Virtual file support (#701)

* Add support for virtual files (eg, not backed by an OS file).

Virtual files are implemented through trait objects, with a default
implementation that tries to behave like on-disk files, but entirely
backed by in-memory structures.

Co-authored-by: Dan Gohman <sunfish@mozilla.com>

crates/test-programs/build.rs

@@ -11,11 +11,19 @@ fn main() {
 #[cfg(feature = "test_programs")]
 mod wasi_tests {
     use std::env;
-    use std::fs::{read_dir, DirEntry, File};
+    use std::fs::{read_dir, File};
     use std::io::{self, Write};
     use std::path::{Path, PathBuf};
     use std::process::{Command, Stdio};
 
+    #[derive(Clone, Copy, Debug)]
+    enum PreopenType {
+        /// Preopens should be satisfied with real OS files.
+        OS,
+        /// Preopens should be satisfied with virtual files.
+        Virtual,
+    }
+
     pub(super) fn build_and_generate_tests() {
         // Validate if any of test sources are present and if they changed
         // This should always work since there is no submodule to init anymore
@@ -103,8 +111,21 @@ mod wasi_tests {
                 .replace("-", "_")
         )?;
         writeln!(out, "    use super::{{runtime, utils, setup_log}};")?;
+        writeln!(out, "    use runtime::PreopenType;")?;
         for dir_entry in dir_entries {
-            write_testsuite_tests(out, dir_entry, testsuite)?;
+            let test_path = dir_entry.path();
+            let stemstr = test_path
+                .file_stem()
+                .expect("file_stem")
+                .to_str()
+                .expect("to_str");
+
+            if no_preopens(testsuite, stemstr) {
+                write_testsuite_tests(out, &test_path, testsuite, PreopenType::OS)?;
+            } else {
+                write_testsuite_tests(out, &test_path, testsuite, PreopenType::OS)?;
+                write_testsuite_tests(out, &test_path, testsuite, PreopenType::Virtual)?;
+            }
         }
         writeln!(out, "}}")?;
         Ok(())
@@ -112,10 +133,10 @@ mod wasi_tests {
 
     fn write_testsuite_tests(
         out: &mut File,
-        dir_entry: DirEntry,
+        path: &Path,
         testsuite: &str,
+        preopen_type: PreopenType,
     ) -> io::Result<()> {
-        let path = dir_entry.path();
         let stemstr = path
             .file_stem()
             .expect("file_stem")
@@ -123,14 +144,19 @@ mod wasi_tests {
             .expect("to_str");
 
         writeln!(out, "    #[test]")?;
-        if ignore(testsuite, stemstr) {
+        let test_fn_name = format!(
+            "{}{}",
+            &stemstr.replace("-", "_"),
+            if let PreopenType::Virtual = preopen_type {
+                "_virtualfs"
+            } else {
+                ""
+            }
+        );
+        if ignore(testsuite, &test_fn_name) {
             writeln!(out, "    #[ignore]")?;
         }
-        writeln!(
+        writeln!(out, "    fn r#{}() -> anyhow::Result<()> {{", test_fn_name,)?;
-            out,
-            "    fn r#{}() -> anyhow::Result<()> {{",
-            &stemstr.replace("-", "_")
-        )?;
         writeln!(out, "        setup_log();")?;
         writeln!(
             out,
@@ -145,16 +171,25 @@ mod wasi_tests {
         let workspace = if no_preopens(testsuite, stemstr) {
             "None"
         } else {
+            match preopen_type {
+                PreopenType::OS => {
                     writeln!(
                         out,
                         "        let workspace = utils::prepare_workspace(&bin_name)?;"
                     )?;
                     "Some(workspace.path())"
+                }
+                PreopenType::Virtual => "Some(std::path::Path::new(&bin_name))",
+            }
         };
         writeln!(
             out,
-            "        runtime::instantiate(&data, &bin_name, {})",
+            "        runtime::instantiate(&data, &bin_name, {}, {})",
-            workspace
+            workspace,
+            match preopen_type {
+                PreopenType::OS => "PreopenType::OS",
+                PreopenType::Virtual => "PreopenType::Virtual",
+            }
         )?;
         writeln!(out, "    }}")?;
         writeln!(out)?;
@@ -164,8 +199,30 @@ mod wasi_tests {
     cfg_if::cfg_if! {
         if #[cfg(not(windows))] {
             /// Ignore tests that aren't supported yet.
-            fn ignore(_testsuite: &str, _name: &str) -> bool {
+            fn ignore(testsuite: &str, name: &str) -> bool {
-                false
+                if testsuite == "wasi-tests" {
+                    match name {
+                        // TODO: virtfs files cannot be poll_oneoff'd yet
+                        "poll_oneoff_virtualfs" => true,
+                        // TODO: virtfs does not support filetimes yet.
+                        "path_filestat_virtualfs" |
+                        "fd_filestat_set_virtualfs" => true,
+                        // TODO: virtfs does not support symlinks yet.
+                        "nofollow_errors_virtualfs" |
+                        "path_link_virtualfs" |
+                        "readlink_virtualfs" |
+                        "readlink_no_buffer_virtualfs" |
+                        "dangling_symlink_virtualfs" |
+                        "symlink_loop_virtualfs" |
+                        "path_symlink_trailing_slashes_virtualfs" => true,
+                        // TODO: virtfs does not support rename yet.
+                        "path_rename_trailing_slashes_virtualfs" |
+                        "path_rename_virtualfs" => true,
+                        _ => false,
+                    }
+                } else {
+                    unreachable!()
+                }
             }
         } else {
             /// Ignore tests that aren't supported yet.
@@ -178,6 +235,22 @@ mod wasi_tests {
                         "truncation_rights" => true,
                         "path_link" => true,
                         "dangling_fd" => true,
+                        // TODO: virtfs files cannot be poll_oneoff'd yet
+                        "poll_oneoff_virtualfs" => true,
+                        // TODO: virtfs does not support filetimes yet.
+                        "path_filestat_virtualfs" |
+                        "fd_filestat_set_virtualfs" => true,
+                        // TODO: virtfs does not support symlinks yet.
+                        "nofollow_errors_virtualfs" |
+                        "path_link_virtualfs" |
+                        "readlink_virtualfs" |
+                        "readlink_no_buffer_virtualfs" |
+                        "dangling_symlink_virtualfs" |
+                        "symlink_loop_virtualfs" |
+                        "path_symlink_trailing_slashes_virtualfs" => true,
+                        // TODO: virtfs does not support rename yet.
+                        "path_rename_trailing_slashes_virtualfs" |
+                        "path_rename_virtualfs" => true,
                         _ => false,
                     }
                 } else {

crates/test-programs/tests/wasm_tests/runtime.rs

@@ -1,30 +1,44 @@
 use anyhow::{bail, Context};
 use std::fs::File;
 use std::path::Path;
+use wasi_common::VirtualDirEntry;
 use wasmtime::{Instance, Module, Store};
 
-pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> anyhow::Result<()> {
+#[derive(Clone, Copy, Debug)]
+pub enum PreopenType {
+    /// Preopens should be satisfied with real OS files.
+    OS,
+    /// Preopens should be satisfied with virtual files.
+    Virtual,
+}
+
+pub fn instantiate(
+    data: &[u8],
+    bin_name: &str,
+    workspace: Option<&Path>,
+    preopen_type: PreopenType,
+) -> anyhow::Result<()> {
     let store = Store::default();
 
-    let get_preopens = |workspace: Option<&Path>| -> anyhow::Result<Vec<_>> {
-        if let Some(workspace) = workspace {
-            let preopen_dir = wasi_common::preopen_dir(workspace)
-                .context(format!("error while preopening {:?}", workspace))?;
-
-            Ok(vec![(".".to_owned(), preopen_dir)])
-        } else {
-            Ok(vec![])
-        }
-    };
-
     // Create our wasi context with pretty standard arguments/inheritance/etc.
-    // Additionally register andy preopened directories if we have them.
+    // Additionally register any preopened directories if we have them.
     let mut builder = wasi_common::WasiCtxBuilder::new();
 
     builder.arg(bin_name).arg(".").inherit_stdio();
 
-    for (dir, file) in get_preopens(workspace)? {
+    if let Some(workspace) = workspace {
-        builder.preopened_dir(file, dir);
+        match preopen_type {
+            PreopenType::OS => {
+                let preopen_dir = wasi_common::preopen_dir(workspace)
+                    .context(format!("error while preopening {:?}", workspace))?;
+                builder.preopened_dir(preopen_dir, ".");
+            }
+            PreopenType::Virtual => {
+                // we can ignore the workspace path for virtual preopens because virtual preopens
+                // don't exist in the filesystem anyway - no name conflict concerns.
+                builder.preopened_virt(VirtualDirEntry::empty_directory(), ".");
+            }
+        }
     }
 
     // The nonstandard thing we do with `WasiCtxBuilder` is to ensure that

crates/test-programs/wasi-tests/src/bin/fd_flags_set.rs

@@ -50,7 +50,7 @@ unsafe fn test_fd_fdstat_set_flags(dir_fd: wasi::Fd) {
         )
         .expect("reading file"),
         buffer.len(),
-        "shoudl read {} bytes",
+        "should read {} bytes",
         buffer.len()
     );
 
@@ -87,7 +87,7 @@ unsafe fn test_fd_fdstat_set_flags(dir_fd: wasi::Fd) {
         )
         .expect("reading file"),
         buffer.len(),
-        "shoudl read {} bytes",
+        "should read {} bytes",
         buffer.len()
     );
 
@@ -126,7 +126,7 @@ unsafe fn test_fd_fdstat_set_flags(dir_fd: wasi::Fd) {
         )
         .expect("reading file"),
         buffer.len(),
-        "shoudl read {} bytes",
+        "should read {} bytes",
         buffer.len()
     );
 

crates/test-programs/wasi-tests/src/bin/remove_directory_trailing_slashes.rs

@@ -11,14 +11,14 @@ unsafe fn test_remove_directory_trailing_slashes(dir_fd: wasi::Fd) {
 
     wasi::path_create_directory(dir_fd, "dir").expect("creating a directory");
 
-    // Test that removing it with a trailing flash succeeds.
+    // Test that removing it with a trailing slash succeeds.
     wasi::path_remove_directory(dir_fd, "dir/")
         .expect("remove_directory with a trailing slash on a directory should succeed");
 
     // Create a temporary file.
     create_file(dir_fd, "file");
 
-    // Test that removing it with no trailing flash fails.
+    // Test that removing it with no trailing slash fails.
     assert_eq!(
         wasi::path_remove_directory(dir_fd, "file")
             .expect_err("remove_directory without a trailing slash on a file should fail")
@@ -27,7 +27,7 @@ unsafe fn test_remove_directory_trailing_slashes(dir_fd: wasi::Fd) {
         "errno should be ERRNO_NOTDIR"
     );
 
-    // Test that removing it with a trailing flash fails.
+    // Test that removing it with a trailing slash fails.
     assert_eq!(
         wasi::path_remove_directory(dir_fd, "file/")
             .expect_err("remove_directory with a trailing slash on a file should fail")

crates/wasi-common/src/ctx.rs

@@ -1,4 +1,6 @@
-use crate::fdentry::FdEntry;
+use crate::fdentry::{Descriptor, FdEntry};
+use crate::sys::fdentry_impl::OsHandle;
+use crate::virtfs::{VirtualDir, VirtualDirEntry};
 use crate::{wasi, Error, Result};
 use std::borrow::Borrow;
 use std::collections::HashMap;
@@ -61,7 +63,7 @@ impl PendingCString {
 /// A builder allowing customizable construction of `WasiCtx` instances.
 pub struct WasiCtxBuilder {
     fds: Option<HashMap<wasi::__wasi_fd_t, PendingFdEntry>>,
-    preopens: Option<Vec<(PathBuf, File)>>,
+    preopens: Option<Vec<(PathBuf, Descriptor)>>,
     args: Option<Vec<PendingCString>>,
     env: Option<HashMap<PendingCString, PendingCString>>,
 }
@@ -222,10 +224,46 @@ impl WasiCtxBuilder {
 
