Options to pass env vars from OCI Spec to WASI Context

[Mossaka]

Wasm containers are different from Linux containers in many ways, one of them being their handling of environment variables. A Wasm runtime, by default, does not grant any environment variables to Wasm modules unless explicitly passed to the WASI Context (e.g., using wasmtime's API). This contrasts with Linux containers, which can utilize all the environment variables available to them.

Given this difference, it seems unnecessary for libcontainer to modify the main process's environment variables before fork-exec container processes (as seen here) in Wasm use cases. Moreover, mutating environment variables before calling Executor::exec complicates the delineation between the Containerd shim and Wasm containers (see more in this issue from runwasi). The runwasi shim sets up the Wasm runtime (e.g., Spin, Wasmtime, WasmEdge, etc.) within the container namespace, but this setup is not part of the user's Wasm container code. Runwasi introduces a third layer, the Wasm runtime, between the traditional shim and Linux container layers.

We propose that container environment variables, such as those from the PodSpec in Kubernetes, should be directly passed to the WASI Context instead of the shim process, which is still configuring the Wasm runtime.

I am not sure how exactly we can achieve this, but I have shared some ideas in this issue. I welcome your thoughts on this matter.

[Mossaka]

It would be nice for libcontainer to provide an API to the Executor to set the environment variables.

Linux executor -> remains the same
Wasm executor -> environment variables should be passed to the execute and down to Wasm runtime's initialziation of the WASI context.

[utam0k]

Expanding Executor makes sense to me. But most cases are enough to clear the host's environments, aren't they? So, why don't we expand a new method in Executor related to handling environments for special cases, such as Wasm, and implement the default behavior to clear all ends on the host in the new method we expand?
https://doc.rust-lang.org/book/ch10-02-traits.html#default-implementations

[utam0k]

@Mossaka If you don't mind, I'd like to make this issue a good first issue.

[utam0k]

cc @musaprg

[musaprg]

Hi, I'd love to pick this up. Let me try.

[utam0k]

Sure!