Add dir_perms and file_perms to WasiConfig.preopen_dir

[ajalt]

The rust preopened_dir method has parameters for setting permissions to allow mapping the directory or its files as read-only. The python preopen_dir doesn't currently expose these parameters, so we can only map directories as read-write.

Is it possible to add the ability to set permissions on mapped directories?

[alexcrichton]

Thanks for the report! While this functionality is present in Rust it's not currently reflected in the C API which this package is built on. That doesn't mean it's not possible, just means that the sequence of steps to fix this would be to (a) update the C APi to enable plumbing these permissions bits and (b) update wasmtime-py to use the new bindings. If you're able to help that'd be much appreciated! If not however that's also ok too.

[jder]

I'm also interested in this functionality. Looks like the next step is to update the version of wasmtime this package depends on, then update the preopen_dir python wrapper? @alexcrichton or @ajalt, are either of you already working on this first part, or would it be helpful for me to take it on this week?

[alexcrichton]

I'm not working on this myself, and if @ajalt isn't already taking a crack at it please feel free to!

The line to update is this one and set that to "dev" to download the latest build which will have the changes necessary.

[jder]

Thanks! I started working on the wasmtime 26 update as a first step, and ran into something I'm not sure how to handle.

In particular for the bindings generation code the wasmparser::Validator type used to be exposed (because environ exposed wasmtime_types::*, which included wasmparser), but it no longer is. Should environ re-export wasmparser (perhaps under a feature like wasmtime itself does now)?

[alexcrichton]

Ah the wasmtime-types crate was folded into wasmtime-environ, and for now you can probably add an explicit dependency on wasmparser but otherwise yeah adding it as an export from wasmtime-environ I think would make sense.

[jder]

For posterity, I am working on this, WIP here which we can land once wasmtime 27 is released.

[simonw]

I just came looking for this feature myself. I want to be able to run the equivalent of this (but in Python):

wget https://github.com/brettcannon/cpython-wasi-build/releases/download/v3.13.0/python-3.13.0-wasi_sdk-24.zip
unzip python-3.13.0-wasi_sdk-24.zip
wasmtime run --dir .::/ python.wasm -c 'print("hello world")'

But I want the directory to be read-only, so that things like this don't break everything:

wasmtime run --dir .::/ python.wasm -c 'open("python.wasm", "wb").write(b"blah")'

[alexcrichton]

@jder thanks for this! If you'd like feel free to go ahead and open a PR and update ci/download-wasmtime.py to using the dev tag. We can land that and then switch to the 27.0.0 release artifacts once they're available.

@simonw I believe that this issue will solve your need for that, and if you're interested in seeing such a readonly flag for Wasmtime mind opening an issue over there for that? I don't think we currently have an issue tracking that.

[simonw]

Thanks! Posted that feature request here:

• CLI option for read-only directory mounting wasmtime#9608