     /// Add a preopened directory.
     pub fn preopened_dir<P: AsRef<Path>>(&mut self, dir: File, guest_path: P) -> &mut Self {
+        self.preopens.as_mut().unwrap().push((
+            guest_path.as_ref().to_owned(),
+            Descriptor::OsHandle(OsHandle::from(dir)),
+        ));
+        self
+    }
+
+    /// Add a preopened virtual directory.
+    pub fn preopened_virt<P: AsRef<Path>>(
+        &mut self,
+        dir: VirtualDirEntry,
+        guest_path: P,
+    ) -> &mut Self {
+        fn populate_directory(virtentry: HashMap<String, VirtualDirEntry>, dir: &mut VirtualDir) {
+            for (path, entry) in virtentry.into_iter() {
+                match entry {
+                    VirtualDirEntry::Directory(dir_entries) => {
+                        let mut subdir = VirtualDir::new(true);
+                        populate_directory(dir_entries, &mut subdir);
+                        dir.add_dir(subdir, path);
+                    }
+                    VirtualDirEntry::File(content) => {
+                        dir.add_file(content, path);
+                    }
+                }
+            }
+        }
+
+        let dir = if let VirtualDirEntry::Directory(entries) = dir {
+            let mut dir = VirtualDir::new(true);
+            populate_directory(entries, &mut dir);
+            Box::new(dir)
+        } else {
+            panic!("the root of a VirtualDirEntry tree must be a VirtualDirEntry::Directory");
+        };
+
         self.preopens
             .as_mut()
             .unwrap()
-            .push((guest_path.as_ref().to_owned(), dir));
+            .push((guest_path.as_ref().to_owned(), Descriptor::VirtualFile(dir)));
         self
     }
 
@@ -271,7 +309,7 @@ impl WasiCtxBuilder {
                     fds.insert(fd, f()?);
                 }
                 PendingFdEntry::File(f) => {
-                    fds.insert(fd, FdEntry::from(f)?);
+                    fds.insert(fd, FdEntry::from(Descriptor::OsHandle(OsHandle::from(f)))?);
                 }
             }
         }
@@ -284,9 +322,21 @@ impl WasiCtxBuilder {
             // unnecessarily if we have exactly the maximum number of file descriptors.
             preopen_fd = preopen_fd.checked_add(1).ok_or(Error::ENFILE)?;
 
-            if !dir.metadata()?.is_dir() {
+            match &dir {
+                Descriptor::OsHandle(handle) => {
+                    if !handle.metadata()?.is_dir() {
                         return Err(Error::EBADF);
                     }
+                }
+                Descriptor::VirtualFile(virt) => {
+                    if virt.get_file_type() != wasi::__WASI_FILETYPE_DIRECTORY {
+                        return Err(Error::EBADF);
+                    }
+                }
+                Descriptor::Stdin | Descriptor::Stdout | Descriptor::Stderr => {
+                    panic!("implementation error, stdin/stdout/stderr shouldn't be in the list of preopens");
+                }
+            }
 
             // We don't currently allow setting file descriptors other than 0-2, but this will avoid
             // collisions if we restore that functionality in the future.

crates/wasi-common/src/fdentry.rs

@@ -2,36 +2,75 @@ use crate::sys::dev_null;
 use crate::sys::fdentry_impl::{
     descriptor_as_oshandle, determine_type_and_access_rights, OsHandle,
 };
+use crate::virtfs::VirtualFile;
 use crate::{wasi, Error, Result};
 use std::marker::PhantomData;
 use std::mem::ManuallyDrop;
 use std::ops::{Deref, DerefMut};
 use std::path::PathBuf;
-use std::{fs, io};
+use std::{fmt, fs, io};
 
-#[derive(Debug)]
 pub(crate) enum Descriptor {
     OsHandle(OsHandle),
+    VirtualFile(Box<dyn VirtualFile>),
     Stdin,
     Stdout,
     Stderr,
 }
 
-impl Descriptor {
+impl From<OsHandle> for Descriptor {
-    /// Return a reference to the `OsHandle` treating it as an actual file/dir, and
+    fn from(handle: OsHandle) -> Self {
-    /// allowing operations which require an actual file and not just a stream or
+        Descriptor::OsHandle(handle)
-    /// socket file descriptor.
+    }
-    pub(crate) fn as_file(&self) -> Result<&OsHandle> {
+}
+
+impl From<Box<dyn VirtualFile>> for Descriptor {
+    fn from(virt: Box<dyn VirtualFile>) -> Self {
+        Descriptor::VirtualFile(virt)
+    }
+}
+
+impl fmt::Debug for Descriptor {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self {
-            Self::OsHandle(file) => Ok(file),
+            Descriptor::OsHandle(handle) => write!(f, "{:?}", handle),
+            Descriptor::VirtualFile(_) => write!(f, "VirtualFile"),
+            Descriptor::Stdin => write!(f, "Stdin"),
+            Descriptor::Stdout => write!(f, "Stdout"),
+            Descriptor::Stderr => write!(f, "Stderr"),
+        }
+    }
+}
+
+impl Descriptor {
+    pub(crate) fn try_clone(&self) -> io::Result<Descriptor> {
+        match self {
+            Descriptor::OsHandle(file) => file.try_clone().map(|f| OsHandle::from(f).into()),
+            Descriptor::VirtualFile(virt) => virt.try_clone().map(Descriptor::VirtualFile),
+            Descriptor::Stdin => Ok(Descriptor::Stdin),
+            Descriptor::Stdout => Ok(Descriptor::Stdout),
+            Descriptor::Stderr => Ok(Descriptor::Stderr),
+        }
+    }
+
+    /// Return a reference to the `OsHandle` or `VirtualFile` treating it as an
+    /// actual file/dir, and allowing operations which require an actual file and
+    /// not just a stream or socket file descriptor.
+    pub(crate) fn as_file<'descriptor>(&'descriptor self) -> Result<&'descriptor Descriptor> {
+        match self {
+            Self::OsHandle(_) => Ok(self),
+            Self::VirtualFile(_) => Ok(self),
             _ => Err(Error::EBADF),
         }
     }
 
     /// Like `as_file`, but return a mutable reference.
-    pub(crate) fn as_file_mut(&mut self) -> Result<&mut OsHandle> {
+    pub(crate) fn as_file_mut<'descriptor>(
+        &'descriptor mut self,
+    ) -> Result<&'descriptor mut Descriptor> {
         match self {
-            Self::OsHandle(file) => Ok(file),
+            Self::OsHandle(_) => Ok(self),
+            Self::VirtualFile(_) => Ok(self),
             _ => Err(Error::EBADF),
         }
     }
@@ -61,16 +100,33 @@ pub(crate) struct FdEntry {
 }
 
 impl FdEntry {
-    pub(crate) fn from(file: fs::File) -> Result<Self> {
+    pub(crate) fn from(file: Descriptor) -> Result<Self> {
-        unsafe { determine_type_and_access_rights(&file) }.map(
+        match file {
-            |(file_type, rights_base, rights_inheriting)| Self {
+            Descriptor::OsHandle(handle) => unsafe { determine_type_and_access_rights(&handle) }
+                .map(|(file_type, rights_base, rights_inheriting)| Self {
                     file_type,
-                descriptor: Descriptor::OsHandle(OsHandle::from(file)),
+                    descriptor: handle.into(),
                     rights_base,
                     rights_inheriting,
                     preopen_path: None,
-            },
+                }),
-        )
+            Descriptor::VirtualFile(virt) => {
+                let file_type = virt.get_file_type();
+                let rights_base = virt.get_rights_base();
+                let rights_inheriting = virt.get_rights_inheriting();
+
+                Ok(Self {
+                    file_type,
+                    descriptor: virt.into(),
+                    rights_base,
+                    rights_inheriting,
+                    preopen_path: None,
+                })
+            }
+            Descriptor::Stdin | Descriptor::Stdout | Descriptor::Stderr => {
+                panic!("implementation error, stdin/stdout/stderr FdEntry must not be constructed from FdEntry::from");
+            }
+        }
     }
 
     pub(crate) fn duplicate_stdin() -> Result<Self> {
@@ -110,7 +166,7 @@ impl FdEntry {
     }
 
     pub(crate) fn null() -> Result<Self> {
-        Self::from(dev_null()?)
+        Self::from(OsHandle::from(dev_null()?).into())
     }
 
     /// Convert this `FdEntry` into a host `Descriptor` object provided the specified

crates/wasi-common/src/host.rs

@@ -22,9 +22,9 @@ pub(crate) unsafe fn iovec_to_host_mut(iovec: &mut __wasi_iovec_t) -> io::IoSlic
 }
 
 #[allow(dead_code)] // trouble with sockets
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 #[repr(u8)]
-pub(crate) enum FileType {
+pub enum FileType {
     Unknown = __WASI_FILETYPE_UNKNOWN,
     BlockDevice = __WASI_FILETYPE_BLOCK_DEVICE,
     CharacterDevice = __WASI_FILETYPE_CHARACTER_DEVICE,
@@ -39,10 +39,25 @@ impl FileType {
     pub(crate) fn to_wasi(&self) -> __wasi_filetype_t {
         *self as __wasi_filetype_t
     }
+
+    pub(crate) fn from_wasi(wasi_filetype: u8) -> Option<Self> {
+        use FileType::*;
+        match wasi_filetype {
+            __WASI_FILETYPE_UNKNOWN => Some(Unknown),
+            __WASI_FILETYPE_BLOCK_DEVICE => Some(BlockDevice),
+            __WASI_FILETYPE_CHARACTER_DEVICE => Some(CharacterDevice),
+            __WASI_FILETYPE_DIRECTORY => Some(Directory),
+            __WASI_FILETYPE_REGULAR_FILE => Some(RegularFile),
+            __WASI_FILETYPE_SOCKET_DGRAM => Some(SocketDgram),
+            __WASI_FILETYPE_SOCKET_STREAM => Some(SocketStream),
+            __WASI_FILETYPE_SYMBOLIC_LINK => Some(Symlink),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Debug, Clone)]
-pub(crate) struct Dirent {
+pub struct Dirent {
     pub name: String,
     pub ftype: FileType,
     pub ino: u64,

crates/wasi-common/src/hostcalls_impl/fs.rs

@@ -3,6 +3,7 @@ use super::fs_helpers::path_get;
 use crate::ctx::WasiCtx;
 use crate::fdentry::{Descriptor, FdEntry};
 use crate::helpers::*;
+use crate::host::Dirent;
 use crate::memory::*;
 use crate::sandboxed_tty_writer::SandboxedTTYWriter;
 use crate::sys::hostcalls_impl::fs_helpers::path_open_rights;
@@ -10,7 +11,7 @@ use crate::sys::{host_impl, hostcalls_impl};
 use crate::{helpers, host, wasi, wasi32, Error, Result};
 use filetime::{set_file_handle_times, FileTime};
 use log::trace;
-use std::fs::File;
+use std::convert::TryInto;
 use std::io::{self, Read, Seek, SeekFrom, Write};
 use std::ops::DerefMut;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
@@ -40,12 +41,15 @@ pub(crate) unsafe fn fd_datasync(
 ) -> Result<()> {
     trace!("fd_datasync(fd={:?})", fd);
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry(fd)?
-        .as_descriptor(wasi::__WASI_RIGHTS_FD_DATASYNC, 0)?
+        .as_descriptor(wasi::__WASI_RIGHTS_FD_DATASYNC, 0)?;
-        .as_file()?;
 
