How do I give the program access to cwd?

[Cloudef]

I'm trying to add wasix support to one of my projects, but I can't figure out how to give wasmer access to cwd.
I've tried --dir ., --mapdir /:., nothing seems to work. Does wasmer reserve some fds to itself, what's the cwd fd?

2025-02-11T07:14:14.171971Z TRACE ThreadId(01) wasmer_run:execute_wasm:execute_wasi_module:path_open: wasmer_wasix::syscalls::wasi::path_open: return=Ok(Errno::notdir) dirfd=3 follow_symlinks=true path=".zig-cache"

With wasmtime, --dir . works as expected.

[xdoardo]

Hello! Could you give us a minimal reproducible example and info about the tools you're using to build the module?

[Cloudef]

git clone https://github.com/Cloudef/zig-aio
cd zig-aio
# zig version 0.14.0-dev.3267+59dc15fa0
# ignore the errors you see
zig build install -Dtarget=wasm32-wasi-none
# from the project directory
wasmer --dir . ./zig-out/example/aio_immediate.was

the result with wasmer is:

error: FileNotFound
Unable to dump stack trace: not implemented for Wasm

wasmtime prints the contents of flake.nix

[xdoardo]

I can reproduce the error when using --dir .. However, --mapdir=/:. works:

λ zig-aio on (master) wasmer --mapdir=/:. ./zig-out/example/aio_immediate.wasm
info(aio_immediate): {
  description = "Zig project flake";

  inputs = {
    zig2nix.url = "github:Cloudef/zig2nix";
  };

  outputs = { zig2nix, ... }: let
    flake-utils = zig2nix.inputs.flake-utils;
  in (flake-utils.lib.eachDefaultSystem (system: let
      # Zig flake helper
      # Check the flake.nix in zig2nix project for more options:
      # <https://github.com/Cloudef/zig2nix/blob/master/flake.nix>
      env = zig2nix.outputs.zig-env.${system} {
        zig = zig2nix.outputs.packages.${system}.zig.master.bin;
      };
    in with builtins; with env.lib; with env.pkgs.lib; {
      # nix run .
      apps.default = env.app [] "zig build example -- \"$@\"";

      # nix run .#build
      apps.build = env.app [] "zig build \"$@\"";

      # nix run .#test
      apps.test = with env.pkgs; env.app [wasmtime] ''
        set -e
        zig version
        if [[ "$(uname)" == "Linux" ]]; then
          echo "zig build test bug -Daio:posix=disable -Dsanitize=true"
          zig build test bug -Daio:posix=disable -Dsanitize=true
          echo "zig build test bug -Daio:posix=disable -Dsingle-threaded=true"
          zig build test bug -Daio:posix=disable -Dsingle-threaded=true
          echo "zig build test bug -Daio:posix=force -Dsanitize=true"
          zig build test bug -Daio:posix=force -Dsanitize=true
          echo "zig build test bug -Daio:posix=force -Dsingle-threaded=true"
          zig build test bug -Daio:posix=force -Dsingle-threaded=true
          echo "zig build test:aio test:minilib -Dtarget=wasm32-wasi-none"
          zig build -Doptimize=ReleaseSafe test:aio test:minilib -Dtarget=wasm32-wasi-none
        elif [[ "$(uname)" == "Darwin" ]]; then
          echo "zig build test bug"
          zig build test bug
          echo "zig build test bug -Dsingle-threaded=true"
          zig build test bug -Dsingle-threaded=true
          echo "zig build test bug -Dtarget=x86_64-macos"
          zig build test bug -Dtarget=x86_64-macos
          echo "zig build test bug -Dtarget=x86_64-macos -Dsingle-threaded=true"
          zig build test bug -Dtarget=x86_64-macos -Dsingle-threaded=true
        else
          echo "zig build test bug"
          zig build test bug
          echo "zig build test bug -Dsingle-threaded=true"
          zig build test bug -Dsingle-threaded=true
        fi
      '';

