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wiggle: copy guest strings from shared memory
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Along the same lines as #5471, this change adds a new smart pointer,
`GuestStrCow`, to copy the string bytes over from Wasm memory to the
host when the string is found in shared memory. This is necessary to
maintain Rust guarantees: with shared memory, the bytes backing a
`GuestStr` could be altered by another thread and this would invalidate
the assumption that we can dereference at any point to `&str`.
`GuestStrCow` is essentially a wrapper around `GuestStr` when the memory
is not shared but copies the memory region into a `String` when the
memory is shared.
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@abrown
abrown committed Dec 19, 2022
1 parent 66b930c commit 9c343b3
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Showing 2 changed files with 78 additions and 39 deletions.
24 changes: 10 additions & 14 deletions crates/wasi-common/src/snapshots/preview_1.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -663,7 +663,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
self.table()
.get_dir(u32::from(dirfd))?
.get_cap(DirCaps::CREATE_DIRECTORY)?
.create_dir(path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref())
.create_dir(&path.as_cow()?)
.await
}

Expand All @@ -678,7 +678,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
.get_dir(u32::from(dirfd))?
.get_cap(DirCaps::PATH_FILESTAT_GET)?
.get_path_filestat(
path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref(),
&path.as_cow()?,
flags.contains(types::Lookupflags::SYMLINK_FOLLOW),
)
.await?;
Expand All @@ -705,7 +705,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
.get_dir(u32::from(dirfd))?
.get_cap(DirCaps::PATH_FILESTAT_SET_TIMES)?
.set_times(
path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref(),
&path.as_cow()?,
atim,
mtim,
flags.contains(types::Lookupflags::SYMLINK_FOLLOW),
Expand Down Expand Up @@ -736,9 +736,9 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {

src_dir
.hard_link(
src_path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref(),
&src_path.as_cow()?,
target_dir.deref(),
target_path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref(),
&target_path.as_cow()?,
)
.await
}
Expand All @@ -764,7 +764,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {

let oflags = OFlags::from(&oflags);
let fdflags = FdFlags::from(fdflags);
let path = path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)");
let path = path.as_cow()?;
if oflags.contains(OFlags::DIRECTORY) {
if oflags.contains(OFlags::CREATE)
|| oflags.contains(OFlags::EXCLUSIVE)
Expand Down Expand Up @@ -813,7 +813,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
.table()
.get_dir(u32::from(dirfd))?
.get_cap(DirCaps::READLINK)?
.read_link(path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref())
.read_link(&path.as_cow()?)
.await?
.into_os_string()
.into_string()
Expand All @@ -835,7 +835,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
self.table()
.get_dir(u32::from(dirfd))?
.get_cap(DirCaps::REMOVE_DIRECTORY)?
.remove_dir(path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref())
.remove_dir(&path.as_cow()?)
.await
}

Expand All @@ -854,11 +854,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
.get_dir(u32::from(dest_fd))?
.get_cap(DirCaps::RENAME_TARGET)?;
src_dir
.rename(
src_path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref(),
dest_dir.deref(),
dest_path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref(),
)
.rename(&src_path.as_cow()?, dest_dir.deref(), &dest_path.as_cow()?)
.await
}

Expand All @@ -871,7 +867,7 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
self.table()
.get_dir(u32::from(dirfd))?
.get_cap(DirCaps::SYMLINK)?
.symlink(src_path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref(), dest_path.as_str()?.expect("cannot use with shared memories; see https://github.com/bytecodealliance/wasmtime/issues/5235 (TODO)").deref())
.symlink(&src_path.as_cow()?, &dest_path.as_cow()?)
.await
}

Expand Down
93 changes: 68 additions & 25 deletions crates/wiggle/src/lib.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -703,7 +703,8 @@ impl<'a> GuestPtr<'a, str> {
/// `GuestError` will be returned.
///
/// Additionally, because it is `unsafe` to have a `GuestStr` of shared
/// memory, this function will return `None` in this case.
/// memory, this function will return `None` in this case (see
/// [`GuestPtr<'_, str>::as_cow`]).
pub fn as_str(&self) -> Result<Option<GuestStr<'a>>, GuestError> {
match self.as_bytes().as_unsafe_slice_mut()?.shared_borrow() {
UnsafeBorrowResult::Ok(s) => Ok(Some(s.try_into()?)),
Expand Down Expand Up @@ -731,6 +732,24 @@ impl<'a> GuestPtr<'a, str> {
UnsafeBorrowResult::Err(e) => Err(e),
}
}