-    fd.sync_data().map_err(Into::into)
+    match file {
+        Descriptor::OsHandle(fd) => fd.sync_data().map_err(Into::into),
+        Descriptor::VirtualFile(virt) => virt.datasync(),
+        other => other.as_os_handle().sync_data().map_err(Into::into),
+    }
 }
 
 pub(crate) unsafe fn fd_pread(
@@ -66,7 +70,7 @@ pub(crate) unsafe fn fd_pread(
         nread
     );
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry(fd)?
         .as_descriptor(wasi::__WASI_RIGHTS_FD_READ | wasi::__WASI_RIGHTS_FD_SEEK, 0)?
         .as_file()?;
@@ -76,9 +80,28 @@ pub(crate) unsafe fn fd_pread(
     if offset > i64::max_value() as u64 {
         return Err(Error::EIO);
     }
-    let buf_size = iovs.iter().map(|v| v.buf_len).sum();
+    let buf_size = iovs
-    let mut buf = vec![0; buf_size];
+        .iter()
-    let host_nread = hostcalls_impl::fd_pread(fd, &mut buf, offset)?;
+        .map(|iov| {
+            let cast_iovlen: wasi32::size_t = iov
+                .buf_len
+                .try_into()
+                .expect("iovec are bounded by wasi max sizes");
+            cast_iovlen
+        })
+        .fold(Some(0u32), |len, iov| len.and_then(|x| x.checked_add(iov)))
+        .ok_or(Error::EINVAL)?;
+    let mut buf = vec![0; buf_size as usize];
+    let host_nread = match file {
+        Descriptor::OsHandle(fd) => hostcalls_impl::fd_pread(&fd, &mut buf, offset)?,
+        Descriptor::VirtualFile(virt) => virt.pread(&mut buf, offset)?,
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    };
+
     let mut buf_offset = 0;
     let mut left = host_nread;
     for iov in &iovs {
@@ -115,7 +138,7 @@ pub(crate) unsafe fn fd_pwrite(
         nwritten
     );
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry(fd)?
         .as_descriptor(
             wasi::__WASI_RIGHTS_FD_WRITE | wasi::__WASI_RIGHTS_FD_SEEK,
@@ -127,15 +150,33 @@ pub(crate) unsafe fn fd_pwrite(
     if offset > i64::max_value() as u64 {
         return Err(Error::EIO);
     }
-    let buf_size = iovs.iter().map(|v| v.buf_len).sum();
+    let buf_size = iovs
-    let mut buf = Vec::with_capacity(buf_size);
+        .iter()
+        .map(|iov| {
+            let cast_iovlen: wasi32::size_t = iov
+                .buf_len
+                .try_into()
+                .expect("iovec are bounded by wasi max sizes");
+            cast_iovlen
+        })
+        .fold(Some(0u32), |len, iov| len.and_then(|x| x.checked_add(iov)))
+        .ok_or(Error::EINVAL)?;
+    let mut buf = Vec::with_capacity(buf_size as usize);
     for iov in &iovs {
         buf.extend_from_slice(std::slice::from_raw_parts(
             iov.buf as *const u8,
             iov.buf_len,
         ));
     }
-    let host_nwritten = hostcalls_impl::fd_pwrite(fd, &buf, offset)?;
+    let host_nwritten = match file {
+        Descriptor::OsHandle(fd) => hostcalls_impl::fd_pwrite(&fd, &buf, offset)?,
+        Descriptor::VirtualFile(virt) => virt.pwrite(buf.as_mut(), offset)?,
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    };
 
     trace!("     | *nwritten={:?}", host_nwritten);
 
@@ -168,8 +209,9 @@ pub(crate) unsafe fn fd_read(
         .get_fd_entry_mut(fd)?
         .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_READ, 0)?
     {
-        Descriptor::OsHandle(file) => file.read_vectored(&mut iovs),
+        Descriptor::OsHandle(file) => file.read_vectored(&mut iovs).map_err(Into::into),
-        Descriptor::Stdin => io::stdin().read_vectored(&mut iovs),
+        Descriptor::VirtualFile(virt) => virt.read_vectored(&mut iovs),
+        Descriptor::Stdin => io::stdin().read_vectored(&mut iovs).map_err(Into::into),
         _ => return Err(Error::EBADF),
     };
 
@@ -232,7 +274,7 @@ pub(crate) unsafe fn fd_seek(
     } else {
         wasi::__WASI_RIGHTS_FD_SEEK | wasi::__WASI_RIGHTS_FD_TELL
     };
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry_mut(fd)?
         .as_descriptor_mut(rights, 0)?
         .as_file_mut()?;
@@ -243,7 +285,15 @@ pub(crate) unsafe fn fd_seek(
         wasi::__WASI_WHENCE_SET => SeekFrom::Start(offset as u64),
         _ => return Err(Error::EINVAL),
     };
-    let host_newoffset = fd.seek(pos)?;
+    let host_newoffset = match file {
+        Descriptor::OsHandle(fd) => fd.seek(pos)?,
+        Descriptor::VirtualFile(virt) => virt.seek(pos)?,
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    };
 
     trace!("     | *newoffset={:?}", host_newoffset);
 
@@ -258,12 +308,20 @@ pub(crate) unsafe fn fd_tell(
 ) -> Result<()> {
     trace!("fd_tell(fd={:?}, newoffset={:#x?})", fd, newoffset);
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry_mut(fd)?
         .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_TELL, 0)?
         .as_file_mut()?;
 
-    let host_offset = fd.seek(SeekFrom::Current(0))?;
+    let host_offset = match file {
+        Descriptor::OsHandle(fd) => fd.seek(SeekFrom::Current(0))?,
+        Descriptor::VirtualFile(virt) => virt.seek(SeekFrom::Current(0))?,
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    };
 
     trace!("     | *newoffset={:?}", host_offset);
 
@@ -279,12 +337,13 @@ pub(crate) unsafe fn fd_fdstat_get(
     trace!("fd_fdstat_get(fd={:?}, fdstat_ptr={:#x?})", fd, fdstat_ptr);
 
     let mut fdstat = dec_fdstat_byref(memory, fdstat_ptr)?;
-    let host_fd = wasi_ctx
+    let wasi_file = wasi_ctx.get_fd_entry(fd)?.as_descriptor(0, 0)?;
-        .get_fd_entry(fd)?
-        .as_descriptor(0, 0)?
-        .as_os_handle();
 
-    let fs_flags = hostcalls_impl::fd_fdstat_get(&host_fd)?;
+    let fs_flags = match wasi_file {
+        Descriptor::OsHandle(wasi_fd) => hostcalls_impl::fd_fdstat_get(&wasi_fd)?,
+        Descriptor::VirtualFile(virt) => virt.fdstat_get(),
+        other => hostcalls_impl::fd_fdstat_get(&other.as_os_handle())?,
+    };
 
     let fe = wasi_ctx.get_fd_entry(fd)?;
     fdstat.fs_filetype = fe.file_type;
@@ -309,11 +368,28 @@ pub(crate) unsafe fn fd_fdstat_set_flags(
         .get_fd_entry_mut(fd)?
         .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_FDSTAT_SET_FLAGS, 0)?;
 
-    if let Some(new_handle) =
+    match descriptor {
-        hostcalls_impl::fd_fdstat_set_flags(&descriptor.as_os_handle(), fdflags)?
+        Descriptor::OsHandle(handle) => {
-    {
+            let set_result =
-        *descriptor = Descriptor::OsHandle(new_handle);
+                hostcalls_impl::fd_fdstat_set_flags(&handle, fdflags)?.map(Descriptor::OsHandle);
+
+            if let Some(new_descriptor) = set_result {
+                *descriptor = new_descriptor;
             }
+        }
+        Descriptor::VirtualFile(handle) => {
+            handle.fdstat_set_flags(fdflags)?;
+        }
+        _ => {
+            let set_result =
+                hostcalls_impl::fd_fdstat_set_flags(&descriptor.as_os_handle(), fdflags)?
+                    .map(Descriptor::OsHandle);
+
+            if let Some(new_descriptor) = set_result {
+                *descriptor = new_descriptor;
+            }
+        }
+    };
 
     Ok(())
 }
@@ -351,11 +427,19 @@ pub(crate) unsafe fn fd_sync(
 ) -> Result<()> {
     trace!("fd_sync(fd={:?})", fd);
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry(fd)?
         .as_descriptor(wasi::__WASI_RIGHTS_FD_SYNC, 0)?
         .as_file()?;
-    fd.sync_all().map_err(Into::into)
+    match file {
+        Descriptor::OsHandle(fd) => fd.sync_all().map_err(Into::into),
+        Descriptor::VirtualFile(virt) => virt.sync(),
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    }
 }
 
 pub(crate) unsafe fn fd_write(
@@ -389,6 +473,13 @@ pub(crate) unsafe fn fd_write(
                 file.write_vectored(&iovs)?
             }
         }
+        Descriptor::VirtualFile(virt) => {
+            if isatty {
+                unimplemented!("writes to virtual tty");
+            } else {
+                virt.write_vectored(&iovs)?
+            }
+        }
         Descriptor::Stdin => return Err(Error::EBADF),
         Descriptor::Stdout => {
             // lock for the duration of the scope
@@ -415,7 +506,7 @@ pub(crate) unsafe fn fd_write(
 }
 
 pub(crate) unsafe fn fd_advise(
-    wasi_ctx: &WasiCtx,
+    wasi_ctx: &mut WasiCtx,
     _memory: &mut [u8],
     fd: wasi::__wasi_fd_t,
     offset: wasi::__wasi_filesize_t,
@@ -430,12 +521,20 @@ pub(crate) unsafe fn fd_advise(
         advice
     );
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
-        .get_fd_entry(fd)?
+        .get_fd_entry_mut(fd)?
-        .as_descriptor(wasi::__WASI_RIGHTS_FD_ADVISE, 0)?
+        .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_ADVISE, 0)?
-        .as_file()?;
+        .as_file_mut()?;
 
-    hostcalls_impl::fd_advise(fd, advice, offset, len)
+    match file {
+        Descriptor::OsHandle(fd) => hostcalls_impl::fd_advise(&fd, advice, offset, len),
+        Descriptor::VirtualFile(virt) => virt.advise(advice, offset, len),
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    }
 }
 
 pub(crate) unsafe fn fd_allocate(
@@ -447,11 +546,13 @@ pub(crate) unsafe fn fd_allocate(
 ) -> Result<()> {
     trace!("fd_allocate(fd={:?}, offset={}, len={})", fd, offset, len);
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry(fd)?
         .as_descriptor(wasi::__WASI_RIGHTS_FD_ALLOCATE, 0)?
         .as_file()?;
 
+    match file {
+        Descriptor::OsHandle(fd) => {
             let metadata = fd.metadata()?;
 
             let current_size = metadata.len();
@@ -466,6 +567,14 @@ pub(crate) unsafe fn fd_allocate(
             } else {
                 Ok(())
             }
+        }
+        Descriptor::VirtualFile(virt) => virt.allocate(offset, len),
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    }
 }
 
 pub(crate) unsafe fn path_create_directory(
@@ -490,7 +599,7 @@ pub(crate) unsafe fn path_create_directory(
     let fe = wasi_ctx.get_fd_entry(dirfd)?;
     let resolved = path_get(fe, rights, 0, 0, path, false)?;
 
-    hostcalls_impl::path_create_directory(resolved)
+    resolved.path_create_directory()
 }
 
 pub(crate) unsafe fn path_link(
@@ -607,7 +716,7 @@ pub(crate) unsafe fn path_open(
         read,
         write
     );
-    let fd = hostcalls_impl::path_open(resolved, read, write, oflags, fs_flags)?;
+    let fd = resolved.open_with(read, write, oflags, fs_flags)?;
 
     let mut fe = FdEntry::from(fd)?;
     // We need to manually deny the rights which are not explicitly requested
@@ -652,7 +761,12 @@ pub(crate) unsafe fn path_readlink(
 
     let mut buf = dec_slice_of_mut_u8(memory, buf_ptr, buf_len)?;
 