      # nix run .#check
      apps.check = env.app [] ''
        echo "checking the formatting with zig fmt --check"
        zig fmt --check .
      '';

      # nix run .#bench
      apps.bench = with env.pkgs; env.app ([git coreutils gnutar] ++ optionals (stdenv.isLinux) [poop]) ''
        set -e

        tmpdir="$(mktemp -d)"
        trap 'rm -rf "$tmpdir"' EXIT

        [[ "$(uname)" == "Linux" ]] && zig build -Daio:posix=disable -Dsingle-threaded=true -p "$tmpdir/new-uring"
        zig build -Daio:posix=force -Dsingle-threaded=true -p "$tmpdir/new-posix"

        git archive "''${COMMIT:-HEAD~1}" | tar x -C "$tmpdir"
        cd "$tmpdir"

        [[ "$(uname)" == "Linux" ]] && zig build -Daio:posix=disable -Dsingle-threaded=true -p "$tmpdir/old-uring"
        zig build -Daio:posix=force -Dsingle-threaded=true -p "$tmpdir/old-posix"

        all=(old-posix/bench/*)
        for bench in "''${@:-''${all[@]}}"; do
          base="$(basename "$bench")"
          [[ -f "new-posix/bench/$base" ]] || continue
          if [[ "''${POOP:-0}" == 1 ]]; then
            poop "old-uring/bench/$base" "new-uring/bench/$base"
            poop "old-posix/bench/$base" "new-posix/bench/$base"
          else
            if [[ "$(uname)" == "Linux" ]]; then
              echo "$base: old-uring"
              "old-uring/bench/$base"
              echo "$base: new-uring"
              "new-uring/bench/$base"
            fi
            echo "$base: old-posix"
            "old-posix/bench/$base"
            echo "$base: new-posix"
            "new-posix/bench/$base"
          fi
        done
      '';

      # nix run .#deps
      apps.deps = env.showExternalDeps;

      # nix run .#zon2json
      apps.zon2json = env.app [env.zon2json] "zon2json \"$@\"";

      # nix
info(aio_immediate): .{
    .name = "zig-aio",
    .version = "0.0.0",
    .dependencies = .{
        .zigwin32 = .{
            .url = "git+https://github.com/marlersoft/zigwin32.git#0d804dce7632d0912245b1a7aa0384a21629f453",
            .hash = "1220fbd42a455a94ce2134c4080a16c99074fb28ca94df87dde3cfecccc5c8401ca9",
            .lazy = true,
        },
    },
    .paths = .{
        "build.zig",
        "build.zig.zon",
        "src",
        "examples",
        "LICENSE",
        "README.md",
    },
}

info(aio_immediate): 0

We will look into the problem with --dir=..

[Cloudef]

Seems the --mapdir works here, it seems to not work with tests which seem to access a directory. I'll make a branch which will reproduce that issue as well.

[Arshia001]

Note: --dir . is not the same as --mapdir /:.. --dir . mounts the current dir at /home, and sets the host-side CWD to /home as well, but the guest would be responsible for reading that and taking it into account. This already happens in more recent versions of wasix-libc (from a couple of months ago at the very least) but I'm not sure which version of that gets incorporated into a zig application?

[Arshia001]

Oh, I'm asking the wrong question. Zig only supports WASI, not WASIX. As WASI doesn't support the concept of host-side CWD, there's no way this can work correctly. @xdoardo I believe the correct fix would be to mount . on / by default for WASI apps, and do /home only for WASIX?

[Cloudef]

Here is problem with the tests:

git clone -b wasix https://github.com/Cloudef/zig-aio
cd zig-aio
zig build test:aio -Dtarget=wasm32-wasi-none -Daio:wasi=wasix

It fails to open the .zig-cache directory.
Args passed to wasmer are --mapdir /:. -vvv

Zig only supports WASI, not WASIX

I want to be able to use some wasix functions such as fd_pipe
https://github.com/Cloudef/zig-aio/blob/master/src/aio/posix/wasi.zig#L166

[Arshia001]

To be honest, I have no idea what will happen if you add a few wasix imports to a module built against the wasi-libc... See, by default, when you build for WASI, you're building against wasi-libc. When you build for WASIX, on the other hand, you're building against wasix-libc. While the two are similar, they're not the same; WASIX provides many features that are missing from WASI, and some of those have cross-cutting implications across the entire application.

