wasmtime/runtime/
memory.rs

1use crate::Trap;
2use crate::prelude::*;
3use crate::runtime::vm::{self, VMStore};
4use crate::store::{StoreInstanceId, StoreOpaque, StoreResourceLimiter};
5use crate::trampoline::generate_memory_export;
6use crate::{AsContext, AsContextMut, Engine, MemoryType, StoreContext, StoreContextMut};
7use core::cell::UnsafeCell;
8use core::fmt;
9use core::slice;
10use core::time::Duration;
11use wasmtime_environ::DefinedMemoryIndex;
12
13pub use crate::runtime::vm::WaitResult;
14
15/// Error for out of bounds [`Memory`] access.
16#[derive(Debug)]
17#[non_exhaustive]
18pub struct MemoryAccessError {
19    // Keep struct internals private for future extensibility.
20    _private: (),
21}
22
23impl fmt::Display for MemoryAccessError {
24    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
25        write!(f, "out of bounds memory access")
26    }
27}
28
29impl core::error::Error for MemoryAccessError {}
30
31/// A WebAssembly linear memory.
32///
33/// WebAssembly memories represent a contiguous array of bytes that have a size
34/// that is always a multiple of the WebAssembly page size, currently 64
35/// kilobytes.
36///
37/// WebAssembly memory is used for global data (not to be confused with wasm
38/// `global` items), statics in C/C++/Rust, shadow stack memory, etc. Accessing
39/// wasm memory is generally quite fast.
40///
41/// Memories, like other wasm items, are owned by a [`Store`](crate::Store).
42///
43/// # `Memory` and Safety
44///
45/// Linear memory is a lynchpin of safety for WebAssembly. In Wasmtime there are
46/// safe methods of interacting with a [`Memory`]:
47///
48/// * [`Memory::read`]
49/// * [`Memory::write`]
50/// * [`Memory::data`]
51/// * [`Memory::data_mut`]
52///
53/// Note that all of these consider the entire store context as borrowed for the
54/// duration of the call or the duration of the returned slice. This largely
55/// means that while the function is running you'll be unable to borrow anything
56/// else from the store. This includes getting access to the `T` on
57/// [`Store<T>`](crate::Store), but it also means that you can't recursively
58/// call into WebAssembly for instance.
59///
60/// If you'd like to dip your toes into handling [`Memory`] in a more raw
61/// fashion (e.g. by using raw pointers or raw slices), then there's a few
62/// important points to consider when doing so:
63///
64/// * Any recursive calls into WebAssembly can possibly modify any byte of the
65///   entire memory. This means that whenever wasm is called Rust can't have any
66///   long-lived borrows live across the wasm function call. Slices like `&mut
67///   [u8]` will be violated because they're not actually exclusive at that
68///   point, and slices like `&[u8]` are also violated because their contents
69///   may be mutated.
70///
71/// * WebAssembly memories can grow, and growth may change the base pointer.
72///   This means that even holding a raw pointer to memory over a wasm function
73///   call is also incorrect. Anywhere in the function call the base address of
74///   memory may change. Note that growth can also be requested from the
75///   embedding API as well.
76///
77/// As a general rule of thumb it's recommended to stick to the safe methods of
78/// [`Memory`] if you can. It's not advised to use raw pointers or `unsafe`
79/// operations because of how easy it is to accidentally get things wrong.
80///
81/// Some examples of safely interacting with memory are:
82///
83/// ```rust
84/// use wasmtime::{Memory, Store, MemoryAccessError};
85///
86/// // Memory can be read and written safely with the `Memory::read` and
87/// // `Memory::write` methods.
88/// // An error is returned if the copy did not succeed.
89/// fn safe_examples(mem: Memory, store: &mut Store<()>) -> Result<(), MemoryAccessError> {
90///     let offset = 5;
91///     mem.write(&mut *store, offset, b"hello")?;
92///     let mut buffer = [0u8; 5];
93///     mem.read(&store, offset, &mut buffer)?;
94///     assert_eq!(b"hello", &buffer);
95///
96///     // Note that while this is safe care must be taken because the indexing
97///     // here may panic if the memory isn't large enough.