-    let host_bufused = hostcalls_impl::path_readlink(resolved, &mut buf)?;
+    let host_bufused = match resolved.dirfd() {
+        Descriptor::VirtualFile(_virt) => {
+            unimplemented!("virtual readlink");
+        }
+        _ => hostcalls_impl::path_readlink(resolved, &mut buf)?,
+    };
 
     trace!("     | (buf_ptr,*buf_used)={:?}", buf);
     trace!("     | *buf_used={:?}", host_bufused);
@@ -708,7 +822,15 @@ pub(crate) unsafe fn path_rename(
     log::debug!("path_rename resolved_old={:?}", resolved_old);
     log::debug!("path_rename resolved_new={:?}", resolved_new);
 
+    if let (Descriptor::OsHandle(_), Descriptor::OsHandle(_)) =
+        (resolved_old.dirfd(), resolved_new.dirfd())
+    {
         hostcalls_impl::path_rename(resolved_old, resolved_new)
+    } else {
+        // Virtual files do not support rename, at the moment, and streams don't have paths to
+        // rename, so any combination of Descriptor that gets here is an error in the making.
+        panic!("path_rename with one or more non-OS files");
+    }
 }
 
 pub(crate) unsafe fn fd_filestat_get(
@@ -727,7 +849,15 @@ pub(crate) unsafe fn fd_filestat_get(
         .get_fd_entry(fd)?
         .as_descriptor(wasi::__WASI_RIGHTS_FD_FILESTAT_GET, 0)?
         .as_file()?;
-    let host_filestat = hostcalls_impl::fd_filestat_get(fd)?;
+    let host_filestat = match fd {
+        Descriptor::OsHandle(fd) => hostcalls_impl::fd_filestat_get(&fd)?,
+        Descriptor::VirtualFile(virt) => virt.filestat_get()?,
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    };
 
     trace!("     | *filestat_ptr={:?}", host_filestat);
 
@@ -755,11 +885,11 @@ pub(crate) unsafe fn fd_filestat_set_times(
         .as_descriptor(wasi::__WASI_RIGHTS_FD_FILESTAT_SET_TIMES, 0)?
         .as_file()?;
 
-    fd_filestat_set_times_impl(fd, st_atim, st_mtim, fst_flags)
+    fd_filestat_set_times_impl(&fd, st_atim, st_mtim, fst_flags)
 }
 
 pub(crate) fn fd_filestat_set_times_impl(
-    fd: &File,
+    file: &Descriptor,
     st_atim: wasi::__wasi_timestamp_t,
     st_mtim: wasi::__wasi_timestamp_t,
     fst_flags: wasi::__wasi_fstflags_t,
@@ -791,7 +921,15 @@ pub(crate) fn fd_filestat_set_times_impl(
     } else {
         None
     };
-    set_file_handle_times(fd, atim, mtim).map_err(Into::into)
+    match file {
+        Descriptor::OsHandle(fd) => set_file_handle_times(fd, atim, mtim).map_err(Into::into),
+        Descriptor::VirtualFile(virt) => virt.filestat_set_times(atim, mtim),
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    }
 }
 
 pub(crate) unsafe fn fd_filestat_set_size(
@@ -802,7 +940,7 @@ pub(crate) unsafe fn fd_filestat_set_size(
 ) -> Result<()> {
     trace!("fd_filestat_set_size(fd={:?}, st_size={})", fd, st_size);
 
-    let fd = wasi_ctx
+    let file = wasi_ctx
         .get_fd_entry(fd)?
         .as_descriptor(wasi::__WASI_RIGHTS_FD_FILESTAT_SET_SIZE, 0)?
         .as_file()?;
@@ -811,7 +949,15 @@ pub(crate) unsafe fn fd_filestat_set_size(
     if st_size > i64::max_value() as u64 {
         return Err(Error::E2BIG);
     }
-    fd.set_len(st_size).map_err(Into::into)
+    match file {
+        Descriptor::OsHandle(fd) => fd.set_len(st_size).map_err(Into::into),
+        Descriptor::VirtualFile(virt) => virt.filestat_set_size(st_size),
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    }
 }
 
 pub(crate) unsafe fn path_filestat_get(
@@ -845,7 +991,12 @@ pub(crate) unsafe fn path_filestat_get(
         path,
         false,
     )?;
-    let host_filestat = hostcalls_impl::path_filestat_get(resolved, dirflags)?;
+    let host_filestat = match resolved.dirfd() {
+        Descriptor::VirtualFile(virt) => virt
+            .openat(std::path::Path::new(resolved.path()), false, false, 0, 0)?
+            .filestat_get()?,
+        _ => hostcalls_impl::path_filestat_get(resolved, dirflags)?,
+    };
 
     trace!("     | *filestat_ptr={:?}", host_filestat);
 
@@ -887,7 +1038,14 @@ pub(crate) unsafe fn path_filestat_set_times(
         false,
     )?;
 
+    match resolved.dirfd() {
+        Descriptor::VirtualFile(_virt) => {
+            unimplemented!("virtual filestat_set_times");
+        }
+        _ => {
             hostcalls_impl::path_filestat_set_times(resolved, dirflags, st_atim, st_mtim, fst_flags)
+        }
+    }
 }
 
 pub(crate) unsafe fn path_symlink(
@@ -917,7 +1075,12 @@ pub(crate) unsafe fn path_symlink(
     let fe = wasi_ctx.get_fd_entry(dirfd)?;
     let resolved_new = path_get(fe, wasi::__WASI_RIGHTS_PATH_SYMLINK, 0, 0, new_path, true)?;
 
-    hostcalls_impl::path_symlink(old_path, resolved_new)
+    match resolved_new.dirfd() {
+        Descriptor::VirtualFile(_virt) => {
+            unimplemented!("virtual path_symlink");
+        }
+        _non_virtual => hostcalls_impl::path_symlink(old_path, resolved_new),
+    }
 }
 
 pub(crate) unsafe fn path_unlink_file(
@@ -941,7 +1104,10 @@ pub(crate) unsafe fn path_unlink_file(
     let fe = wasi_ctx.get_fd_entry(dirfd)?;
     let resolved = path_get(fe, wasi::__WASI_RIGHTS_PATH_UNLINK_FILE, 0, 0, path, false)?;
 
-    hostcalls_impl::path_unlink_file(resolved)
+    match resolved.dirfd() {
+        Descriptor::VirtualFile(virt) => virt.unlink_file(resolved.path()),
+        _ => hostcalls_impl::path_unlink_file(resolved),
+    }
 }
 
 pub(crate) unsafe fn path_remove_directory(
@@ -974,7 +1140,10 @@ pub(crate) unsafe fn path_remove_directory(
 
     log::debug!("path_remove_directory resolved={:?}", resolved);
 
-    hostcalls_impl::path_remove_directory(resolved)
+    match resolved.dirfd() {
+        Descriptor::VirtualFile(virt) => virt.remove_directory(resolved.path()),
+        _ => hostcalls_impl::path_remove_directory(resolved),
+    }
 }
 
 pub(crate) unsafe fn fd_prestat_get(
@@ -1068,11 +1237,14 @@ pub(crate) unsafe fn fd_readdir(
         .get_fd_entry_mut(fd)?
         .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_READDIR, 0)?
         .as_file_mut()?;
-    let mut host_buf = dec_slice_of_mut_u8(memory, buf, buf_len)?;
+    let host_buf = dec_slice_of_mut_u8(memory, buf, buf_len)?;
 
     trace!("     | (buf,buf_len)={:?}", host_buf);
 
-    let iter = hostcalls_impl::fd_readdir(file, cookie)?;
+    fn copy_entities<T: Iterator<Item = Result<Dirent>>>(
+        iter: T,
+        mut host_buf: &mut [u8],
+    ) -> Result<usize> {
         let mut host_bufused = 0;
         for dirent in iter {
             let dirent_raw = dirent?.to_wasi_raw()?;
@@ -1085,6 +1257,20 @@ pub(crate) unsafe fn fd_readdir(
                 host_buf = &mut host_buf[offset..];
             }
         }
+        Ok(host_bufused)
+    }
+
+    let host_bufused = match file {
+        Descriptor::OsHandle(file) => {
+            copy_entities(hostcalls_impl::fd_readdir(file, cookie)?, host_buf)?
+        }
+        Descriptor::VirtualFile(virt) => copy_entities(virt.readdir(cookie)?, host_buf)?,
+        _ => {
+            unreachable!(
+                "implementation error: fd should have been checked to not be a stream already"
+            );
+        }
+    };
 
     trace!("     | *buf_used={:?}", host_bufused);
 

crates/wasi-common/src/hostcalls_impl/fs_helpers.rs

@@ -1,24 +1,100 @@
 #![allow(non_camel_case_types)]
+use crate::sys::fdentry_impl::OsHandle;
 use crate::sys::host_impl;
 use crate::sys::hostcalls_impl::fs_helpers::*;
-use crate::{error::WasiError, fdentry::FdEntry, wasi, Error, Result};
+use crate::{error::WasiError, fdentry::Descriptor, fdentry::FdEntry, wasi, Error, Result};
-use std::fs::File;
 use std::path::{Component, Path};
 
 #[derive(Debug)]
 pub(crate) struct PathGet {
-    dirfd: File,
+    dirfd: Descriptor,
     path: String,
 }
 
 impl PathGet {
-    pub(crate) fn dirfd(&self) -> &File {
+    pub(crate) fn dirfd(&self) -> &Descriptor {
         &self.dirfd
     }
 
     pub(crate) fn path(&self) -> &str {
         &self.path
     }
+
+    pub(crate) fn path_create_directory(self) -> Result<()> {
+        match &self.dirfd {
+            Descriptor::OsHandle(file) => {
+                crate::sys::hostcalls_impl::path_create_directory(&file, &self.path)
+            }
+            Descriptor::VirtualFile(virt) => virt.create_directory(&Path::new(&self.path)),
+            other => {
+                panic!("invalid descriptor to create directory: {:?}", other);
+            }
+        }
+    }
+
+    pub(crate) fn open_with(
+        self,
+        read: bool,
+        write: bool,
+        oflags: u16,
+        fs_flags: u16,
+    ) -> Result<Descriptor> {
+        match &self.dirfd {
+            Descriptor::OsHandle(_) => {
+                crate::sys::hostcalls_impl::path_open(self, read, write, oflags, fs_flags)
+                    .map_err(Into::into)
+            }
+            Descriptor::VirtualFile(virt) => virt
+                .openat(Path::new(&self.path), read, write, oflags, fs_flags)
+                .map(|file| Descriptor::VirtualFile(file)),
+            other => {
+                panic!("invalid descriptor to path_open: {:?}", other);
+            }
+        }
+    }
+}
+
+struct PathRef<'a, 'b> {
+    dirfd: &'a Descriptor,
+    path: &'b str,
+}
+
+impl<'a, 'b> PathRef<'a, 'b> {
+    fn new(dirfd: &'a Descriptor, path: &'b str) -> Self {
+        PathRef { dirfd, path }
+    }
+
+    fn open(&self) -> Result<Descriptor> {
+        match self.dirfd {
+            Descriptor::OsHandle(file) => Ok(Descriptor::OsHandle(OsHandle::from(openat(
+                &file, &self.path,
+            )?))),
+            Descriptor::VirtualFile(virt) => virt
+                .openat(
+                    Path::new(&self.path),
+                    false,
+                    false,
+                    wasi::__WASI_OFLAGS_DIRECTORY,
+                    0,
+                )
+                .map(|file| Descriptor::VirtualFile(file)),
+            other => {
+                panic!("invalid descriptor for open: {:?}", other);
+            }
+        }
+    }
+
+    fn readlink(&self) -> Result<String> {
+        match self.dirfd {
+            Descriptor::OsHandle(file) => readlinkat(file, self.path),
+            Descriptor::VirtualFile(virt) => {
+                virt.readlinkat(Path::new(self.path)).map_err(Into::into)
+            }
+            other => {
+                panic!("invalid descriptor for readlink: {:?}", other);
+            }
+        }
+    }
 }
 
