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use crate::prelude::*;
use crate::runtime::vm::{RuntimeLinearMemory, VMMemoryImport};
use crate::store::{StoreData, StoreOpaque, Stored};
use crate::trampoline::generate_memory_export;
use crate::Trap;
use crate::{AsContext, AsContextMut, Engine, MemoryType, StoreContext, StoreContextMut};
use core::cell::UnsafeCell;
use core::fmt;
use core::ops::Range;
use core::slice;
use core::time::Duration;
use wasmtime_environ::MemoryPlan;
pub use crate::runtime::vm::WaitResult;
/// Error for out of bounds [`Memory`] access.
#[derive(Debug)]
#[non_exhaustive]
pub struct MemoryAccessError {
// Keep struct internals private for future extensibility.
_private: (),
}
impl fmt::Display for MemoryAccessError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "out of bounds memory access")
}
}
#[cfg(feature = "std")]
impl std::error::Error for MemoryAccessError {}
/// A WebAssembly linear memory.
///
/// WebAssembly memories represent a contiguous array of bytes that have a size
/// that is always a multiple of the WebAssembly page size, currently 64
/// kilobytes.
///
/// WebAssembly memory is used for global data (not to be confused with wasm
/// `global` items), statics in C/C++/Rust, shadow stack memory, etc. Accessing
/// wasm memory is generally quite fast.
///
/// Memories, like other wasm items, are owned by a [`Store`](crate::Store).
///
/// # `Memory` and Safety
///
/// Linear memory is a lynchpin of safety for WebAssembly. In Wasmtime there are
/// safe methods of interacting with a [`Memory`]:
///
/// * [`Memory::read`]
/// * [`Memory::write`]
/// * [`Memory::data`]
/// * [`Memory::data_mut`]
///
/// Note that all of these consider the entire store context as borrowed for the
/// duration of the call or the duration of the returned slice. This largely
/// means that while the function is running you'll be unable to borrow anything
/// else from the store. This includes getting access to the `T` on
/// [`Store<T>`](crate::Store), but it also means that you can't recursively
/// call into WebAssembly for instance.
///
/// If you'd like to dip your toes into handling [`Memory`] in a more raw
/// fashion (e.g. by using raw pointers or raw slices), then there's a few
/// important points to consider when doing so:
///
/// * Any recursive calls into WebAssembly can possibly modify any byte of the
/// entire memory. This means that whenever wasm is called Rust can't have any
/// long-lived borrows live across the wasm function call. Slices like `&mut
/// [u8]` will be violated because they're not actually exclusive at that
/// point, and slices like `&[u8]` are also violated because their contents
/// may be mutated.
///
/// * WebAssembly memories can grow, and growth may change the base pointer.
/// This means that even holding a raw pointer to memory over a wasm function
/// call is also incorrect. Anywhere in the function call the base address of
/// memory may change. Note that growth can also be requested from the
/// embedding API as well.
///
/// As a general rule of thumb it's recommended to stick to the safe methods of
/// [`Memory`] if you can. It's not advised to use raw pointers or `unsafe`
/// operations because of how easy it is to accidentally get things wrong.
///
/// Some examples of safely interacting with memory are:
///
/// ```rust
/// use wasmtime::{Memory, Store, MemoryAccessError};
///
/// // Memory can be read and written safely with the `Memory::read` and
/// // `Memory::write` methods.
/// // An error is returned if the copy did not succeed.
/// fn safe_examples(mem: Memory, store: &mut Store<()>) -> Result<(), MemoryAccessError> {
/// let offset = 5;
/// mem.write(&mut *store, offset, b"hello")?;
/// let mut buffer = [0u8; 5];
/// mem.read(&store, offset, &mut buffer)?;
/// assert_eq!(b"hello", &buffer);
///
/// // Note that while this is safe care must be taken because the indexing
/// // here may panic if the memory isn't large enough.
/// assert_eq!(&mem.data(&store)[offset..offset + 5], b"hello");
/// mem.data_mut(&mut *store)[offset..offset + 5].copy_from_slice(b"bye!!");
///
/// Ok(())
/// }
/// ```
///
/// It's worth also, however, covering some examples of **incorrect**,
/// **unsafe** usages of `Memory`. Do not do these things!
///
/// ```rust
/// # use anyhow::Result;
/// use wasmtime::{Memory, Store};
///
/// // NOTE: All code in this function is not safe to execute and may cause
/// // segfaults/undefined behavior at runtime. Do not copy/paste these examples
/// // into production code!
/// unsafe fn unsafe_examples(mem: Memory, store: &mut Store<()>) -> Result<()> {
/// // First and foremost, any borrow can be invalidated at any time via the
/// // `Memory::grow` function. This can relocate memory which causes any
/// // previous pointer to be possibly invalid now.
/// let pointer: &u8 = &*mem.data_ptr(&store);
/// mem.grow(&mut *store, 1)?; // invalidates `pointer`!
/// // println!("{}", *pointer); // FATAL: use-after-free
///
/// // Note that the use-after-free also applies to slices, whether they're
/// // slices of bytes or strings.