/// Attempts to create a [`GuestStrCow<'_>`] from this pointer, performing
/// bounds checks and utf-8 checks. Whereas [`GuestPtr::as_str`] will fail
/// with `None` if attempting to access Wasm shared memory, this call will
/// succeed: if used on shared memory, this function will copy the string
/// into [`GuestStrCow::Copied`]. If the memory is non-shared, this returns
/// a [`GuestStrCow::Borrowed`] (a thin wrapper over [`GuestStr<'_, T>]`).
pub fn as_cow(&self) -> Result<GuestStrCow<'a>, GuestError> {
match self.as_bytes().as_unsafe_slice_mut()?.shared_borrow() {
UnsafeBorrowResult::Ok(s) => Ok(GuestStrCow::Borrowed(GuestStr(s))),
UnsafeBorrowResult::Shared(_) => {
let copied = self.as_bytes().to_vec()?;
let utf8_string = String::from_utf8(copied).map_err(|e| e.utf8_error())?;
Ok(GuestStrCow::Copied(utf8_string))
}
UnsafeBorrowResult::Err(e) => Err(e),
}
}
}

impl<'a> GuestPtr<'a, [u8]> {
Expand All @@ -755,30 +774,6 @@ impl<T: ?Sized + Pointee> fmt::Debug for GuestPtr<'_, T> {
}
}

/// A smart pointer for distinguishing between different kinds of Wasm memory:
/// shared and non-shared.
///
/// As with `GuestSlice`, this is usable as a `&'a [T]` via [`std::ops::Deref`].
/// The major difference is that, for shared memories, the memory will be copied
/// out of Wasm linear memory to avoid the possibility of concurrent mutation by
/// another thread. This extra copy exists solely to maintain the Rust
/// guarantees regarding `&[T]`.
pub enum GuestCow<'a, T> {
Borrowed(GuestSlice<'a, T>),
Copied(Vec<T>),
}

impl<'a, T> std::ops::Deref for GuestCow<'a, T> {
type Target = [T];

fn deref(&self) -> &Self::Target {
match self {
GuestCow::Borrowed(s) => s,
GuestCow::Copied(s) => s,
}
}
}

/// A smart pointer to an shareable slice in guest memory.
///
/// Usable as a `&'a [T]` via [`std::ops::Deref`].
Expand Down Expand Up @@ -848,6 +843,30 @@ impl<'a, T> Drop for GuestSliceMut<'a, T> {
}
}

/// A smart pointer for distinguishing between different kinds of Wasm memory:
/// shared and non-shared.
///
/// As with `GuestSlice`, this is usable as a `&'a [T]` via [`std::ops::Deref`].
/// The major difference is that, for shared memories, the memory will be copied
/// out of Wasm linear memory to avoid the possibility of concurrent mutation by
/// another thread. This extra copy exists solely to maintain the Rust
/// guarantees regarding `&[T]`.
pub enum GuestCow<'a, T> {
Borrowed(GuestSlice<'a, T>),
Copied(Vec<T>),
}

impl<'a, T> std::ops::Deref for GuestCow<'a, T> {
type Target = [T];

fn deref(&self) -> &Self::Target {
match self {
GuestCow::Borrowed(s) => s,
GuestCow::Copied(s) => s,
}
}
}

/// A smart pointer to an `unsafe` slice in guest memory.
///
/// Accessing guest memory (e.g., WebAssembly linear memory) is inherently
Expand Down Expand Up @@ -1059,6 +1078,30 @@ impl<'a> std::ops::DerefMut for GuestStrMut<'a> {
}
}

/// A smart pointer to a `str` for distinguishing between different kinds of
/// Wasm memory: shared and non-shared.
///
/// As with `GuestStr`, this is usable as a `&'a str` via [`std::ops::Deref`].
/// The major difference is that, for shared memories, the string will be copied
/// out of Wasm linear memory to avoid the possibility of concurrent mutation by
/// another thread. This extra copy exists solely to maintain the Rust
/// guarantees regarding `&str`.
pub enum GuestStrCow<'a> {
Borrowed(GuestStr<'a>),
Copied(String),
}

impl<'a> std::ops::Deref for GuestStrCow<'a> {
type Target = str;

fn deref(&self) -> &Self::Target {
match self {
GuestStrCow::Borrowed(s) => s,
GuestStrCow::Copied(s) => s,
}
}
}

mod private {
pub trait Sealed {}
impl<T> Sealed for T {}
Expand Down

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