 /// Normalizes a path to ensure that the target path is located under the directory provided.
@@ -112,7 +188,9 @@ pub(crate) fn path_get(
                         }
 
                         if !path_stack.is_empty() || (ends_with_slash && !needs_final_component) {
-                            match openat(dir_stack.last().ok_or(Error::ENOTCAPABLE)?, &head) {
+                            match PathRef::new(dir_stack.last().ok_or(Error::ENOTCAPABLE)?, &head)
+                                .open()
+                            {
                                 Ok(new_dir) => {
                                     dir_stack.push(new_dir);
                                 }
@@ -125,10 +203,11 @@ pub(crate) fn path_get(
                                         // this with ENOTDIR because of the O_DIRECTORY flag.
                                         {
                                             // attempt symlink expansion
-                                            let mut link_path = readlinkat(
+                                            let mut link_path = PathRef::new(
                                                 dir_stack.last().ok_or(Error::ENOTCAPABLE)?,
                                                 &head,
-                                            )?;
+                                            )
+                                            .readlink()?;
 
                                             symlink_expansions += 1;
                                             if symlink_expansions > MAX_SYMLINK_EXPANSIONS {
@@ -159,7 +238,9 @@ pub(crate) fn path_get(
                         {
                             // if there's a trailing slash, or if `LOOKUP_SYMLINK_FOLLOW` is set, attempt
                             // symlink expansion
-                            match readlinkat(dir_stack.last().ok_or(Error::ENOTCAPABLE)?, &head) {
+                            match PathRef::new(dir_stack.last().ok_or(Error::ENOTCAPABLE)?, &head)
+                                .readlink()
+                            {
                                 Ok(mut link_path) => {
                                     symlink_expansions += 1;
                                     if symlink_expansions > MAX_SYMLINK_EXPANSIONS {

crates/wasi-common/src/lib.rs

@@ -32,6 +32,8 @@ mod memory;
 pub mod old;
 mod sandboxed_tty_writer;
 mod sys;
+mod virtfs;
+pub use virtfs::{FileContents, VirtualDirEntry};
 pub mod wasi;
 pub mod wasi32;
 

crates/wasi-common/src/memory.rs

@@ -207,7 +207,7 @@ pub(crate) fn dec_ciovec_slice(
         .iter()
         .map(|raw_iov| {
             let len = dec_usize(PrimInt::from_le(raw_iov.buf_len));
-            let buf = PrimInt::from_le(raw_iov.buf);
+            let buf: u32 = PrimInt::from_le(raw_iov.buf);
             Ok(host::__wasi_ciovec_t {
                 buf: dec_ptr(memory, buf, len)? as *const u8,
                 buf_len: len,

crates/wasi-common/src/sys/unix/fdentry_impl.rs

@@ -11,6 +11,7 @@ impl AsRawFd for Descriptor {
     fn as_raw_fd(&self) -> RawFd {
         match self {
             Self::OsHandle(file) => file.as_raw_fd(),
+            Self::VirtualFile(_) => panic!("virtual files do not have a raw fd"),
             Self::Stdin => io::stdin().as_raw_fd(),
             Self::Stdout => io::stdout().as_raw_fd(),
             Self::Stderr => io::stderr().as_raw_fd(),

crates/wasi-common/src/sys/unix/hostcalls_impl/fs.rs

@@ -1,5 +1,6 @@
 #![allow(non_camel_case_types)]
 #![allow(unused_unsafe)]
+use crate::fdentry::Descriptor;
 use crate::host::Dirent;
 use crate::hostcalls_impl::PathGet;
 use crate::sys::{fdentry_impl::OsHandle, host_impl, unix::sys_impl};
@@ -60,16 +61,9 @@ pub(crate) fn fd_advise(
     unsafe { posix_fadvise(file.as_raw_fd(), offset, len, host_advice) }.map_err(Into::into)
 }
 
-pub(crate) fn path_create_directory(resolved: PathGet) -> Result<()> {
+pub(crate) fn path_create_directory(base: &File, path: &str) -> Result<()> {
     use yanix::file::{mkdirat, Mode};
-    unsafe {
+    unsafe { mkdirat(base.as_raw_fd(), path, Mode::from_bits_truncate(0o777)) }.map_err(Into::into)
-        mkdirat(
-            resolved.dirfd().as_raw_fd(),
-            resolved.path(),
-            Mode::from_bits_truncate(0o777),
-        )
-    }
-    .map_err(Into::into)
 }
 
 pub(crate) fn path_link(resolved_old: PathGet, resolved_new: PathGet) -> Result<()> {
@@ -92,7 +86,7 @@ pub(crate) fn path_open(
     write: bool,
     oflags: wasi::__wasi_oflags_t,
     fs_flags: wasi::__wasi_fdflags_t,
-) -> Result<File> {
+) -> Result<Descriptor> {
     use yanix::file::{fstatat, openat, AtFlag, FileType, Mode, OFlag};
 
     let mut nix_all_oflags = if read && write {
@@ -119,14 +113,15 @@ pub(crate) fn path_open(
     log::debug!("path_open resolved = {:?}", resolved);
     log::debug!("path_open oflags = {:?}", nix_all_oflags);
 
-    let new_fd = match unsafe {
+    let fd_no = unsafe {
         openat(
             resolved.dirfd().as_raw_fd(),
             resolved.path(),
             nix_all_oflags,
             Mode::from_bits_truncate(0o666),
         )
-    } {
+    };
+    let new_fd = match fd_no {
         Ok(fd) => fd,
         Err(e) => {
             if let yanix::Error::Io(ref err) = e {
@@ -188,7 +183,7 @@ pub(crate) fn path_open(
     log::debug!("path_open (host) new_fd = {:?}", new_fd);
 
     // Determine the type of the new file descriptor and which rights contradict with this type
-    Ok(unsafe { File::from_raw_fd(new_fd) })
+    Ok(OsHandle::from(unsafe { File::from_raw_fd(new_fd) }).into())
 }
 
 pub(crate) fn path_readlink(resolved: PathGet, buf: &mut [u8]) -> Result<usize> {
@@ -263,7 +258,7 @@ pub(crate) fn path_filestat_set_times(
     };
 
     utimensat(
-        resolved.dirfd(),
+        &resolved.dirfd().as_os_handle(),
         resolved.path(),
         atim,
         mtim,
@@ -274,6 +269,7 @@ pub(crate) fn path_filestat_set_times(
 
 pub(crate) fn path_remove_directory(resolved: PathGet) -> Result<()> {
     use yanix::file::{unlinkat, AtFlag};
+
     unsafe {
         unlinkat(
             resolved.dirfd().as_raw_fd(),

crates/wasi-common/src/sys/unix/linux/hostcalls_impl.rs

@@ -1,3 +1,4 @@
+use crate::fdentry::Descriptor;
 use crate::hostcalls_impl::PathGet;
 use crate::Result;
 use std::os::unix::prelude::AsRawFd;
@@ -26,15 +27,22 @@ pub(crate) fn path_symlink(old_path: &str, resolved: PathGet) -> Result<()> {
 
 pub(crate) fn path_rename(resolved_old: PathGet, resolved_new: PathGet) -> Result<()> {
     use yanix::file::renameat;
+    match (resolved_old.dirfd(), resolved_new.dirfd()) {
+        (Descriptor::OsHandle(resolved_old_file), Descriptor::OsHandle(resolved_new_file)) => {
             unsafe {
                 renameat(
-            resolved_old.dirfd().as_raw_fd(),
+                    resolved_old_file.as_raw_fd(),
                     resolved_old.path(),
-            resolved_new.dirfd().as_raw_fd(),
+                    resolved_new_file.as_raw_fd(),
                     resolved_new.path(),
                 )
             }
             .map_err(Into::into)
+        }
+        _ => {
+            unimplemented!("path_link with one or more virtual files");
+        }
+    }
 }
 
 pub(crate) mod fd_readdir_impl {

crates/wasi-common/src/sys/windows/fdentry_impl.rs

@@ -39,6 +39,9 @@ impl AsRawHandle for Descriptor {
     fn as_raw_handle(&self) -> RawHandle {
         match self {
             Self::OsHandle(file) => file.as_raw_handle(),
+            Self::VirtualFile(_file) => {
+                unimplemented!("virtual as_raw_handle");
+            }
             Self::Stdin => io::stdin().as_raw_handle(),
             Self::Stdout => io::stdout().as_raw_handle(),
             Self::Stderr => io::stderr().as_raw_handle(),

crates/wasi-common/src/sys/windows/hostcalls_impl/fs.rs

@@ -2,7 +2,7 @@
 #![allow(unused)]
 use super::fs_helpers::*;
 use crate::ctx::WasiCtx;
-use crate::fdentry::FdEntry;
+use crate::fdentry::{Descriptor, FdEntry};
 use crate::host::{Dirent, FileType};
 use crate::hostcalls_impl::{fd_filestat_set_times_impl, PathGet};
 use crate::sys::fdentry_impl::{determine_type_rights, OsHandle};
@@ -119,8 +119,8 @@ pub(crate) fn fd_advise(
     Ok(())
 }
 
-pub(crate) fn path_create_directory(resolved: PathGet) -> Result<()> {
+pub(crate) fn path_create_directory(file: &File, path: &str) -> Result<()> {
-    let path = resolved.concatenate()?;
+    let path = concatenate(file, path)?;
     std::fs::create_dir(&path).map_err(Into::into)
 }
 
@@ -134,7 +134,7 @@ pub(crate) fn path_open(
     write: bool,
     oflags: wasi::__wasi_oflags_t,
     fdflags: wasi::__wasi_fdflags_t,
-) -> Result<File> {
+) -> Result<Descriptor> {
     use winx::file::{AccessMode, CreationDisposition, Flags};
 
     let is_trunc = oflags & wasi::__WASI_OFLAGS_TRUNC != 0;
@@ -207,6 +207,7 @@ pub(crate) fn path_open(
     opts.access_mode(access_mode.bits())
         .custom_flags(file_flags_from_fdflags(fdflags).bits())
         .open(&path)
+        .map(|f| OsHandle::from(f).into())
         .map_err(Into::into)
 }
 
@@ -371,7 +372,7 @@ pub(crate) fn path_readlink(resolved: PathGet, buf: &mut [u8]) -> Result<usize>
     // we need to strip the prefix from the absolute path
     // as otherwise we will error out since WASI is not capable
     // of dealing with absolute paths
-    let dir_path = get_file_path(resolved.dirfd())?;
+    let dir_path = get_file_path(&resolved.dirfd().as_os_handle())?;
     let dir_path = PathBuf::from(strip_extended_prefix(dir_path));
     let target_path = target_path
         .strip_prefix(dir_path)
@@ -401,7 +402,7 @@ pub(crate) fn path_readlink(resolved: PathGet, buf: &mut [u8]) -> Result<usize>
 fn strip_trailing_slashes_and_concatenate(resolved: &PathGet) -> Result<Option<PathBuf>> {
     if resolved.path().ends_with('/') {
         let suffix = resolved.path().trim_end_matches('/');
-        concatenate(resolved.dirfd(), Path::new(suffix)).map(Some)
+        concatenate(&resolved.dirfd().as_os_handle(), Path::new(suffix)).map(Some)
     } else {
         Ok(None)
     }
@@ -492,14 +493,15 @@ pub(crate) fn path_filestat_set_times(
     let file = OpenOptions::new()
         .access_mode(AccessMode::FILE_WRITE_ATTRIBUTES.bits())
         .open(path)?;
-    fd_filestat_set_times_impl(&file, st_atim, st_mtim, fst_flags)
+    let modifiable_fd = Descriptor::OsHandle(OsHandle::from(file));
+    fd_filestat_set_times_impl(&modifiable_fd, st_atim, st_mtim, fst_flags)
 }
 
 pub(crate) fn path_symlink(old_path: &str, resolved: PathGet) -> Result<()> {
     use std::os::windows::fs::{symlink_dir, symlink_file};
     use winx::winerror::WinError;
 