/// let mem_slice = std::slice::from_raw_parts(
/// mem.data_ptr(&store),
/// mem.data_size(&store),
/// );
/// let slice: &[u8] = &mem_slice[0x100..0x102];
/// mem.grow(&mut *store, 1)?; // invalidates `slice`!
/// // println!("{:?}", slice); // FATAL: use-after-free
///
/// // The `Memory` type may be stored in other locations, so if you hand
/// // off access to the `Store` then those locations may also call
/// // `Memory::grow` or similar, so it's not enough to just audit code for
/// // calls to `Memory::grow`.
/// let pointer: &u8 = &*mem.data_ptr(&store);
/// some_other_function(store); // may invalidate `pointer` through use of `store`
/// // println!("{:?}", pointer); // FATAL: maybe a use-after-free
///
/// // An especially subtle aspect of accessing a wasm instance's memory is
/// // that you need to be extremely careful about aliasing. Anyone at any
/// // time can call `data_unchecked()` or `data_unchecked_mut()`, which
/// // means you can easily have aliasing mutable references:
/// let ref1: &u8 = &*mem.data_ptr(&store).add(0x100);
/// let ref2: &mut u8 = &mut *mem.data_ptr(&store).add(0x100);
/// // *ref2 = *ref1; // FATAL: violates Rust's aliasing rules
///
/// Ok(())
/// }
/// # fn some_other_function(store: &mut Store<()>) {}
/// ```
///
/// Overall there's some general rules of thumb when unsafely working with
/// `Memory` and getting raw pointers inside of it:
///
/// * If you never have a "long lived" pointer into memory, you're likely in the
/// clear. Care still needs to be taken in threaded scenarios or when/where
/// data is read, but you'll be shielded from many classes of issues.
/// * Long-lived pointers must always respect Rust'a aliasing rules. It's ok for
/// shared borrows to overlap with each other, but mutable borrows must
/// overlap with nothing.
/// * Long-lived pointers are only valid if they're not invalidated for their
/// lifetime. This means that [`Store`](crate::Store) isn't used to reenter
/// wasm or the memory itself is never grown or otherwise modified/aliased.
///
/// At this point it's worth reiterating again that unsafely working with
/// `Memory` is pretty tricky and not recommended! It's highly recommended to
/// use the safe methods to interact with [`Memory`] whenever possible.
///
/// ## `Memory` Safety and Threads
///
/// Currently the `wasmtime` crate does not implement the wasm threads proposal,
/// but it is planned to do so. It may be interesting to readers to see how this
/// affects memory safety and what was previously just discussed as well.
///
/// Once threads are added into the mix, all of the above rules still apply.
/// There's an additional consideration that all reads and writes can happen
/// concurrently, though. This effectively means that any borrow into wasm
/// memory are virtually never safe to have.
///
/// Mutable pointers are fundamentally unsafe to have in a concurrent scenario
/// in the face of arbitrary wasm code. Only if you dynamically know for sure
/// that wasm won't access a region would it be safe to construct a mutable
/// pointer. Additionally even shared pointers are largely unsafe because their
/// underlying contents may change, so unless `UnsafeCell` in one form or
/// another is used everywhere there's no safety.
///
/// One important point about concurrency is that while [`Memory::grow`] can
/// happen concurrently it will never relocate the base pointer. Shared
/// memories must always have a maximum size and they will be preallocated such
/// that growth will never relocate the base pointer. The current size of the
/// memory may still change over time though.
///
/// Overall the general rule of thumb for shared memories is that you must
/// atomically read and write everything. Nothing can be borrowed and everything
/// must be eagerly copied out. This means that [`Memory::data`] and
/// [`Memory::data_mut`] won't work in the future (they'll probably return an
/// error) for shared memories when they're implemented. When possible it's
/// recommended to use [`Memory::read`] and [`Memory::write`] which will still
/// be provided.
#[derive(Copy, Clone, Debug)]
#[repr(transparent)] // here for the C API
pub struct Memory(Stored<crate::runtime::vm::ExportMemory>);
impl Memory {
/// Creates a new WebAssembly memory given the configuration of `ty`.
///
/// The `store` argument will be the owner of the returned [`Memory`]. All
/// WebAssembly memory is initialized to zero.
///
/// # Panics
///
/// This function will panic if the [`Store`](`crate::Store`) has a
/// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`) (see also:
/// [`Store::limiter_async`](`crate::Store::limiter_async`)). When
/// using an async resource limiter, use [`Memory::new_async`] instead.
///
/// # Examples
///
/// ```
/// # use wasmtime::*;
/// # fn main() -> anyhow::Result<()> {
/// let engine = Engine::default();
/// let mut store = Store::new(&engine, ());
///
/// let memory_ty = MemoryType::new(1, None);
/// let memory = Memory::new(&mut store, memory_ty)?;
///
/// let module = Module::new(&engine, "(module (memory (import \"\" \"\") 1))")?;
/// let instance = Instance::new(&mut store, &module, &[memory.into()])?;
/// // ...
