wag

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Wag is a WebAssembly compiler implemented as a Go package.

• License: 3-clause BSD
• Author: Timo Savola [email protected]

Features

• The input is a wasm binary module.

• The output is machine code.

• It is only a compiler. A runtime environment for the compiled program, including all import functions, needs to be implemented separately. Wag has been developed for the Gate runtime. See wasys for a simple example runtime.

• Single-pass, fast ahead-of-time compilation. Early functions can be executed while the latter functions are still being compiled, even while the source is still being downloaded.

• The generated code requires minimal runtime support; it's designed to be executed in an isolated environment. Calling standard library ABIs is not directly supported, but see wasys for an example which exposes syscalls as WebAssembly import functions.

• Supports snapshot-and-restore across compiler versions and CPU architectures.

• Supports breakpoint debugging via recompilation.

• Cross-compilation is supported via Go build tags. If wagamd64 is specified, the x86-64 code generator is used regardless of host architecture, and CPU feature detection is disabled with pessimistic assumptions. Likewise for wagarm64 (but feature detection is not used for ARM64).

Status

• Supports WebAssembly version 1 (wasm32). No wasm extensions are supported.

• Supports x86-64 and ARM64 code generation.

• Generated x86-64 code requires SSE4.1 floating-point instructions (available since 2007).

Security

Spectre variant 1: Out-of-bounds linear memory access detection requires that addressable but unallocated memory is inaccessible. It naturally prevents conditional branch exploitation.

Spectre variant 2: On x86-64, Retpoline is used to protect the runtime environment (although user programs shouldn't be able to inject arbitrary addresses into the branch target buffer).

Testing

Requires Linux, Make, Go, Python, Capstone, and a recent version of WABT. The applicable parts of the WebAssembly spec testsuite are run^*. Code execution tests are implemented in a separate Go module in the testsuite subdirectory (to work around circular dependencies). All tests can be run by checking out Git submodules and running make check.

Screenshot

$ make
$ bin/wasys -v testdata/hello.wasm
import write(i32, i32, i32) i32
import openat(i32, i32, i32, i32) i32
import read(i32, i32, i32) i32
import close(i32) i32
import pipe2(i32, i32) i32
import _exit(i32)
hello, world