-    let old_path = concatenate(resolved.dirfd(), Path::new(old_path))?;
+    let old_path = concatenate(&resolved.dirfd().as_os_handle(), Path::new(old_path))?;
     let new_path = resolved.concatenate()?;
 
     // try creating a file symlink

crates/wasi-common/src/sys/windows/hostcalls_impl/fs_helpers.rs

@@ -1,4 +1,5 @@
 #![allow(non_camel_case_types)]
+use crate::fdentry::Descriptor;
 use crate::hostcalls_impl::PathGet;
 use crate::{wasi, Error, Result};
 use std::ffi::{OsStr, OsString};
@@ -12,7 +13,15 @@ pub(crate) trait PathGetExt {
 
 impl PathGetExt for PathGet {
     fn concatenate(&self) -> Result<PathBuf> {
-        concatenate(self.dirfd(), Path::new(self.path()))
+        match self.dirfd() {
+            Descriptor::OsHandle(file) => concatenate(file, Path::new(self.path())),
+            Descriptor::VirtualFile(_virt) => {
+                panic!("concatenate on a virtual base");
+            }
+            Descriptor::Stdin | Descriptor::Stdout | Descriptor::Stderr => {
+                unreachable!("streams do not have paths and should not be accessible via PathGet");
+            }
+        }
     }
 }
 
@@ -126,7 +135,7 @@ pub(crate) fn strip_extended_prefix<P: AsRef<OsStr>>(path: P) -> OsString {
     }
 }
 
-pub(crate) fn concatenate<P: AsRef<Path>>(dirfd: &File, path: P) -> Result<PathBuf> {
+pub(crate) fn concatenate<P: AsRef<Path>>(file: &File, path: P) -> Result<PathBuf> {
     use winx::file::get_file_path;
 
     // WASI is not able to deal with absolute paths
@@ -135,7 +144,7 @@ pub(crate) fn concatenate<P: AsRef<Path>>(dirfd: &File, path: P) -> Result<PathB
         return Err(Error::ENOTCAPABLE);
     }
 
-    let dir_path = get_file_path(dirfd)?;
+    let dir_path = get_file_path(file)?;
     // concatenate paths
     let mut out_path = PathBuf::from(dir_path);
     out_path.push(path.as_ref());

crates/wasi-common/src/sys/windows/hostcalls_impl/misc.rs

@@ -222,6 +222,9 @@ fn handle_rw_event(event: FdEventData, out_events: &mut Vec<wasi::__wasi_event_t
         Descriptor::Stdin => Ok(1),
         // On Unix, ioctl(FIONREAD) will return 0 for stdout/stderr. Emulate the same behavior on Windows.
         Descriptor::Stdout | Descriptor::Stderr => Ok(0),
+        Descriptor::VirtualFile(_) => {
+            panic!("virtual files do not get rw events");
+        }
     };
 
     let new_event = make_rw_event(&event, size);
@@ -295,6 +298,9 @@ pub(crate) fn poll_oneoff(
                     unreachable!();
                 }
             }
+            Descriptor::VirtualFile(_) => {
+                panic!("virtual files do not get rw events");
+            }
         }
     }
 