/// # Ok(())
/// # }
/// ```
pub fn new(mut store: impl AsContextMut, ty: MemoryType) -> Result<Memory> {
Self::_new(store.as_context_mut().0, ty)
}
/// Async variant of [`Memory::new`]. You must use this variant with
/// [`Store`](`crate::Store`)s which have a
/// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`).
///
/// # Panics
///
/// This function will panic when used with a non-async
/// [`Store`](`crate::Store`).
#[cfg(feature = "async")]
pub async fn new_async<T>(
mut store: impl AsContextMut<Data = T>,
ty: MemoryType,
) -> Result<Memory>
where
T: Send,
{
let mut store = store.as_context_mut();
assert!(
store.0.async_support(),
"cannot use `new_async` without enabling async support on the config"
);
store.on_fiber(|store| Self::_new(store.0, ty)).await?
}
/// Helper function for attaching the memory to a "frankenstein" instance
fn _new(store: &mut StoreOpaque, ty: MemoryType) -> Result<Memory> {
unsafe {
let export = generate_memory_export(store, &ty, None)?;
Ok(Memory::from_wasmtime_memory(export, store))
}
}
/// Returns the underlying type of this memory.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
///
/// # Examples
///
/// ```
/// # use wasmtime::*;
/// # fn main() -> anyhow::Result<()> {
/// let engine = Engine::default();
/// let mut store = Store::new(&engine, ());
/// let module = Module::new(&engine, "(module (memory (export \"mem\") 1))")?;
/// let instance = Instance::new(&mut store, &module, &[])?;
/// let memory = instance.get_memory(&mut store, "mem").unwrap();
/// let ty = memory.ty(&store);
/// assert_eq!(ty.minimum(), 1);
/// # Ok(())
/// # }
/// ```
pub fn ty(&self, store: impl AsContext) -> MemoryType {
let store = store.as_context();
let ty = &store[self.0].memory.memory;
MemoryType::from_wasmtime_memory(&ty)
}
/// Safely reads memory contents at the given offset into a buffer.
///
/// The entire buffer will be filled.
///
/// If `offset + buffer.len()` exceed the current memory capacity, then the
/// buffer is left untouched and a [`MemoryAccessError`] is returned.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn read(
&self,
store: impl AsContext,
offset: usize,
buffer: &mut [u8],
) -> Result<(), MemoryAccessError> {
let store = store.as_context();
let slice = self
.data(&store)
.get(offset..)
.and_then(|s| s.get(..buffer.len()))
.ok_or(MemoryAccessError { _private: () })?;
buffer.copy_from_slice(slice);
Ok(())
}
/// Safely writes contents of a buffer to this memory at the given offset.
///
/// If the `offset + buffer.len()` exceeds the current memory capacity, then
/// none of the buffer is written to memory and a [`MemoryAccessError`] is
/// returned.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn write(
&self,
mut store: impl AsContextMut,
offset: usize,
buffer: &[u8],
) -> Result<(), MemoryAccessError> {
let mut context = store.as_context_mut();
self.data_mut(&mut context)
.get_mut(offset..)
.and_then(|s| s.get_mut(..buffer.len()))
.ok_or(MemoryAccessError { _private: () })?
.copy_from_slice(buffer);
Ok(())
}
/// Returns this memory as a native Rust slice.
///
/// Note that this method will consider the entire store context provided as
/// borrowed for the duration of the lifetime of the returned slice.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn data<'a, T: 'a>(&self, store: impl Into<StoreContext<'a, T>>) -> &'a [u8] {
unsafe {
let store = store.into();
let definition = &*store[self.0].definition;
debug_assert!(!self.ty(store).is_shared());
slice::from_raw_parts(definition.base, definition.current_length())
}
}
/// Returns this memory as a native Rust mutable slice.
///
/// Note that this method will consider the entire store context provided as
/// borrowed for the duration of the lifetime of the returned slice.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn data_mut<'a, T: 'a>(&self, store: impl Into<StoreContextMut<'a, T>>) -> &'a mut [u8] {
unsafe {
let store = store.into();
let definition = &*store[self.0].definition;
debug_assert!(!self.ty(store).is_shared());
slice::from_raw_parts_mut(definition.base, definition.current_length())
}
}
/// Same as [`Memory::data_mut`], but also returns the `T` from the
/// [`StoreContextMut`].
///
/// This method can be used when you want to simultaneously work with the
/// `T` in the store as well as the memory behind this [`Memory`]. Using
/// [`Memory::data_mut`] would consider the entire store borrowed, whereas
/// this method allows the Rust compiler to see that the borrow of this
/// memory and the borrow of `T` are disjoint.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn data_and_store_mut<'a, T: 'a>(
&self,
store: impl Into<StoreContextMut<'a, T>>,
) -> (&'a mut [u8], &'a mut T) {
// Note the unsafety here. Our goal is to simultaneously borrow the
// memory and custom data from `store`, and the store it's connected
// to. Rust will not let us do that, however, because we must call two
// separate methods (both of which borrow the whole `store`) and one of
// our borrows is mutable (the custom data).
