denoland/rusty_v8

Rusty V8 Binding

V8 Version: 15.2.124.1

Goals

1. Provide high quality Rust bindings to V8's C++ API. The API should match the original API as closely as possible.

2. Do not introduce additional call overhead. (For example, previous attempts at Rust V8 bindings forced the use of Persistent handles.)

3. Do not rely on a binary libv8.a built outside of cargo. V8 is a very large project (over 600,000 lines of C++) which often takes 30 minutes to compile. Furthermore, V8 relies on Chromium's bespoke build system (gn + ninja) which is not easy to use outside of Chromium. For this reason many attempts to bind to V8 rely on pre-built binaries that are built separately from the binding itself. While this is simple, it makes upgrading V8 difficult, it makes CI difficult, it makes producing builds with different configurations difficult, and it is a security concern since binary blobs can hide malicious code. For this reason we believe it is imperative to build V8 from source code during "cargo build".

4. Publish the crate on crates.io and allow docs.rs to generate documentation. Due to the complexity and size of V8's build, this is nontrivial. For example the crate size must be kept under 10 MiB in order to publish.

Versioning

Rusty V8's major version aligns with Chrome's major version, which corresponds to a specific V8 release. For example, Rusty V8 129.0.0 maps to Chrome 129.x.y.z, which uses V8 12.9.a.b. While the minor and patch numbers between Chrome and V8 may differ, Rusty V8 will follow Chrome's release schedule, with a new major version every 4 weeks.

As a Rust crate, Rusty V8 follows semantic versioning (semver) and will not introduce breaking changes within a major version. However, major version bumps will occur regularly to stay in sync with Chrome's release cycle.

Binary Build

V8 is very large and takes a long time to compile. Many users will prefer to use a prebuilt version of V8. We publish static libs for every version of rusty v8 on Github.

Binary builds are the default: cargo build will initiate a download from github to get the static lib. To build V8 from source instead, set the V8_FROM_SOURCE environment variable to 1 (true and yes also work). Any other value, or leaving it unset, uses the prebuilt lib.

When making changes to rusty_v8 itself, it should be tested by build from source. The CI always builds from source.

The V8_FORCE_DEBUG environment variable

By default rusty_v8 will link against release builds of v8, if you want to use a debug build of v8 set V8_FORCE_DEBUG=true.

We default to release builds of v8 due to performance & CI reasons in deno.

The RUSTY_V8_MIRROR environment variable

Tells the build script where to get binary builds from. Understands http:// and https:// URLs, and file paths. The default is https://github.com/denoland/rusty_v8/releases.

File-based mirrors are good for using cached downloads. First, point the environment variable to a suitable location:

# you might want to add this to your .bashrc
$ export RUSTY_V8_MIRROR=$HOME/.cache/rusty_v8

Then populate the cache:

#!/bin/bash

# see https://github.com/denoland/rusty_v8/releases

for REL in v0.13.0 v0.12.0; do
  mkdir -p $RUSTY_V8_MIRROR/$REL
  for FILE in \
    librusty_v8_debug_x86_64-unknown-linux-gnu.a \
    librusty_v8_release_x86_64-unknown-linux-gnu.a \
  ; do
    if [ ! -f $RUSTY_V8_MIRROR/$REL/$FILE ]; then
      wget -O $RUSTY_V8_MIRROR/$REL/$FILE \
        https://github.com/denoland/rusty_v8/releases/download/$REL/$FILE
    fi
  done
done

The RUSTY_V8_ARCHIVE environment variable

Tell the build script to use a specific v8 library. This can be an URL or a path. This is useful when you have a prebuilt archive somewhere:

export RUSTY_V8_ARCHIVE=/path/to/custom_archive.a
cargo build

Build V8 from Source

Use V8_FROM_SOURCE=1 cargo build -vv to build the crate completely from source.

The build scripts require Python 3 to be available as python3 in your PATH. If you want to specify the exact binary of Python to use, you should use the PYTHON environment variable.

The build also requires curl to be installed on your system.

For linux builds: glib-2.0 development files need to be installed such that pkg-config can find them. On Ubuntu, run sudo apt install libglib2.0-dev to install them.

Additionally, building from source requires libclang 21.1+ for bindgen:

sudo apt install libclang-21-dev
export LIBCLANG_PATH=/usr/lib/llvm-21/lib

For Windows builds: the 64-bit toolchain needs to be used. 32-bit targets are not supported. The default source build downloads Chromium's pinned libclang automatically. If $CLANG_BASE_PATH is set to a custom LLVM installation, $LIBCLANG_PATH must point to the directory containing libclang.dll. The tools/win submodule is skipped because its standalone mirror is unreliable, so source builds must populate its pinned debugger visualizers:

mkdir -p tools/win
curl -fL https://chromium.googlesource.com/chromium/src/tools/win/+archive/faefd1b6fa9eeb033ad6fe60368ccb9bf908cbd0.tar.gz |
  tar -xz -C tools/win

For Mac builds: You'll need Xcode and Xcode CLT installed. Recent macOS versions will also require you to pass PYTHON=python3 because macOS no longer ships with python simlinked to Python 3.