98///     assert_eq!(&mem.data(&store)[offset..offset + 5], b"hello");
99///     mem.data_mut(&mut *store)[offset..offset + 5].copy_from_slice(b"bye!!");
100///
101///     Ok(())
102/// }
103/// ```
104///
105/// It's worth also, however, covering some examples of **incorrect**,
106/// **unsafe** usages of `Memory`. Do not do these things!
107///
108/// ```rust
109/// # use anyhow::Result;
110/// use wasmtime::{Memory, Store};
111///
112/// // NOTE: All code in this function is not safe to execute and may cause
113/// // segfaults/undefined behavior at runtime. Do not copy/paste these examples
114/// // into production code!
115/// unsafe fn unsafe_examples(mem: Memory, store: &mut Store<()>) -> Result<()> {
116///     // First and foremost, any borrow can be invalidated at any time via the
117///     // `Memory::grow` function. This can relocate memory which causes any
118///     // previous pointer to be possibly invalid now.
119///     unsafe {
120///         let pointer: &u8 = &*mem.data_ptr(&store);
121///         mem.grow(&mut *store, 1)?; // invalidates `pointer`!
122///         // println!("{}", *pointer); // FATAL: use-after-free
123///     }
124///
125///     // Note that the use-after-free also applies to slices, whether they're
126///     // slices of bytes or strings.
127///     unsafe {
128///         let mem_slice = std::slice::from_raw_parts(
129///             mem.data_ptr(&store),
130///             mem.data_size(&store),
131///         );
132///         let slice: &[u8] = &mem_slice[0x100..0x102];
133///         mem.grow(&mut *store, 1)?; // invalidates `slice`!
134///         // println!("{:?}", slice); // FATAL: use-after-free
135///     }
136///
137///     // The `Memory` type may be stored in other locations, so if you hand
138///     // off access to the `Store` then those locations may also call
139///     // `Memory::grow` or similar, so it's not enough to just audit code for
140///     // calls to `Memory::grow`.
141///     unsafe {
142///         let pointer: &u8 = &*mem.data_ptr(&store);
143///         some_other_function(store); // may invalidate `pointer` through use of `store`
144///         // println!("{:?}", pointer); // FATAL: maybe a use-after-free
145///     }
146///
147///     // An especially subtle aspect of accessing a wasm instance's memory is
148///     // that you need to be extremely careful about aliasing. Anyone at any
149///     // time can call `data_unchecked()` or `data_unchecked_mut()`, which
150///     // means you can easily have aliasing mutable references:
151///     unsafe {
152///         let ref1: &u8 = &*mem.data_ptr(&store).add(0x100);
153///         let ref2: &mut u8 = &mut *mem.data_ptr(&store).add(0x100);
154///         // *ref2 = *ref1; // FATAL: violates Rust's aliasing rules
155///     }
156///
157///     Ok(())
158/// }
159/// # fn some_other_function(store: &mut Store<()>) {}
160/// ```
161///
162/// Overall there's some general rules of thumb when unsafely working with
163/// `Memory` and getting raw pointers inside of it:
164///
165/// * If you never have a "long lived" pointer into memory, you're likely in the
166///   clear. Care still needs to be taken in threaded scenarios or when/where
167///   data is read, but you'll be shielded from many classes of issues.
168/// * Long-lived pointers must always respect Rust'a aliasing rules. It's ok for
169///   shared borrows to overlap with each other, but mutable borrows must
170///   overlap with nothing.
171/// * Long-lived pointers are only valid if they're not invalidated for their
172///   lifetime. This means that [`Store`](crate::Store) isn't used to reenter
173///   wasm or the memory itself is never grown or otherwise modified/aliased.
174///
175/// At this point it's worth reiterating again that unsafely working with
176/// `Memory` is pretty tricky and not recommended! It's highly recommended to
177/// use the safe methods to interact with [`Memory`] whenever possible.