//
// This operation, however, is safe because these borrows do not overlap
// and in the process of borrowing them mutability doesn't actually
// touch anything. This is akin to mutably borrowing two indices in an
// array, which is safe so long as the indices are separate.
unsafe {
let mut store = store.into();
let data = &mut *(store.data_mut() as *mut T);
(self.data_mut(store), data)
}
}
/// Returns the base pointer, in the host's address space, that the memory
/// is located at.
///
/// For more information and examples see the documentation on the
/// [`Memory`] type.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn data_ptr(&self, store: impl AsContext) -> *mut u8 {
unsafe { (*store.as_context()[self.0].definition).base }
}
/// Returns the byte length of this memory.
///
/// WebAssembly memories are made up of a whole number of pages, so the byte
/// size returned will always be a multiple of this memory's page size. Note
/// that different Wasm memories may have different page sizes. You can get
/// a memory's page size via the [`Memory::page_size`] method.
///
/// By default the page size is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or
/// `65536`) but [the custom-page-sizes proposal] allows a memory to opt
/// into a page size of `1`. Future extensions might allow any power of two
/// as a page size.
///
/// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
///
/// For more information and examples see the documentation on the
/// [`Memory`] type.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn data_size(&self, store: impl AsContext) -> usize {
self.internal_data_size(store.as_context().0)
}
pub(crate) fn internal_data_size(&self, store: &StoreOpaque) -> usize {
unsafe { (*store[self.0].definition).current_length() }
}
/// Returns the size, in units of pages, of this Wasm memory.
///
/// WebAssembly memories are made up of a whole number of pages, so the byte
/// size returned will always be a multiple of this memory's page size. Note
/// that different Wasm memories may have different page sizes. You can get
/// a memory's page size via the [`Memory::page_size`] method.
///
/// By default the page size is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or
/// `65536`) but [the custom-page-sizes proposal] allows a memory to opt
/// into a page size of `1`. Future extensions might allow any power of two
/// as a page size.
///
/// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
pub fn size(&self, store: impl AsContext) -> u64 {
self.internal_size(store.as_context().0)
}
pub(crate) fn internal_size(&self, store: &StoreOpaque) -> u64 {
let byte_size = self.internal_data_size(store);
let page_size = usize::try_from(self._page_size(store)).unwrap();
u64::try_from(byte_size / page_size).unwrap()
}
/// Returns the size of a page, in bytes, for this memory.
///
/// WebAssembly memories are made up of a whole number of pages, so the byte
/// size (as returned by [`Memory::data_size`]) will always be a multiple of
/// their page size. Different Wasm memories may have different page sizes.
///
/// By default this is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or `65536`)
/// but [the custom-page-sizes proposal] allows opting into a page size of
/// `1`. Future extensions might allow any power of two as a page size.
///
/// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
pub fn page_size(&self, store: impl AsContext) -> u64 {
self._page_size(store.as_context().0)
}
pub(crate) fn _page_size(&self, store: &StoreOpaque) -> u64 {
store[self.0].memory.memory.page_size()
}
/// Returns the log2 of this memory's page size, in bytes.
///
/// WebAssembly memories are made up of a whole number of pages, so the byte
/// size (as returned by [`Memory::data_size`]) will always be a multiple of
/// their page size. Different Wasm memories may have different page sizes.
///
/// By default the page size is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or
/// `65536`) but [the custom-page-sizes proposal] allows opting into a page
/// size of `1`. Future extensions might allow any power of two as a page
/// size.
///
/// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
pub fn page_size_log2(&self, store: impl AsContext) -> u8 {
self._page_size_log2(store.as_context().0)
}
pub(crate) fn _page_size_log2(&self, store: &StoreOpaque) -> u8 {
store[self.0].memory.memory.page_size_log2
}
/// Grows this WebAssembly memory by `delta` pages.
///
/// This will attempt to add `delta` more pages of memory on to the end of
/// this `Memory` instance. If successful this may relocate the memory and
/// cause [`Memory::data_ptr`] to return a new value. Additionally any
/// unsafely constructed slices into this memory may no longer be valid.
///
/// On success returns the number of pages this memory previously had
/// before the growth succeeded.
///
/// Note that, by default, a WebAssembly memory's page size is 64KiB (aka
/// 65536 or 2<sup>16</sup>). The [custom-page-sizes proposal] allows Wasm
/// memories to opt into a page size of one byte (and this may be further
/// relaxed to any power of two in a future extension).
///
/// [custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
///
/// # Errors
///
/// Returns an error if memory could not be grown, for example if it exceeds
/// the maximum limits of this memory. A
/// [`ResourceLimiter`](crate::ResourceLimiter) is another example of
/// preventing a memory to grow.
///
/// # Panics
///
/// Panics if this memory doesn't belong to `store`.
///
/// This function will panic if the [`Store`](`crate::Store`) has a
/// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`) (see also:
/// [`Store::limiter_async`](`crate::Store::limiter_async`). When using an
/// async resource limiter, use [`Memory::grow_async`] instead.