For Android builds: You'll need to cross compile from a x86_64 host to the aarch64 or x64 android. You can use the following commands:

rustup target add aarch64-linux-android  # or x86_64-linux-android
V8_FROM_SOURCE=1 cargo build -vv --target aarch64-linux-android
# or with cross
docker build --build-arg CROSS_BASE_IMAGE=ghcr.io/cross-rs/aarch64-linux-android:0.2.5 -t cross-rusty_v8:aarch64-linux-android .
V8_FROM_SOURCE=1 cross build -vv --target aarch64-linux-android

For iOS builds: cross compile from an arm64 macOS host. The simulator target keeps the JIT; the device target (aarch64-apple-ios) is built jitless, since iOS denies the JIT entitlement to non-WebKit apps (WebAssembly is also disabled in this configuration). build.rs selects these settings automatically per target — no extra GN args required:

rustup target add aarch64-apple-ios-sim  # simulator
rustup target add aarch64-apple-ios      # device (jitless)
V8_FROM_SOURCE=1 cargo build -vv --target aarch64-apple-ios-sim

The build depends on several binary tools: gn, ninja and clang. The tools will automatically be downloaded, if they are not detected in the environment.

Specifying the $GN and $NINJA environmental variables can be used to skip the download of gn and ninja. The clang download can be skipped by setting $CLANG_BASE_PATH to the directory containing a llvm/clang installation. V8 is known to rely on bleeding edge features, so LLVM v8.0+ or Apple clang 11.0+ is recommended.

Arguments can be passed to gn by setting the $GN_ARGS environmental variable.

For Linux targets, rusty_v8 now defaults to defining V8_TLS_USED_IN_LIBRARY via GN args when building from source so the produced static archive can be linked into downstream cdylib/shared-library targets. The default injected argument is:

GN_ARGS='extra_cflags=["-DV8_TLS_USED_IN_LIBRARY"]'

Linux prebuilt release archives published by this repository are built with this shared-library-compatible TLS mode.

Env vars used in when building from source: SCCACHE, CCACHE, GN, NINJA, CLANG_BASE_PATH, GN_ARGS

FAQ

Building V8 takes over 30 minutes, this is too slow for me to use this crate. What should I do?

Install sccache or ccache. Our build scripts will detect and use them. Set the $SCCACHE or $CCACHE environmental variable if it's not in your path.

What are all these random directories for like build and buildtools are these really necessary?

In order to build V8 from source code, we must provide a certain directory structure with some git submodules from Chromium. We welcome any simplifications to the code base, but this is a structure we have found after many failed attempts that carefully balances the requirements of cargo crates and GN/Ninja.

V8 has a very large API with hundreds of methods. Why don't you automate the generation of this binding code?

We have actually started down this route several times, however due to many eccentric features of the V8 API, this has not proven successful. Therefore we are proceeding in a brute-force fashion for now, focusing on solving our stated goals first. We hope to auto-generate bindings in the future.

Why are you building this?

This is to support the Deno project. We previously have gotten away with a simpler high-level Rust binding to V8 called libdeno. But as Deno has matured we've found ourselves continually needing access to an increasing amount of V8's API in Rust.

When building I get unknown argument: '-gno-inline-line-tables'

Use export GN_ARGS="no_inline_line_tables=false" during build.

My program crashes when initializing on non-main thread

Initializing V8 on a non-main thread with the CPUs PKU feature enabled might lead to crashes. You can work around this problem by using v8::new_unprotected_default_platform.

See https://github.com/denoland/rusty_v8/issues/1381

Download cache

The v8 archives used for linking in prebuilt mode can be cached to avoid re-downloading archives when switching between branches that otherwise change the current rusty_v8 version.

To populate the cache by hand, you'll need to place the files in the appropriate location in your .cargo folder. Running cargo build -v -v will print two lines that you can use to determine the correct file and cache location:

[v8 0.87.0] static lib URL: https://github.com/denoland/rusty_v8/releases/download/v0.87.0/librusty_v8_release_aarch64-apple-darwin.a.gz
[v8 0.87.0] Looking for download in '"/Users/<name>/.cargo/.rusty_v8/https___github_com_denoland_rusty_v8_releases_download_v0_87_0_librusty_v8_release_aarch64_apple_darwin_a_gz"'

Given the above log output, use curl to download the file like so:

curl -L https://github.com/denoland/rusty_v8/releases/download/v0.87.0/librusty_v8_release_aarch64-apple-darwin.a.gz >
  /Users/<name>/.cargo/.rusty_v8/https___github_com_denoland_rusty_v8_releases_download_v0_87_0_librusty_v8_release_aarch64_apple_darwin_a_gz

For maintainers

Cut a release

Create a PR to bump the release version (e.g. https://github.com/denoland/rusty_v8/pull/1415).

Create a new release/tag after the bump PR is landed. CI will publish the crate and upload release binaries. You will need to manually upload binary archives for M1 build.

$ V8_FROM_SOURCE=1 cargo build
$ V8_FROM_SOURCE=1 cargo build --release

Experimental Features

rusty_v8 includes experimental support for certain feature(s) that may be useful in security focused contexts but are not as well tested and do not undergo any sort of CI related testing or prebuilt archives. Due to their experimental status, these features require either V8_FROM_SOURCE=1 to be set or the use of a custom-built archive of v8.

• v8_enable_sandbox: Enables v8 sandbox mode. The v8 sandbox enables improved safety while executing potentially malicious JavaScript code through the use of memory cages. Note that the v8 sandbox will allocate ~1TB of virtual memory (although this should not be an issue as many operating systems allow 128-256TB of virtual memory per process). Creating isolates with the sandbox enabled comes with API limitations and may have increased overhead. Note that enabling the V8 sandbox also implies pointer compression to be enabled as well.