178///
179/// ## `Memory` Safety and Threads
180///
181/// Currently the `wasmtime` crate does not implement the wasm threads proposal,
182/// but it is planned to do so. It may be interesting to readers to see how this
183/// affects memory safety and what was previously just discussed as well.
184///
185/// Once threads are added into the mix, all of the above rules still apply.
186/// There's an additional consideration that all reads and writes can happen
187/// concurrently, though. This effectively means that any borrow into wasm
188/// memory are virtually never safe to have.
189///
190/// Mutable pointers are fundamentally unsafe to have in a concurrent scenario
191/// in the face of arbitrary wasm code. Only if you dynamically know for sure
192/// that wasm won't access a region would it be safe to construct a mutable
193/// pointer. Additionally even shared pointers are largely unsafe because their
194/// underlying contents may change, so unless `UnsafeCell` in one form or
195/// another is used everywhere there's no safety.
196///
197/// One important point about concurrency is that while [`Memory::grow`] can
198/// happen concurrently it will never relocate the base pointer. Shared
199/// memories must always have a maximum size and they will be preallocated such
200/// that growth will never relocate the base pointer. The current size of the
201/// memory may still change over time though.
202///
203/// Overall the general rule of thumb for shared memories is that you must
204/// atomically read and write everything. Nothing can be borrowed and everything
205/// must be eagerly copied out. This means that [`Memory::data`] and
206/// [`Memory::data_mut`] won't work in the future (they'll probably return an
207/// error) for shared memories when they're implemented. When possible it's
208/// recommended to use [`Memory::read`] and [`Memory::write`] which will still
209/// be provided.
210#[derive(Copy, Clone, Debug)]
211#[repr(C)] // here for the C API
212pub struct Memory {
213    /// The internal store instance that this memory belongs to.
214    instance: StoreInstanceId,
215    /// The index of the memory, within `instance` above, that this memory
216    /// refers to.
217    index: DefinedMemoryIndex,
218}
219
220// Double-check that the C representation in `extern.h` matches our in-Rust
221// representation here in terms of size/alignment/etc.
222const _: () = {
223    #[repr(C)]
224    struct Tmp(u64, u32);
225    #[repr(C)]
226    struct C(Tmp, u32);
227    assert!(core::mem::size_of::<C>() == core::mem::size_of::<Memory>());
228    assert!(core::mem::align_of::<C>() == core::mem::align_of::<Memory>());
229    assert!(core::mem::offset_of!(Memory, instance) == 0);
230};
231
232impl Memory {
233    /// Creates a new WebAssembly memory given the configuration of `ty`.
234    ///
235    /// The `store` argument will be the owner of the returned [`Memory`]. All
236    /// WebAssembly memory is initialized to zero.
237    ///
238    /// # Panics
239    ///
240    /// This function will panic if the [`Store`](`crate::Store`) has a
241    /// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`) (see also:
242    /// [`Store::limiter_async`](`crate::Store::limiter_async`)). When
243    /// using an async resource limiter, use [`Memory::new_async`] instead.
244    ///
245    /// # Examples
246    ///
247    /// ```
248    /// # use wasmtime::*;
249    /// # fn main() -> anyhow::Result<()> {
250    /// let engine = Engine::default();
251    /// let mut store = Store::new(&engine, ());
252    ///
253    /// let memory_ty = MemoryType::new(1, None);
254    /// let memory = Memory::new(&mut store, memory_ty)?;
255    ///
256    /// let module = Module::new(&engine, "(module (memory (import \"\" \"\") 1))")?;
257    /// let instance = Instance::new(&mut store, &module, &[memory.into()])?;
258    /// // ...
259    /// # Ok(())
260    /// # }
261    /// ```
262    pub fn new(mut store: impl AsContextMut, ty: MemoryType) -> Result<Memory> {
263        let (mut limiter, store) = store.as_context_mut().0.resource_limiter_and_store_opaque();
264        vm::one_poll(Self::_new(store, limiter.as_mut(), ty))
265            .expect("must use `new_async` when async resource limiters are in use")
266    }
267
268    /// Async variant of [`Memory::new`]. You must use this variant with
269    /// [`Store`](`crate::Store`)s which have a
270    /// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`).
