️ all modules marked with assert_unlinkable are accepted

The relevant tests can be foundat tests/z_spec.rs.

You can install the CLI using cargo:

$ cargo install wadec-cli

Note: We do not distribute binaries yet, but we may do so in the future.

To use the library, similarly:

$ cargo add wadec

# a module that encodes an invalid Value Type marker byte (0xAA) in its
# Type section
$ hexdump -C type_section_invalid_valtype_marker.wasm
00000000  00 61 73 6d 01 00 00 00  01 05 01 60 01 aa 00     |.asm.......`...|
0000000f

$ wadec --verbose type_section_invalid_valtype_marker.wasm
ERROR: failed decoding Type section

Caused by:
    0: failed parsing vector element at position 0
    1: failed decoding Parameters
    2: failed parsing vector element at position 0
    3: invalid ValType marker byte - expected one of 0x7F (Num(Int32)), 0x7E (Num(Int64)), 0x7D (Num(Float32)), 0x7C (Num(Float64)), 0x7B (Vec(V128)), 0x70 (Ref(Func)), 0x6F (Ref(Extern)); got 0xAA

DEBUG OUTPUT:
DecodeTypeSection(
    DecodeVector(
        ParseElement {
            position: 0,
            source: DecodeFuncTypeError::DecodeParameterTypes(
                DecodeVector(
                    ParseElement {
                        position: 0,
                        source: DecodeValTypeError::InvalidMarkerByte(
                            InvalidValTypeMarkerError(
                                0xAA,
                            ),
                        ),
                    },
                ),
            ),
        },
    ),
)

• Implement the Decoding phase for specification version 2
• Implement the Decoding phase for specification version 3
• Implement the Validation phase
• Optional support for WebAssembly extensions

wadec is licensed under the Apache 2.0 license.