Document WebAssembly workload format

Fixes enarx-archive#17.

00001-workload-package-format/README.md

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+# workload-package-format
+- Authors: Daiki Ueno
+- Status: [PROPOSED](/README.md#proposed)
+- Since: 2020-05-19
+- Status Note: Under discussion
+- Supersedes: None
+- Start Date: 2020-05-19
+- Tags: feature, protocol
+
+## Summary
+
+Typical applications are accompanied with read-only resources used at
+run time, such as configuration files or precomputed binary data. This
+RFC proposes a way to provision those resources along with the
+application executable into a Keep.
+
+## Motivation
+
+Unlike normal applications running on the host, an application
+deployed in a Keep must be careful about accessing external resources,
+so that it doesn't expose any confidential information to the host. On
+the other hand, applications practically require external resources to
+achieve their tasks. There needs to be a way to provision such
+resources into a Keep so that the applications can access those files
+in a secure manner.
+
+## Tutorial
+
+This RFC introduces a new [custom section] in the WebAssembly binary
+format, identified by the name `.enarx.resources`. The payload of the
+custom section is in the `tar` file format that may contain any number
+of resource files.
+
+The name of each file entry MUST be a canonical path name, meaning
+that it is a relative path name from the current directory, and
+symbolic links or any special path components that affect directory
+traversal (`.` and `..`) are resolved in advance.
+
+When running the WebAssembly binary executable, each file entry is
+mapped on an in-memory virtual filesystem exposed to the WASI
+instance. The executable is allowed to access those files through the
+standard WASI system calls such as [`path_open`].
+
+## Reference
+
+[wasi-common] provides a way to expose in-memory data as a file on
+virtual filesystem with [VirtualDirEntry]. Once the bundled resources
+are mapped to a VirtualDirEntry, it can be associated with the WASI
+instance through `WasiCtxBuilder::preopened_virt()` call.
+
+To create a WebAssembly executable with resources, we will provide a
+command-line tool that bundles files with the executable, after proper
+checks and conversions of the path names.
+
+## Drawbacks
+
+No drawbacks currently known.
+
+## Rationale and alternatives
+
+For simplicity, this RFC is extending the WebAssembly binary format to
+be able to embed resources in it. However, there is a possibility to
+use a container format directly for packaging, such as bundling the
+executable along with other data files in the same archive, or using
+the [OCI image format]. The benefits of this approach are:
+- The application bundle can be built using a standard tools
+- May bring additional flexiblity in composition, such as versioning
+
+## Prior art
+
+[GResource] in glib provides a similar mechanism using a custom
+section in the ELF format (`.gresources.*`).
+
+## Unresolved questions
+
+This proposal assumes that bundled resources fit in the Keep's memory.
+It is still under discussion what approach should be taken to
+provision non-confidential data as well, in particular the sizes of such
+resources are large and cause unreasonable memory consumption.
+
+## Implementations
+
+No implementation yet available (2020-05-19).
+
+## Future Possibilities
+
+It might be useful to convey other information than the resources,
+such as environment variables or the default command-line arguments,
+as a new custom section holding metadata.
+
+[`path_open`]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md#path_open
+[custom section]: https://webassembly.github.io/spec/core/binary/modules.html#custom-section
+[wasi-common]: https://crates.io/crates/wasi-common
+[VirtualDirEntry]: https://docs.rs/wasi-common/0.16.0/wasi_common/enum.VirtualDirEntry.html
+[GResource]: https://developer.gnome.org/gio/stable/glib-compile-resources.html
+[OCI image format]: https://github.com/opencontainers/image-spec