///
/// # Examples
///
/// ```
/// # use wasmtime::*;
/// # fn main() -> anyhow::Result<()> {
/// let engine = Engine::default();
/// let mut store = Store::new(&engine, ());
/// let module = Module::new(&engine, "(module (memory (export \"mem\") 1 2))")?;
/// let instance = Instance::new(&mut store, &module, &[])?;
/// let memory = instance.get_memory(&mut store, "mem").unwrap();
///
/// assert_eq!(memory.size(&store), 1);
/// assert_eq!(memory.grow(&mut store, 1)?, 1);
/// assert_eq!(memory.size(&store), 2);
/// assert!(memory.grow(&mut store, 1).is_err());
/// assert_eq!(memory.size(&store), 2);
/// assert_eq!(memory.grow(&mut store, 0)?, 2);
/// # Ok(())
/// # }
/// ```
pub fn grow(&self, mut store: impl AsContextMut, delta: u64) -> Result<u64> {
let store = store.as_context_mut().0;
let mem = self.wasmtime_memory(store);
unsafe {
match (*mem).grow(delta, Some(store))? {
Some(size) => {
let vm = (*mem).vmmemory();
*store[self.0].definition = vm;
let page_size = (*mem).page_size();
Ok(u64::try_from(size).unwrap() / page_size)
}
None => bail!("failed to grow memory by `{}`", delta),
}
}
}
/// Async variant of [`Memory::grow`]. Required when using a
/// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`).
///
/// # Panics
///
/// This function will panic when used with a non-async
/// [`Store`](`crate::Store`).
#[cfg(feature = "async")]
pub async fn grow_async<T>(
&self,
mut store: impl AsContextMut<Data = T>,
delta: u64,
) -> Result<u64>
where
T: Send,
{
let mut store = store.as_context_mut();
assert!(
store.0.async_support(),
"cannot use `grow_async` without enabling async support on the config"
);
store.on_fiber(|store| self.grow(store, delta)).await?
}
fn wasmtime_memory(&self, store: &mut StoreOpaque) -> *mut crate::runtime::vm::Memory {
unsafe {
let export = &store[self.0];
crate::runtime::vm::Instance::from_vmctx(export.vmctx, |handle| {
handle.get_defined_memory(export.index)
})
}
}
pub(crate) unsafe fn from_wasmtime_memory(
wasmtime_export: crate::runtime::vm::ExportMemory,
store: &mut StoreOpaque,
) -> Memory {
Memory(store.store_data_mut().insert(wasmtime_export))
}
pub(crate) fn wasmtime_ty<'a>(&self, store: &'a StoreData) -> &'a wasmtime_environ::Memory {
&store[self.0].memory.memory
}
pub(crate) fn vmimport(&self, store: &StoreOpaque) -> crate::runtime::vm::VMMemoryImport {
let export = &store[self.0];
crate::runtime::vm::VMMemoryImport {
from: export.definition,
vmctx: export.vmctx,
index: export.index,
}
}
pub(crate) fn comes_from_same_store(&self, store: &StoreOpaque) -> bool {
store.store_data().contains(self.0)
}
/// Get a stable hash key for this memory.
///
/// Even if the same underlying memory definition is added to the
/// `StoreData` multiple times and becomes multiple `wasmtime::Memory`s,
/// this hash key will be consistent across all of these memories.
pub(crate) fn hash_key(&self, store: &StoreOpaque) -> impl core::hash::Hash + Eq {
store[self.0].definition as usize
}
}
/// A linear memory. This trait provides an interface for raw memory buffers
/// which are used by wasmtime, e.g. inside ['Memory']. Such buffers are in
/// principle not thread safe. By implementing this trait together with
/// MemoryCreator, one can supply wasmtime with custom allocated host managed
/// memory.
///
/// # Safety
///
/// The memory should be page aligned and a multiple of page size.
/// To prevent possible silent overflows, the memory should be protected by a
/// guard page. Additionally the safety concerns explained in ['Memory'], for
/// accessing the memory apply here as well.
///
/// Note that this is a relatively new and experimental feature and it is
/// recommended to be familiar with wasmtime runtime code to use it.
pub unsafe trait LinearMemory: Send + Sync + 'static {
/// Returns the number of allocated bytes which are accessible at this time.
fn byte_size(&self) -> usize;
/// Returns the maximum number of bytes the memory can grow to.
///
/// Returns `None` if the memory is unbounded, or `Some` if memory cannot
/// grow beyond a specified limit.
fn maximum_byte_size(&self) -> Option<usize>;
/// Grows this memory to have the `new_size`, in bytes, specified.
///
/// Returns `Err` if memory can't be grown by the specified amount
/// of bytes. The error may be downcastable to `std::io::Error`.