271    ///
272    /// # Panics
273    ///
274    /// This function will panic when used with a non-async
275    /// [`Store`](`crate::Store`).
276    #[cfg(feature = "async")]
277    pub async fn new_async(mut store: impl AsContextMut, ty: MemoryType) -> Result<Memory> {
278        let (mut limiter, store) = store.as_context_mut().0.resource_limiter_and_store_opaque();
279        Self::_new(store, limiter.as_mut(), ty).await
280    }
281
282    /// Helper function for attaching the memory to a "frankenstein" instance
283    async fn _new(
284        store: &mut StoreOpaque,
285        limiter: Option<&mut StoreResourceLimiter<'_>>,
286        ty: MemoryType,
287    ) -> Result<Memory> {
288        generate_memory_export(store, limiter, &ty, None).await
289    }
290
291    /// Returns the underlying type of this memory.
292    ///
293    /// # Panics
294    ///
295    /// Panics if this memory doesn't belong to `store`.
296    ///
297    /// # Examples
298    ///
299    /// ```
300    /// # use wasmtime::*;
301    /// # fn main() -> anyhow::Result<()> {
302    /// let engine = Engine::default();
303    /// let mut store = Store::new(&engine, ());
304    /// let module = Module::new(&engine, "(module (memory (export \"mem\") 1))")?;
305    /// let instance = Instance::new(&mut store, &module, &[])?;
306    /// let memory = instance.get_memory(&mut store, "mem").unwrap();
307    /// let ty = memory.ty(&store);
308    /// assert_eq!(ty.minimum(), 1);
309    /// # Ok(())
310    /// # }
311    /// ```
312    pub fn ty(&self, store: impl AsContext) -> MemoryType {
313        let store = store.as_context();
314        MemoryType::from_wasmtime_memory(self.wasmtime_ty(store.0))
315    }
316
317    /// Safely reads memory contents at the given offset into a buffer.
318    ///
319    /// The entire buffer will be filled.
320    ///
321    /// If `offset + buffer.len()` exceed the current memory capacity, then the
322    /// buffer is left untouched and a [`MemoryAccessError`] is returned.
323    ///
324    /// # Panics
325    ///
326    /// Panics if this memory doesn't belong to `store`.
327    pub fn read(
328        &self,
329        store: impl AsContext,
330        offset: usize,
331        buffer: &mut [u8],
332    ) -> Result<(), MemoryAccessError> {
333        let store = store.as_context();
334        let slice = self
335            .data(&store)
336            .get(offset..)
337            .and_then(|s| s.get(..buffer.len()))
338            .ok_or(MemoryAccessError { _private: () })?;
339        buffer.copy_from_slice(slice);
340        Ok(())
341    }
342
343    /// Safely writes contents of a buffer to this memory at the given offset.
344    ///
345    /// If the `offset + buffer.len()` exceeds the current memory capacity, then
346    /// none of the buffer is written to memory and a [`MemoryAccessError`] is
347    /// returned.
348    ///
349    /// # Panics
350    ///
351    /// Panics if this memory doesn't belong to `store`.
352    pub fn write(
353        &self,
354        mut store: impl AsContextMut,
355        offset: usize,
356        buffer: &[u8],
357    ) -> Result<(), MemoryAccessError> {
358        let mut context = store.as_context_mut();
359        self.data_mut(&mut context)
360            .get_mut(offset..)
361            .and_then(|s| s.get_mut(..buffer.len()))
362            .ok_or(MemoryAccessError { _private: () })?
363            .copy_from_slice(buffer);
364        Ok(())
365    }
366
367    /// Returns this memory as a native Rust slice.
368    ///
369    /// Note that this method will consider the entire store context provided as
370    /// borrowed for the duration of the lifetime of the returned slice.
371    ///
372    /// # Panics
373    ///
374    /// Panics if this memory doesn't belong to `store`.
