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Explore sandboxed build scripts #108

Open nikomatsakis opened 4 months ago

nikomatsakis commented 4 months ago
Metadata
Owner(s) @weihanglo
Team(s) cargo, compiler
Goal document 2024h2/sandboxed-build-script

Summary

Explore different strategies for sandboxing build script executions in Cargo.

Tasks and status

nikomatsakis commented 4 months ago

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

weihanglo commented 4 months ago

Key developments:

Have been looking into different sandbox runtime choices. Here is a simple version of the comparison of three potential choices:

There are prior research on cross-over between each of these options. I've been busy these two weeks. Will update a more detail post for prior arts afterward.

The biggest challenge I am seeing now is spawning external processes. Most build script usages invoke some external binaries, like pkg-config for building *-sys crates, or protoc for generating protobuf bindings. If process spawning is that common, we need to find a way to provide a fine-grained permission granting scheme. I don't want it to see an “all-or-nothing” mechanism when process spawning is needed.

The other huge headache is setting library search paths. We cannot know every possible path of system libraries ahead of time, but we need to grant access to the runtime.

Blockers:

None.

Help wanted:

None.

weihanglo commented 2 months ago

Having a family urgency. I will be back after RustConf.

weihanglo commented 2 months ago

Key developments:

Building a workable version of wasm-based build script (not yet done). There are some technical difficulties. Not blocking but need to be addressed.

In order to make the development independent of Cargo, RUSTC_WRAPPER was the first approach I tried, though it still lacks some extension point in Cargo:

Some compatibility issues came up when integrating with Cargo:

Blockers:

None.

Help wanted:

None.

weihanglo commented 1 month ago

Have a working-in-process pull request in https://github.com/weihanglo/cargo/pull/66.

Let me copy some texts from there :)

What did we achieve in this experiment?

As you can see, we can easily swap to any sandbox runner with a custom target.

We use wasm32-wasip1 above as an example, as it is the one with smaller footprint, very cross-platform, and pretty popular in the Rust community. However, it turns out that `wasm32-wasip1 doesn't support POSIX process spawning. Use cases of process spawning in build scripts are essential, such as

According to the design axioms,

Restrcting process spawning is the top one axiom. We made that with wasm32-wasip1, but have no way to opt-out. This contradicts to the "ensuring -sys crates be built" axiom.

As a result, it is unlikely to use wasm32-wasip1 as a default sandbox environment with this experiment.

Other possibilities

Have talked to some other folks, there are some potential route we could take if we chose wasi as a default sandbox environment.

The offical build-rs crate to the rescue

The Cargo team recently adopted the build-rs crate. It is going to be the official crate providing API for writing build scripts. We could take the advantage of it, telling everyone instead of using std::process::Command, use something like build_rs::Command. So that Cargo could have a full control over how a build script spawning processes.

While it sounds ideal, this doesn't help the current situation because

A Cargo-flavored wasi standard library

There was a discussion in the GSoC "sandboxed proc-macro with wasm" about shipping a custom verion of the standard library for sandboxed wasm. For Cargo's build script, we could potentially ship a wasm32-wasi-cargo target. The std in this custom target could intercept any exec call or process spawning. Then it calls back to the host process (which is Cargo in our case) to determine how to handle process execution. The host process could either reject, or run the external program and post back the result.

This idea sounds pretty hacky and need more investigations of the communication mechanism between the host process and the wasm runtime. Perhaps via sockets, The WebAssembly Component Model, other host function call mechanism. There is also WASIX project which supprots fork/exec though it is currently not a WASI standard not even a proposal.

Continue with other more mature sandbox runtime choices

Since one of major design space is the user interafce of sandbox configuration, we could leave off sandbox runtimes and explore more on the configuration side.

We could, for example, use docker or eBPF as a temporary default runtime, and explore how the configuration should look like. We may want to take a look at the configuration of Cackle-rs as a starting point. By doing so, we wouldn't block on waiting for wasm runtime to being more mature.

weihanglo commented 1 day ago

Key Developments

Unfortunately, no significant progress has been made. However, we received feedback on the previous experiment: https://github.com/weihanglo/cargo/pull/66.

Alex, who has been working on WebAssembly and is also an honorable Rust project contributor, expressed interest in adding POSIX process support to WASI. However, according to their comments, this seems unlikely in the near future. While the experiment with a Cargo-flavored WASI standard library remains incomplete, it is still on my to-do list. At the very least, I want to give it a try and see how gnarly it would be.

In this comment, Ralf pointed out that supporting process spawning may be somewhat pointless, as one of WASI's key selling points is avoiding the execution of external processes altogether. To me, this feels more like a UX design issue. For example, VS Code prompts users to "trust" a project when it is opened, although most users simply click "trust." With better UX design—such as interactive shell sessions for crate approvals or pre-approval lists from a company (similar to how cargo-vet operates)—an opt-in mechanism would be a better default than allowing everything by default.

Samuel shared a project called build-wrap, which leverages Bubblewrap. While not cross-platform, the configuration interface of Bubblewrap is inspiring and worth exploring, particularly for how it sandboxes system libraries and binaries.

Jeff suggested using the WebAssembly Component Model to allow interaction with a set of well-known programs (e.g., pkg-config and cc), with the host exposing an interface via Wasm components to build scripts. This would enable build scripts to be written in plain Rust while aligning with the use cases of the Wasm Component Model. This idea is particularly promising after the integration of build-rs into Cargo as a workspace member. While build-rs is a lower-level crate, it opens doors to integrating future build script helpers or interfaces.

Not Rolling Over to 2025H1

Despite the valuable feedback and interest, after discussions with the Cargo team, this initiative will not continue as a project goal for 2025H1. Instead, there are alternative approaches worth exploring that may require less effort or depend less on parallel developments in other technologies:

We want to explore various possibilities beyond the Wasm sandbox environment. That said, as mentioned earlier, personally I am still interested in experimenting further with Wasm. Sorry for not making more progress for the past month.