crates/wasi-common/src/virtfs.rs

@@ -0,0 +1,878 @@
+use crate::host::Dirent;
+use crate::host::FileType;
+use crate::{wasi, wasi32, Error, Result};
+use filetime::FileTime;
+use log::trace;
+use std::cell::RefCell;
+use std::collections::hash_map::Entry;
+use std::collections::HashMap;
+use std::convert::TryInto;
+use std::io;
+use std::io::SeekFrom;
+use std::path::{Path, PathBuf};
+use std::rc::Rc;
+
+pub enum VirtualDirEntry {
+    Directory(HashMap<String, VirtualDirEntry>),
+    File(Box<dyn FileContents>),
+}
+
+impl VirtualDirEntry {
+    pub fn empty_directory() -> Self {
+        VirtualDirEntry::Directory(HashMap::new())
+    }
+}
+
+/// Files and directories may be moved, and for implementation reasons retain a reference to their
+/// parent VirtualFile, so files that can be moved must provide an interface to update their parent
+/// reference.
+pub(crate) trait MovableFile {
+    fn set_parent(&self, new_parent: Option<Box<dyn VirtualFile>>);
+}
+
+/// `VirtualFile` encompasses the whole interface of a `File`, `Directory`, or `Stream`-like
+/// object, suitable for forwarding from `wasi-common` public interfaces. `File` and
+/// `Directory`-style objects can be moved, so implemetors of this trait must also implement
+/// `MovableFile`.
+///
+/// Default implementations of functions here fail in ways that are intended to mimic a file-like
+/// object with no permissions, no content, and that cannot be used in any way.
+pub(crate) trait VirtualFile: MovableFile {
+    fn fdstat_get(&self) -> wasi::__wasi_fdflags_t {
+        0
+    }
+
+    fn try_clone(&self) -> io::Result<Box<dyn VirtualFile>>;
+
+    fn readlinkat(&self, _path: &Path) -> Result<String> {
+        Err(Error::EACCES)
+    }
+
+    fn openat(
+        &self,
+        _path: &Path,
+        _read: bool,
+        _write: bool,
+        _oflags: wasi::__wasi_oflags_t,
+        _fd_flags: wasi::__wasi_fdflags_t,
+    ) -> Result<Box<dyn VirtualFile>> {
+        Err(Error::EACCES)
+    }
+
+    fn remove_directory(&self, _path: &str) -> Result<()> {
+        Err(Error::EACCES)
+    }
+
+    fn unlink_file(&self, _path: &str) -> Result<()> {
+        Err(Error::EACCES)
+    }
+
+    fn datasync(&self) -> Result<()> {
+        Err(Error::EINVAL)
+    }
+
+    fn sync(&self) -> Result<()> {
+        Ok(())
+    }
+
+    fn create_directory(&self, _path: &Path) -> Result<()> {
+        Err(Error::EACCES)
+    }
+
+    fn readdir(
+        &self,
+        _cookie: wasi::__wasi_dircookie_t,
+    ) -> Result<Box<dyn Iterator<Item = Result<Dirent>>>> {
+        Err(Error::EBADF)
+    }
+
+    fn write_vectored(&mut self, _iovs: &[io::IoSlice]) -> Result<usize> {
+        Err(Error::EBADF)
+    }
+
+    fn pread(&self, _buf: &mut [u8], _offset: u64) -> Result<usize> {
+        Err(Error::EBADF)
+    }
+
+    fn pwrite(&self, _buf: &mut [u8], _offset: u64) -> Result<usize> {
+        Err(Error::EBADF)
+    }
+
+    fn seek(&mut self, _offset: SeekFrom) -> Result<u64> {
+        Err(Error::EBADF)
+    }
+
+    fn advise(
+        &self,
+        _advice: wasi::__wasi_advice_t,
+        _offset: wasi::__wasi_filesize_t,
+        _len: wasi::__wasi_filesize_t,
+    ) -> Result<()> {
+        Err(Error::EBADF)
+    }
+
+    fn allocate(
+        &self,
+        _offset: wasi::__wasi_filesize_t,
+        _len: wasi::__wasi_filesize_t,
+    ) -> Result<()> {
+        Err(Error::EBADF)
+    }
+
+    fn filestat_get(&self) -> Result<wasi::__wasi_filestat_t> {
+        Err(Error::EBADF)
+    }
+
+    fn filestat_set_times(&self, _atim: Option<FileTime>, _mtim: Option<FileTime>) -> Result<()> {
+        Err(Error::EBADF)
+    }
+
+    fn filestat_set_size(&self, _st_size: wasi::__wasi_filesize_t) -> Result<()> {
+        Err(Error::EBADF)
+    }
+
+    fn fdstat_set_flags(&mut self, _fdflags: wasi::__wasi_fdflags_t) -> Result<()> {
+        Err(Error::EBADF)
+    }
+
+    fn read_vectored(&mut self, _iovs: &mut [io::IoSliceMut]) -> Result<usize> {
+        Err(Error::EBADF)
+    }
+
+    fn get_file_type(&self) -> wasi::__wasi_filetype_t;
+
+    fn get_rights_base(&self) -> wasi::__wasi_rights_t {
+        0
+    }
+
+    fn get_rights_inheriting(&self) -> wasi::__wasi_rights_t {
+        0
+    }
+}
+
+pub trait FileContents {
+    /// The implementation-defined maximum size of the store corresponding to a `FileContents`
+    /// implementation.
+    fn max_size(&self) -> wasi::__wasi_filesize_t;
+    /// The current number of bytes this `FileContents` describes.
+    fn size(&self) -> wasi::__wasi_filesize_t;
+    /// Resize to hold `new_size` number of bytes, or error if this is not possible.
+    fn resize(&mut self, new_size: wasi::__wasi_filesize_t) -> Result<()>;
+    /// Write a list of `IoSlice` starting at `offset`. `offset` plus the total size of all `iovs`
+    /// is guaranteed to not exceed `max_size`. Implementations must not indicate more bytes have
+    /// been written than can be held by `iovs`.
+    fn pwritev(&mut self, iovs: &[io::IoSlice], offset: wasi::__wasi_filesize_t) -> Result<usize>;
+    /// Read from the file from `offset`, filling a list of `IoSlice`. The returend size must not
+    /// be more than the capactiy of `iovs`, and must not exceed the limit reported by
+    /// `self.max_size()`.
+    fn preadv(&self, iovs: &mut [io::IoSliceMut], offset: wasi::__wasi_filesize_t)
+        -> Result<usize>;
+    /// Write contents from `buf` to this file starting at `offset`. `offset` plus the length of
+    /// `buf` is guaranteed to not exceed `max_size`. Implementations must not indicate more bytes
+    /// have been written than the size of `buf`.
+    fn pwrite(&mut self, buf: &[u8], offset: wasi::__wasi_filesize_t) -> Result<usize>;
+    /// Read from the file at `offset`, filling `buf`. The returned size must not be more than the
+    /// capacity of `buf`, and `offset` plus the returned size must not exceed `self.max_size()`.
+    fn pread(&self, buf: &mut [u8], offset: wasi::__wasi_filesize_t) -> Result<usize>;
+}
+
+impl FileContents for VecFileContents {
+    fn max_size(&self) -> wasi::__wasi_filesize_t {
+        std::usize::MAX as wasi::__wasi_filesize_t
+    }
+
+    fn size(&self) -> wasi::__wasi_filesize_t {
+        self.content.len() as wasi::__wasi_filesize_t
+    }
+
+    fn resize(&mut self, new_size: wasi::__wasi_filesize_t) -> Result<()> {
+        let new_size: usize = new_size.try_into().map_err(|_| Error::EINVAL)?;
+        self.content.resize(new_size, 0);
+        Ok(())
+    }
+
+    fn preadv(
+        &self,
+        iovs: &mut [io::IoSliceMut],
+        offset: wasi::__wasi_filesize_t,
+    ) -> Result<usize> {
+        let mut read_total = 0usize;
+        for iov in iovs.iter_mut() {
+            let read = self.pread(iov, offset)?;
+            read_total = read_total.checked_add(read).expect("FileContents::preadv must not be called when reads could total to more bytes than the return value can hold");
+        }
+        Ok(read_total)
+    }
+
+    fn pwritev(&mut self, iovs: &[io::IoSlice], offset: wasi::__wasi_filesize_t) -> Result<usize> {
+        let mut write_total = 0usize;
+        for iov in iovs.iter() {
+            let written = self.pwrite(iov, offset)?;
+            write_total = write_total.checked_add(written).expect("FileContents::pwritev must not be called when writes could total to more bytes than the return value can hold");
+        }
+        Ok(write_total)
+    }
+
+    fn pread(&self, buf: &mut [u8], offset: wasi::__wasi_filesize_t) -> Result<usize> {
+        trace!("     | pread(buf.len={}, offset={})", buf.len(), offset);
+        let offset: usize = offset.try_into().map_err(|_| Error::EINVAL)?;
+
+        let data_remaining = self.content.len().saturating_sub(offset);
+
+        let read_count = std::cmp::min(buf.len(), data_remaining);
+
+        (&mut buf[..read_count]).copy_from_slice(&self.content[offset..][..read_count]);
+
+        let res = Ok(read_count);
+        trace!("     | pread={:?}", res);
+        res
+    }
+
+    fn pwrite(&mut self, buf: &[u8], offset: wasi::__wasi_filesize_t) -> Result<usize> {
+        let offset: usize = offset.try_into().map_err(|_| Error::EINVAL)?;
+
+        let write_end = offset.checked_add(buf.len()).ok_or(Error::EFBIG)?;
+
+        if write_end > self.content.len() {
+            self.content.resize(write_end, 0);
+        }
+
+        (&mut self.content[offset..][..buf.len()]).copy_from_slice(buf);
+
+        Ok(buf.len())
+    }
+}
+
+struct VecFileContents {
+    content: Vec<u8>,
+}
+
+impl VecFileContents {
+    fn new() -> Self {
+        Self {
+            content: Vec::new(),
+        }
+    }
+}
+
+/// An `InMemoryFile` is a shared handle to some underlying data. The relationship is analagous to
+/// a filesystem wherein a file descriptor is one view into a possibly-shared underlying collection
+/// of data and permissions on a filesystem.
+pub struct InMemoryFile {
+    cursor: wasi::__wasi_filesize_t,
+    parent: Rc<RefCell<Option<Box<dyn VirtualFile>>>>,
+    fd_flags: wasi::__wasi_fdflags_t,
+    data: Rc<RefCell<Box<dyn FileContents>>>,
+}
+
+impl InMemoryFile {
+    pub fn memory_backed() -> Self {
+        Self {
+            cursor: 0,
+            parent: Rc::new(RefCell::new(None)),
+            fd_flags: 0,
+            data: Rc::new(RefCell::new(Box::new(VecFileContents::new()))),
+        }
+    }
+
+    pub fn new(contents: Box<dyn FileContents>) -> Self {
+        Self {
+            cursor: 0,
+            fd_flags: 0,
+            parent: Rc::new(RefCell::new(None)),
+            data: Rc::new(RefCell::new(contents)),
+        }
+    }
+}
+
+impl MovableFile for InMemoryFile {
+    fn set_parent(&self, new_parent: Option<Box<dyn VirtualFile>>) {
+        *self.parent.borrow_mut() = new_parent;
+    }
+}
+
+impl VirtualFile for InMemoryFile {
+    fn fdstat_get(&self) -> wasi::__wasi_fdflags_t {
+        self.fd_flags
+    }
+
+    fn try_clone(&self) -> io::Result<Box<dyn VirtualFile>> {
+        Ok(Box::new(InMemoryFile {
+            cursor: 0,
+            fd_flags: self.fd_flags,
+            parent: Rc::clone(&self.parent),
+            data: Rc::clone(&self.data),
+        }))
+    }
+
+    fn readlinkat(&self, _path: &Path) -> Result<String> {
+        // no symlink support, so always say it's invalid.
+        Err(Error::ENOTDIR)
+    }
+
+    fn openat(
+        &self,
+        path: &Path,
+        read: bool,
+        write: bool,
+        oflags: wasi::__wasi_oflags_t,
+        fd_flags: wasi::__wasi_fdflags_t,
+    ) -> Result<Box<dyn VirtualFile>> {
+        log::trace!(
+            "InMemoryFile::openat(path={:?}, read={:?}, write={:?}, oflags={:?}, fd_flags={:?}",
+            path,
+            read,
+            write,
+            oflags,
+            fd_flags
+        );
+
+        if oflags & wasi::__WASI_OFLAGS_DIRECTORY != 0 {
+            log::trace!(
+                "InMemoryFile::openat was passed oflags DIRECTORY, but {:?} is a file.",
+                path
+            );
+            log::trace!("  return ENOTDIR");
+            return Err(Error::ENOTDIR);
+        }
+
+        if path == Path::new(".") {
+            return self.try_clone().map_err(Into::into);
+        } else if path == Path::new("..") {
+            match &*self.parent.borrow() {
+                Some(file) => file.try_clone().map_err(Into::into),
+                None => self.try_clone().map_err(Into::into),
+            }
+        } else {
+            Err(Error::EACCES)
+        }
+    }
+
+    fn remove_directory(&self, _path: &str) -> Result<()> {
+        Err(Error::ENOTDIR)
+    }
+
+    fn unlink_file(&self, _path: &str) -> Result<()> {
+        Err(Error::ENOTDIR)
+    }
+
+    fn fdstat_set_flags(&mut self, fdflags: wasi::__wasi_fdflags_t) -> Result<()> {
+        self.fd_flags = fdflags;
+        Ok(())
+    }
+
+    fn write_vectored(&mut self, iovs: &[io::IoSlice]) -> Result<usize> {
+        trace!("write_vectored(iovs={:?})", iovs);
+        let mut data = self.data.borrow_mut();
+
+        let append_mode = self.fd_flags & wasi::__WASI_FDFLAGS_APPEND != 0;
+        trace!("     | fd_flags={:o}", self.fd_flags);
+
+        // If this file is in append mode, we write to the end.
+        let write_start = if append_mode {
+            data.size()
+        } else {
+            self.cursor
+        };
+
+        let max_size = iovs
+            .iter()
+            .map(|iov| {
+                let cast_iovlen: wasi32::size_t = iov
+                    .len()
+                    .try_into()
+                    .expect("iovec are bounded by wasi max sizes");
+                cast_iovlen
+            })
+            .fold(Some(0u32), |len, iov| len.and_then(|x| x.checked_add(iov)))
+            .expect("write_vectored will not be called with invalid iovs");
+
+        if let Some(end) = write_start.checked_add(max_size as wasi::__wasi_filesize_t) {
+            if end > data.max_size() {
+                return Err(Error::EFBIG);
+            }
+        } else {
+            return Err(Error::EFBIG);
+        }
+
+        trace!("     | *write_start={:?}", write_start);
+        let written = data.pwritev(iovs, write_start)?;
+
+        // If we are not appending, adjust the cursor appropriately for the write, too. This can't
+        // overflow, as we checked against that before writing any data.
+        if !append_mode {
+            self.cursor += written as u64;
+        }
+
+        Ok(written)
+    }
+
+    fn read_vectored(&mut self, iovs: &mut [io::IoSliceMut]) -> Result<usize> {
+        trace!("read_vectored(iovs={:?})", iovs);
+        trace!("     | *read_start={:?}", self.cursor);
+        self.data.borrow_mut().preadv(iovs, self.cursor)
+    }
+
+    fn pread(&self, buf: &mut [u8], offset: wasi::__wasi_filesize_t) -> Result<usize> {
+        self.data.borrow_mut().pread(buf, offset)
+    }
+
+    fn pwrite(&self, buf: &mut [u8], offset: wasi::__wasi_filesize_t) -> Result<usize> {
+        self.data.borrow_mut().pwrite(buf, offset)
+    }
+
+    fn seek(&mut self, offset: SeekFrom) -> Result<wasi::__wasi_filesize_t> {
+        let content_len = self.data.borrow().size();
+        match offset {
+            SeekFrom::Current(offset) => {
+                let new_cursor = if offset < 0 {
+                    self.cursor
+                        .checked_sub(offset.wrapping_neg() as u64)
+                        .ok_or(Error::EINVAL)?
+                } else {
+                    self.cursor
+                        .checked_add(offset as u64)
+                        .ok_or(Error::EINVAL)?
+                };
+                self.cursor = std::cmp::min(content_len, new_cursor);
+            }
+            SeekFrom::End(offset) => {
+                // A negative offset from the end would be past the end of the file,
+                let offset: u64 = offset.try_into().map_err(|_| Error::EINVAL)?;
+                self.cursor = content_len.saturating_sub(offset);
+            }
+            SeekFrom::Start(offset) => {
+                // A negative offset from the end would be before the start of the file.
+                let offset: u64 = offset.try_into().map_err(|_| Error::EINVAL)?;
+                self.cursor = std::cmp::min(content_len, offset);