As an example, CWD is one such case; with WASIX, the host can provide a CWD to the module (which wasmer does when you run with --dir .). When an app built against wasix-libc starts up, it checks the host-side CWD and performs a (logical) chdir if necessary; code is here. With WASI, there's no way for the host to do this, so the module always starts with CWD set to /.

Furthermore, when a module includes any wasix_32v1 imports, wasmer assumes it's a WASIX module (that was built against wasix-libc), and may decide to do things differently, which would break the expectations of a WASI module entirely. While WASIX is a superset of WASI in the sense that it has all the same syscalls as WASI plus some more, it doesn't mean you can run a WASI module as a WASIX module and expect it to work.

The correct solution to this would be for zig to add support for the WASIX target. This is not super-complicated, but the zig maintainers appear to not be in favor of accepting it (it was already proposed before, and rejected); so that option does not exist at this time.

[Cloudef]

I'm not linking to any libc

[Arshia001]

When you build any code for a specific target, you're ultimately linking against that target's libc, which is where all the most basic of functionality is implemented on top of OS syscalls. For non-C languages, the compiler usually does this for you automatically. For example, on linux, you have glibc. wasi-libc and wasix-libc are the same thing.

[Cloudef]

No, I'm not linking to any libc.

[Arshia001]

Zig is doing it for you automatically.

[Cloudef]

It's not. Zig does not link to libc unless asked. Linux target is libcless. Zig exposes wasi functions like this: https://github.com/ziglang/zig/blob/master/lib/std/os/wasi.zig and zig implements "posix like" api in std.posix, it's completely libcless

[Arshia001]

There appears to be a wasi-libc in the zig source, though? I'm not super-familiar with zig tbh, but I don't suppose you can bootstrap a module without wasi-libc (or recreating its functionality in zig).

[Cloudef]

That one isn't used unless you ask to link against libc.

I don't suppose you can bootstrap a module without wasi-libc

The thing is that zig doesn't call libc functions, it calls the wasi functions directly, it implements posix like api with std.posix. If you link to libc, then std.posix will instead call the libc implementations.

[Arshia001]

Interesting... However, regardless of whether zig uses wasi-libc or not, and my general lack of understanding of zig, what I can tell you is that --mapdir . in the presence of WASIX imports won't work unless you specifically link against wasix-libc or recreate its CWD-related functionality.

[Cloudef]

So what this means in practice (by reading wasix-libc) that all relative paths should be joined with getcwd
https://wasix.org/docs/api-reference/wasix/getcwd
Seems bit weird that you can't map / to . in wasix

[Cloudef]

It seems even without any wasix symbols being compiled into the program, it still fails to open (or create) the .zig_cache directory

2025-02-21T13:16:41.129349Z TRACE ThreadId(01) wasmer_run:execute_wasm:execute_wasi_module:path_open: wasmer_wasix::syscalls::wasi::path_open: return=Ok(Errno::notdir) dirfd=3 follow_symlinks=true path=".zig-cache"

Curious is that it's returning notdir rather than noent which is the reason zig's test runner won't try creating the directory
This happens even if i map / to /tmp which does not contain .zig-cache, which makes me believe there's something wrong with wasi::path_open with / paths

[Arshia001]

Are you running with --dir . or --mapdir /:.? --dir . may be broken for WASI anyway, and we'd need to fix it.

[Cloudef]

--mapdir /:.

These steps should reproduce:
(get the master build of zig from https://ziglang.org/download/)

git clone -b wasix https://github.com/Cloudef/zig-aio
cd zig-aio
zig build test:aio -Dtarget=wasm32-wasi-none -Daio:wasi=wasix