/// Returns `Ok` if memory was grown successfully.
fn grow_to(&mut self, new_size: usize) -> Result<()>;
/// Return the allocated memory as a mutable pointer to u8.
fn as_ptr(&self) -> *mut u8;
/// Returns the range of native addresses that WebAssembly can natively
/// access from this linear memory, including guard pages.
fn wasm_accessible(&self) -> Range<usize>;
}
/// A memory creator. Can be used to provide a memory creator
/// to wasmtime which supplies host managed memory.
///
/// # Safety
///
/// This trait is unsafe, as the memory safety depends on proper implementation
/// of memory management. Memories created by the MemoryCreator should always be
/// treated as owned by wasmtime instance, and any modification of them outside
/// of wasmtime invoked routines is unsafe and may lead to corruption.
///
/// Note that this is a relatively new and experimental feature and it is
/// recommended to be familiar with wasmtime runtime code to use it.
pub unsafe trait MemoryCreator: Send + Sync {
/// Create a new `LinearMemory` object from the specified parameters.
///
/// The type of memory being created is specified by `ty` which indicates
/// both the minimum and maximum size, in wasm pages. The minimum and
/// maximum sizes, in bytes, are also specified as parameters to avoid
/// integer conversion if desired.
///
/// The `reserved_size_in_bytes` value indicates the expected size of the
/// reservation that is to be made for this memory. If this value is `None`
/// than the implementation is free to allocate memory as it sees fit. If
/// the value is `Some`, however, then the implementation is expected to
/// reserve that many bytes for the memory's allocation, plus the guard
/// size at the end. Note that this reservation need only be a virtual
/// memory reservation, physical memory does not need to be allocated
/// immediately. In this case `grow` should never move the base pointer and
/// the maximum size of `ty` is guaranteed to fit within
/// `reserved_size_in_bytes`.
///
/// The `guard_size_in_bytes` parameter indicates how many bytes of space,
/// after the memory allocation, is expected to be unmapped. JIT code will
/// elide bounds checks based on the `guard_size_in_bytes` provided, so for
/// JIT code to work correctly the memory returned will need to be properly
/// guarded with `guard_size_in_bytes` bytes left unmapped after the base
/// allocation.
///
/// Note that the `reserved_size_in_bytes` and `guard_size_in_bytes` options
/// are tuned from the various [`Config`](crate::Config) methods about
/// memory sizes/guards. Additionally these two values are guaranteed to be
/// multiples of the system page size.
///
/// Memory created from this method should be zero filled.
fn new_memory(
&self,
ty: MemoryType,
minimum: usize,
maximum: Option<usize>,
reserved_size_in_bytes: Option<usize>,
guard_size_in_bytes: usize,
) -> Result<Box<dyn LinearMemory>, String>;
}
/// A constructor for externally-created shared memory.
///
/// The [threads proposal] adds the concept of "shared memory" to WebAssembly.
/// This is much the same as a Wasm linear memory (i.e., [`Memory`]), but can be
/// used concurrently by multiple agents. Because these agents may execute in
/// different threads, [`SharedMemory`] must be thread-safe.
///
/// When the threads proposal is enabled, there are multiple ways to construct
/// shared memory:
/// 1. for imported shared memory, e.g., `(import "env" "memory" (memory 1 1
/// shared))`, the user must supply a [`SharedMemory`] with the
/// externally-created memory as an import to the instance--e.g.,
/// `shared_memory.into()`.
/// 2. for private or exported shared memory, e.g., `(export "env" "memory"
/// (memory 1 1 shared))`, Wasmtime will create the memory internally during
/// instantiation--access using `Instance::get_shared_memory()`.
///
/// [threads proposal]:
/// https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md
///
/// # Examples
///
/// ```
/// # use wasmtime::*;
/// # fn main() -> anyhow::Result<()> {
/// let mut config = Config::new();
/// config.wasm_threads(true);
/// let engine = Engine::new(&config)?;
/// let mut store = Store::new(&engine, ());
///
/// let shared_memory = SharedMemory::new(&engine, MemoryType::shared(1, 2))?;
/// let module = Module::new(&engine, r#"(module (memory (import "" "") 1 2 shared))"#)?;
/// let instance = Instance::new(&mut store, &module, &[shared_memory.into()])?;
/// // ...
/// # Ok(())
/// # }
/// ```
#[derive(Clone)]
pub struct SharedMemory {
vm: crate::runtime::vm::SharedMemory,
engine: Engine,
page_size_log2: u8,
}
impl SharedMemory {
/// Construct a [`SharedMemory`] by providing both the `minimum` and
/// `maximum` number of 64K-sized pages. This call allocates the necessary
/// pages on the system.
#[cfg(feature = "threads")]
pub fn new(engine: &Engine, ty: MemoryType) -> Result<Self> {
if !ty.is_shared() {
bail!("shared memory must have the `shared` flag enabled on its memory type")
}
debug_assert!(ty.maximum().is_some());
let tunables = engine.tunables();
let plan = MemoryPlan::for_memory(*ty.wasmtime_memory(), tunables);
let page_size_log2 = plan.memory.page_size_log2;
let memory = crate::runtime::vm::SharedMemory::new(plan)?;
Ok(Self {
vm: memory,
engine: engine.clone(),
page_size_log2,
})
}
/// Return the type of the shared memory.
pub fn ty(&self) -> MemoryType {
MemoryType::from_wasmtime_memory(&self.vm.ty())
}
/// Returns the size, in WebAssembly pages, of this wasm memory.
pub fn size(&self) -> u64 {
let byte_size = u64::try_from(self.data_size()).unwrap();
let page_size = u64::from(self.page_size());
byte_size / page_size
}
/// Returns the size of a page, in bytes, for this memory.