+            }
+        }
+
+        Ok(self.cursor)
+    }
+
+    fn advise(
+        &self,
+        advice: wasi::__wasi_advice_t,
+        _offset: wasi::__wasi_filesize_t,
+        _len: wasi::__wasi_filesize_t,
+    ) -> Result<()> {
+        // we'll just ignore advice for now, unless it's totally invalid
+        match advice {
+            wasi::__WASI_ADVICE_DONTNEED
+            | wasi::__WASI_ADVICE_SEQUENTIAL
+            | wasi::__WASI_ADVICE_WILLNEED
+            | wasi::__WASI_ADVICE_NOREUSE
+            | wasi::__WASI_ADVICE_RANDOM
+            | wasi::__WASI_ADVICE_NORMAL => Ok(()),
+            _ => Err(Error::EINVAL),
+        }
+    }
+
+    fn allocate(
+        &self,
+        offset: wasi::__wasi_filesize_t,
+        len: wasi::__wasi_filesize_t,
+    ) -> Result<()> {
+        let new_limit = offset.checked_add(len).ok_or(Error::EFBIG)?;
+        let mut data = self.data.borrow_mut();
+
+        if new_limit > data.max_size() {
+            return Err(Error::EFBIG);
+        }
+
+        if new_limit > data.size() {
+            data.resize(new_limit)?;
+        }
+
+        Ok(())
+    }
+
+    fn filestat_set_size(&self, st_size: wasi::__wasi_filesize_t) -> Result<()> {
+        let mut data = self.data.borrow_mut();
+        if st_size > data.max_size() {
+            return Err(Error::EFBIG);
+        }
+        data.resize(st_size)
+    }
+
+    fn filestat_get(&self) -> Result<wasi::__wasi_filestat_t> {
+        let stat = wasi::__wasi_filestat_t {
+            dev: 0,
+            ino: 0,
+            nlink: 0,
+            size: self.data.borrow().size(),
+            atim: 0,
+            ctim: 0,
+            mtim: 0,
+            filetype: self.get_file_type(),
+        };
+        Ok(stat)
+    }
+
+    fn get_file_type(&self) -> wasi::__wasi_filetype_t {
+        wasi::__WASI_FILETYPE_REGULAR_FILE
+    }
+
+    fn get_rights_base(&self) -> wasi::__wasi_rights_t {
+        wasi::RIGHTS_REGULAR_FILE_BASE
+    }
+
+    fn get_rights_inheriting(&self) -> wasi::__wasi_rights_t {
+        wasi::RIGHTS_REGULAR_FILE_INHERITING
+    }
+}
+
+/// A clonable read/write directory.
+pub struct VirtualDir {
+    writable: bool,
+    // All copies of this `VirtualDir` must share `parent`, and changes in one copy's `parent`
+    // must be reflected in all handles, so they share `Rc` of an underlying `parent`.
+    parent: Rc<RefCell<Option<Box<dyn VirtualFile>>>>,
+    entries: Rc<RefCell<HashMap<PathBuf, Box<dyn VirtualFile>>>>,
+}
+
+impl VirtualDir {
+    pub fn new(writable: bool) -> Self {
+        VirtualDir {
+            writable,
+            parent: Rc::new(RefCell::new(None)),
+            entries: Rc::new(RefCell::new(HashMap::new())),
+        }
+    }
+
+    #[allow(dead_code)]
+    pub fn with_dir<P: AsRef<Path>>(mut self, dir: VirtualDir, path: P) -> Self {
+        self.add_dir(dir, path);
+        self
+    }
+
+    #[allow(dead_code)]
+    pub fn add_dir<P: AsRef<Path>>(&mut self, dir: VirtualDir, path: P) {
+        let entry = Box::new(dir);
+        entry.set_parent(Some(self.try_clone().expect("can clone self")));
+        self.entries
+            .borrow_mut()
+            .insert(path.as_ref().to_owned(), entry);
+    }
+
+    #[allow(dead_code)]
+    pub fn with_file<P: AsRef<Path>>(mut self, content: Box<dyn FileContents>, path: P) -> Self {
+        self.add_file(content, path);
+        self
+    }
+
+    #[allow(dead_code)]
+    pub fn add_file<P: AsRef<Path>>(&mut self, content: Box<dyn FileContents>, path: P) {
+        let entry = Box::new(InMemoryFile::new(content));
+        entry.set_parent(Some(self.try_clone().expect("can clone self")));
+        self.entries
+            .borrow_mut()
+            .insert(path.as_ref().to_owned(), entry);
+    }
+}
+
+impl MovableFile for VirtualDir {
+    fn set_parent(&self, new_parent: Option<Box<dyn VirtualFile>>) {
+        *self.parent.borrow_mut() = new_parent;
+    }
+}
+
+const SELF_DIR_COOKIE: u32 = 0;
+const PARENT_DIR_COOKIE: u32 = 1;
+
+// This MUST be the number of constants above. This limit is used to prevent allocation of files
+// that would wrap and be mapped to the same dir cookies as `self` or `parent`.
+const RESERVED_ENTRY_COUNT: u32 = 2;
+
+impl VirtualFile for VirtualDir {
+    fn try_clone(&self) -> io::Result<Box<dyn VirtualFile>> {
+        Ok(Box::new(VirtualDir {
+            writable: self.writable,
+            parent: Rc::clone(&self.parent),
+            entries: Rc::clone(&self.entries),
+        }))
+    }
+
+    fn readlinkat(&self, _path: &Path) -> Result<String> {
+        // Files are not symbolic links or directories, faithfully report ENOTDIR.
+        Err(Error::ENOTDIR)
+    }
+
+    fn openat(
+        &self,
+        path: &Path,
+        read: bool,
+        write: bool,
+        oflags: wasi::__wasi_oflags_t,
+        fd_flags: wasi::__wasi_fdflags_t,
+    ) -> Result<Box<dyn VirtualFile>> {
+        log::trace!(
+            "VirtualDir::openat(path={:?}, read={:?}, write={:?}, oflags={:?}, fd_flags={:?}",
+            path,
+            read,
+            write,
+            oflags,
+            fd_flags
+        );
+
+        if path == Path::new(".") {
+            return self.try_clone().map_err(Into::into);
+        } else if path == Path::new("..") {
+            match &*self.parent.borrow() {
+                Some(file) => {
+                    return file.try_clone().map_err(Into::into);
+                }
+                None => {
+                    return self.try_clone().map_err(Into::into);
+                }
+            }
+        }
+
+        // openat may have been passed a path with a trailing slash, but files are mapped to paths
+        // with trailing slashes normalized out.
+        let file_name = path.file_name().ok_or(Error::EINVAL)?;
+        let mut entries = self.entries.borrow_mut();
+        let entry_count = entries.len();
+        match entries.entry(Path::new(file_name).to_path_buf()) {
+            Entry::Occupied(e) => {
+                let creat_excl_mask = wasi::__WASI_OFLAGS_CREAT | wasi::__WASI_OFLAGS_EXCL;
+                if (oflags & creat_excl_mask) == creat_excl_mask {
+                    log::trace!("VirtualDir::openat was passed oflags CREAT|EXCL, but the file {:?} exists.", file_name);
+                    log::trace!("  return EEXIST");
+                    return Err(Error::EEXIST);
+                }
+
+                if (oflags & wasi::__WASI_OFLAGS_DIRECTORY) != 0
+                    && e.get().get_file_type() != wasi::__WASI_FILETYPE_DIRECTORY
+                {
+                    log::trace!(
+                        "VirtualDir::openat was passed oflags DIRECTORY, but {:?} is a file.",
+                        file_name
+                    );
+                    log::trace!("  return ENOTDIR");
+                    return Err(Error::ENOTDIR);
+                }
+
+                e.get().try_clone().map_err(Into::into)
+            }
+            Entry::Vacant(v) => {
+                if self.writable {
+                    // Enforce a hard limit at `u32::MAX - 2` files.
+                    // This is to have a constant limit (rather than target-dependent limit we
+                    // would have with `usize`. The limit is the full `u32` range minus two so we
+                    // can reserve "self" and "parent" cookie values.
+                    if entry_count >= (std::u32::MAX - RESERVED_ENTRY_COUNT) as usize {
+                        return Err(Error::ENOSPC);
+                    }
+
+                    log::trace!(
+                        "VirtualDir::openat creating an InMemoryFile named {}",
+                        path.display()
+                    );
+
+                    let mut file = Box::new(InMemoryFile::memory_backed());
+                    file.fd_flags = fd_flags;
+                    file.set_parent(Some(self.try_clone().expect("can clone self")));
+                    v.insert(file).try_clone().map_err(Into::into)
+                } else {
+                    Err(Error::EACCES)
+                }
+            }
+        }
+    }
+
+    fn remove_directory(&self, path: &str) -> Result<()> {
+        let trimmed_path = path.trim_end_matches('/');
+        let mut entries = self.entries.borrow_mut();
+        match entries.entry(Path::new(trimmed_path).to_path_buf()) {
+            Entry::Occupied(e) => {
+                // first, does this name a directory?
+                if e.get().get_file_type() != wasi::__WASI_FILETYPE_DIRECTORY {
+                    return Err(Error::ENOTDIR);
+                }
+
+                // Okay, but is the directory empty?
+                let iter = e.get().readdir(wasi::__WASI_DIRCOOKIE_START)?;
+                if iter.skip(RESERVED_ENTRY_COUNT as usize).next().is_some() {
+                    return Err(Error::ENOTEMPTY);
+                }
+
+                // Alright, it's an empty directory. We can remove it.
+                let removed = e.remove_entry();
+
+                // And sever the file's parent ref to avoid Rc cycles.
+                removed.1.set_parent(None);
+
+                Ok(())
+            }
+            Entry::Vacant(_) => {
+                log::trace!(
+                    "VirtualDir::remove_directory failed to remove {}, no such entry",
+                    trimmed_path
+                );
+                Err(Error::ENOENT)
+            }
+        }
+    }
+
+    fn unlink_file(&self, path: &str) -> Result<()> {
+        let trimmed_path = path.trim_end_matches('/');
+
+        // Special case: we may be unlinking this directory itself if path is `"."`. In that case,
+        // fail with EISDIR, since this is a directory. Alternatively, we may be unlinking `".."`,
+        // which is bound the same way, as this is by definition contained in a directory.
+        if trimmed_path == "." || trimmed_path == ".." {
+            return Err(Error::EISDIR);
+        }
+
+        let mut entries = self.entries.borrow_mut();
+        match entries.entry(Path::new(trimmed_path).to_path_buf()) {
+            Entry::Occupied(e) => {
+                // Directories must be removed through `remove_directory`, not `unlink_file`.
+                if e.get().get_file_type() == wasi::__WASI_FILETYPE_DIRECTORY {
+                    return Err(Error::EISDIR);
+                }
+
+                let removed = e.remove_entry();
+
+                // Sever the file's parent ref to avoid Rc cycles.
+                removed.1.set_parent(None);
+
+                Ok(())
+            }
+            Entry::Vacant(_) => {
+                log::trace!(
+                    "VirtualDir::unlink_file failed to remove {}, no such entry",
+                    trimmed_path
+                );
+                Err(Error::ENOENT)
+            }
+        }
+    }
+
+    fn create_directory(&self, path: &Path) -> Result<()> {
+        let mut entries = self.entries.borrow_mut();
+        match entries.entry(path.to_owned()) {
+            Entry::Occupied(_) => Err(Error::EEXIST),
+            Entry::Vacant(v) => {
+                if self.writable {
+                    let new_dir = Box::new(VirtualDir::new(true));
+                    new_dir.set_parent(Some(self.try_clone()?));
+                    v.insert(new_dir);
+                    Ok(())
+                } else {
+                    Err(Error::EACCES)
+                }
+            }
+        }
+    }
+
+    fn write_vectored(&mut self, _iovs: &[io::IoSlice]) -> Result<usize> {
+        Err(Error::EBADF)
+    }
+
+    fn readdir(
+        &self,
+        cookie: wasi::__wasi_dircookie_t,
+    ) -> Result<Box<dyn Iterator<Item = Result<Dirent>>>> {
+        struct VirtualDirIter {
+            start: u32,
+            entries: Rc<RefCell<HashMap<PathBuf, Box<dyn VirtualFile>>>>,
+        }
+        impl Iterator for VirtualDirIter {
+            type Item = Result<Dirent>;
+
+            fn next(&mut self) -> Option<Self::Item> {
+                log::trace!("VirtualDirIter::next continuing from {}", self.start);
+                if self.start == SELF_DIR_COOKIE {
+                    self.start += 1;
+                    return Some(Ok(Dirent {
+                        name: ".".to_owned(),
+                        ftype: FileType::from_wasi(wasi::__WASI_FILETYPE_DIRECTORY)
+                            .expect("directories are valid file types"),
+                        ino: 0,
+                        cookie: self.start as u64,
+                    }));
+                }
+                if self.start == PARENT_DIR_COOKIE {
+                    self.start += 1;
+                    return Some(Ok(Dirent {
+                        name: "..".to_owned(),
+                        ftype: FileType::from_wasi(wasi::__WASI_FILETYPE_DIRECTORY)
+                            .expect("directories are valid file types"),
+                        ino: 0,
+                        cookie: self.start as u64,
+                    }));
+                }
+
+                let entries = self.entries.borrow();
+
+                // Adjust `start` to be an appropriate number of HashMap entries.
+                let start = self.start - RESERVED_ENTRY_COUNT;
+                if start as usize >= entries.len() {
+                    return None;
+                }
+
+                self.start += 1;
+
+                let (path, file) = entries
+                    .iter()
+                    .skip(start as usize)
+                    .next()
+                    .expect("seeked less than the length of entries");
+
+                let entry = Dirent {
+                    name: path
+                        .to_str()
+                        .expect("wasi paths are valid utf8 strings")
+                        .to_owned(),
+                    ftype: FileType::from_wasi(file.get_file_type())
+                        .expect("virtfs reports valid wasi file types"),
+                    ino: 0,
+                    cookie: self.start as u64,
+                };
+
+                Some(Ok(entry))
+            }
+        }
+        let cookie = match cookie.try_into() {
+            Ok(cookie) => cookie,
+            Err(_) => {
+                // Cookie is larger than u32. it doesn't seem like there's an explicit error
+                // condition in POSIX or WASI, so just start from the start?
+                0
+            }
+        };
+        Ok(Box::new(VirtualDirIter {
+            start: cookie,
+            entries: Rc::clone(&self.entries),
+        }))
+    }
+
+    fn filestat_get(&self) -> Result<wasi::__wasi_filestat_t> {
+        let stat = wasi::__wasi_filestat_t {
+            dev: 0,
+            ino: 0,
+            nlink: 0,
+            size: 0,
+            atim: 0,
+            ctim: 0,
+            mtim: 0,
+            filetype: self.get_file_type(),
+        };
+        Ok(stat)
+    }
+
+    fn get_file_type(&self) -> wasi::__wasi_filetype_t {
+        wasi::__WASI_FILETYPE_DIRECTORY
+    }
+
+    fn get_rights_base(&self) -> wasi::__wasi_rights_t {
+        wasi::RIGHTS_DIRECTORY_BASE
+    }
+
+    fn get_rights_inheriting(&self) -> wasi::__wasi_rights_t {
+        wasi::RIGHTS_DIRECTORY_INHERITING
+    }
+}