///
/// By default this is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or `65536`)
/// but [the custom-page-sizes proposal] allows opting into a page size of
/// `1`. Future extensions might allow any power of two as a page size.
///
/// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
pub fn page_size(&self) -> u32 {
debug_assert!(self.page_size_log2 == 0 || self.page_size_log2 == 16);
1 << self.page_size_log2
}
/// Returns the byte length of this memory.
///
/// The returned value will be a multiple of the wasm page size, 64k.
///
/// For more information and examples see the documentation on the
/// [`Memory`] type.
pub fn data_size(&self) -> usize {
self.vm.byte_size()
}
/// Return access to the available portion of the shared memory.
///
/// The slice returned represents the region of accessible memory at the
/// time that this function was called. The contents of the returned slice
/// will reflect concurrent modifications happening on other threads.
///
/// # Safety
///
/// The returned slice is valid for the entire duration of the lifetime of
/// this instance of [`SharedMemory`]. The base pointer of a shared memory
/// does not change. This [`SharedMemory`] may grow further after this
/// function has been called, but the slice returned will not grow.
///
/// Concurrent modifications may be happening to the data returned on other
/// threads. The `UnsafeCell<u8>` represents that safe access to the
/// contents of the slice is not possible through normal loads and stores.
///
/// The memory returned must be accessed safely through the `Atomic*` types
/// in the [`std::sync::atomic`] module. Casting to those types must
/// currently be done unsafely.
pub fn data(&self) -> &[UnsafeCell<u8>] {
unsafe {
let definition = &*self.vm.vmmemory_ptr();
slice::from_raw_parts(definition.base.cast(), definition.current_length())
}
}
/// Grows this WebAssembly memory by `delta` pages.
///
/// This will attempt to add `delta` more pages of memory on to the end of
/// this `Memory` instance. If successful this may relocate the memory and
/// cause [`Memory::data_ptr`] to return a new value. Additionally any
/// unsafely constructed slices into this memory may no longer be valid.
///
/// On success returns the number of pages this memory previously had
/// before the growth succeeded.
///
/// # Errors
///
/// Returns an error if memory could not be grown, for example if it exceeds
/// the maximum limits of this memory. A
/// [`ResourceLimiter`](crate::ResourceLimiter) is another example of
/// preventing a memory to grow.
pub fn grow(&self, delta: u64) -> Result<u64> {
match self.vm.grow(delta, None)? {
Some((old_size, _new_size)) => {
// For shared memory, the `VMMemoryDefinition` is updated inside
// the locked region.
Ok(u64::try_from(old_size).unwrap() / u64::from(self.page_size()))
}
None => bail!("failed to grow memory by `{}`", delta),
}
}
/// Equivalent of the WebAssembly `memory.atomic.notify` instruction for
/// this shared memory.
///
/// This method allows embedders to notify threads blocked on the specified
/// `addr`, an index into wasm linear memory. Threads could include
/// wasm threads blocked on a `memory.atomic.wait*` instruction or embedder
/// threads blocked on [`SharedMemory::atomic_wait32`], for example.
///
/// The `count` argument is the number of threads to wake up.
///
/// This function returns the number of threads awoken.
///
/// # Errors
///
/// This function will return an error if `addr` is not within bounds or
/// not aligned to a 4-byte boundary.
pub fn atomic_notify(&self, addr: u64, count: u32) -> Result<u32, Trap> {
self.vm.atomic_notify(addr, count)
}
/// Equivalent of the WebAssembly `memory.atomic.wait32` instruction for
/// this shared memory.
///
/// This method allows embedders to block the current thread until notified
/// via the `memory.atomic.notify` instruction or the
/// [`SharedMemory::atomic_notify`] method, enabling synchronization with
/// the wasm guest as desired.
///
/// The `expected` argument is the expected 32-bit value to be stored at
/// the byte address `addr` specified. The `addr` specified is an index
/// into this linear memory.
///
/// The optional `timeout` argument is the maximum amount of time to block
/// the current thread. If not specified the thread may sleep indefinitely.
///
/// This function returns one of three possible values:
///
/// * `WaitResult::Ok` - this function, loaded the value at `addr`, found
/// it was equal to `expected`, and then blocked (all as one atomic
/// operation). The thread was then awoken with a `memory.atomic.notify`
/// instruction or the [`SharedMemory::atomic_notify`] method.
/// * `WaitResult::Mismatch` - the value at `addr` was loaded but was not
/// equal to `expected` so the thread did not block and immediately
/// returned.
/// * `WaitResult::TimedOut` - all the steps of `Ok` happened, except this
/// thread was woken up due to a timeout.
///
/// This function will not return due to spurious wakeups.
///
/// # Errors
///
/// This function will return an error if `addr` is not within bounds or
/// not aligned to a 4-byte boundary.
pub fn atomic_wait32(
&self,
addr: u64,
expected: u32,
timeout: Option<Duration>,
) -> Result<WaitResult, Trap> {
self.vm.atomic_wait32(addr, expected, timeout)
}
/// Equivalent of the WebAssembly `memory.atomic.wait64` instruction for
/// this shared memory.
///
/// For more information see [`SharedMemory::atomic_wait32`].
///
/// # Errors
///
/// Returns the same error as [`SharedMemory::atomic_wait32`] except that
/// the specified address must be 8-byte aligned instead of 4-byte aligned.
pub fn atomic_wait64(
&self,
addr: u64,
expected: u64,
timeout: Option<Duration>,
) -> Result<WaitResult, Trap> {
self.vm.atomic_wait64(addr, expected, timeout)
}
/// Return a reference to the [`Engine`] used to configure the shared
/// memory.
pub(crate) fn engine(&self) -> &Engine {
&self.engine
}
/// Construct a single-memory instance to provide a way to import
/// [`SharedMemory`] into other modules.
pub(crate) fn vmimport(&self, store: &mut StoreOpaque) -> crate::runtime::vm::VMMemoryImport {
let export_memory = generate_memory_export(store, &self.ty(), Some(&self.vm)).unwrap();
VMMemoryImport {
from: export_memory.definition,
vmctx: export_memory.vmctx,
index: export_memory.index,
}
}
/// Create a [`SharedMemory`] from an [`ExportMemory`] definition. This
/// function is available to handle the case in which a Wasm module exports
/// shared memory and the user wants host-side access to it.
pub(crate) unsafe fn from_wasmtime_memory(
wasmtime_export: crate::runtime::vm::ExportMemory,
store: &mut StoreOpaque,
) -> Self {
#[cfg_attr(not(feature = "threads"), allow(unused_variables, unreachable_code))]
crate::runtime::vm::Instance::from_vmctx(wasmtime_export.vmctx, |handle| {
let memory_index = handle.module().memory_index(wasmtime_export.index);
let page_size = handle.memory_page_size(memory_index);
debug_assert!(page_size.is_power_of_two());
let page_size_log2 = u8::try_from(page_size.ilog2()).unwrap();
let memory = handle
.get_defined_memory(wasmtime_export.index)
.as_mut()
.unwrap();
match memory.as_shared_memory() {
Some(mem) => Self {
vm: mem.clone(),
engine: store.engine().clone(),
page_size_log2,
},
None => panic!("unable to convert from a shared memory"),
}
})
}
}
impl fmt::Debug for SharedMemory {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("SharedMemory").finish_non_exhaustive()
}
}
#[cfg(test)]
mod tests {
use crate::*;
// Assert that creating a memory via `Memory::new` respects the limits/tunables
// in `Config`.
#[test]
fn respect_tunables() {
let mut cfg = Config::new();
cfg.static_memory_maximum_size(0)
.dynamic_memory_guard_size(0);
let mut store = Store::new(&Engine::new(&cfg).unwrap(), ());
let ty = MemoryType::new(1, None);
let mem = Memory::new(&mut store, ty).unwrap();
let store = store.as_context();
assert_eq!(store[mem.0].memory.offset_guard_size, 0);
match &store[mem.0].memory.style {
wasmtime_environ::MemoryStyle::Dynamic { .. } => {}
other => panic!("unexpected style {other:?}"),
}
}
#[test]
fn hash_key_is_stable_across_duplicate_store_data_entries() -> Result<()> {
let mut store = Store::<()>::default();
let module = Module::new(
store.engine(),
r#"
(module
(memory (export "m") 1 1)
)
"#,
)?;
let instance = Instance::new(&mut store, &module, &[])?;
// Each time we `get_memory`, we call `Memory::from_wasmtime` which adds
// a new entry to `StoreData`, so `g1` and `g2` will have different
// indices into `StoreData`.
let m1 = instance.get_memory(&mut store, "m").unwrap();
let m2 = instance.get_memory(&mut store, "m").unwrap();
// That said, they really point to the same memory.
assert_eq!(m1.data(&store)[0], 0);
assert_eq!(m2.data(&store)[0], 0);
m1.data_mut(&mut store)[0] = 42;
assert_eq!(m1.data(&mut store)[0], 42);
assert_eq!(m2.data(&mut store)[0], 42);
// And therefore their hash keys are the same.
assert!(m1.hash_key(&store.as_context().0) == m2.hash_key(&store.as_context().0));
// But the hash keys are different from different memories.
let instance2 = Instance::new(&mut store, &module, &[])?;
let m3 = instance2.get_memory(&mut store, "m").unwrap();
assert!(m1.hash_key(&store.as_context().0) != m3.hash_key(&store.as_context().0));
Ok(())
}
}