375    pub fn data<'a, T: 'static>(&self, store: impl Into<StoreContext<'a, T>>) -> &'a [u8] {
376        unsafe {
377            let store = store.into();
378            let definition = store[self.instance].memory(self.index);
379            debug_assert!(!self.ty(store).is_shared());
380            slice::from_raw_parts(definition.base.as_ptr(), definition.current_length())
381        }
382    }
383
384    /// Returns this memory as a native Rust mutable slice.
385    ///
386    /// Note that this method will consider the entire store context provided as
387    /// borrowed for the duration of the lifetime of the returned slice.
388    ///
389    /// # Panics
390    ///
391    /// Panics if this memory doesn't belong to `store`.
392    pub fn data_mut<'a, T: 'static>(
393        &self,
394        store: impl Into<StoreContextMut<'a, T>>,
395    ) -> &'a mut [u8] {
396        unsafe {
397            let store = store.into();
398            let definition = store[self.instance].memory(self.index);
399            debug_assert!(!self.ty(store).is_shared());
400            slice::from_raw_parts_mut(definition.base.as_ptr(), definition.current_length())
401        }
402    }
403
404    /// Same as [`Memory::data_mut`], but also returns the `T` from the
405    /// [`StoreContextMut`].
406    ///
407    /// This method can be used when you want to simultaneously work with the
408    /// `T` in the store as well as the memory behind this [`Memory`]. Using
409    /// [`Memory::data_mut`] would consider the entire store borrowed, whereas
410    /// this method allows the Rust compiler to see that the borrow of this
411    /// memory and the borrow of `T` are disjoint.
412    ///
413    /// # Panics
414    ///
415    /// Panics if this memory doesn't belong to `store`.
416    pub fn data_and_store_mut<'a, T: 'static>(
417        &self,
418        store: impl Into<StoreContextMut<'a, T>>,
419    ) -> (&'a mut [u8], &'a mut T) {
420        // Note the unsafety here. Our goal is to simultaneously borrow the
421        // memory and custom data from `store`, and the store it's connected
422        // to. Rust will not let us do that, however, because we must call two
423        // separate methods (both of which borrow the whole `store`) and one of
424        // our borrows is mutable (the custom data).
425        //
426        // This operation, however, is safe because these borrows do not overlap
427        // and in the process of borrowing them mutability doesn't actually
428        // touch anything. This is akin to mutably borrowing two indices in an
429        // array, which is safe so long as the indices are separate.
430        unsafe {
431            let mut store = store.into();
432            let data = &mut *(store.data_mut() as *mut T);
433            (self.data_mut(store), data)
434        }
435    }
436
437    /// Returns the base pointer, in the host's address space, that the memory
438    /// is located at.
439    ///
440    /// For more information and examples see the documentation on the
441    /// [`Memory`] type.
442    ///
443    /// # Panics
444    ///
445    /// Panics if this memory doesn't belong to `store`.
446    pub fn data_ptr(&self, store: impl AsContext) -> *mut u8 {
447        store.as_context()[self.instance]
448            .memory(self.index)
449            .base
450            .as_ptr()
451    }
452
453    /// Returns the byte length of this memory.
454    ///
455    /// WebAssembly memories are made up of a whole number of pages, so the byte
456    /// size returned will always be a multiple of this memory's page size. Note
457    /// that different Wasm memories may have different page sizes. You can get
458    /// a memory's page size via the [`Memory::page_size`] method.
459    ///
460    /// By default the page size is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or
461    /// `65536`) but [the custom-page-sizes proposal] allows a memory to opt
462    /// into a page size of `1`. Future extensions might allow any power of two
463    /// as a page size.
464    ///
465    /// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
466    ///
467    /// For more information and examples see the documentation on the
468    /// [`Memory`] type.
469    ///
470    /// # Panics
471    ///
472    /// Panics if this memory doesn't belong to `store`.
473    pub fn data_size(&self, store: impl AsContext) -> usize {
474        self.internal_data_size(store.as_context().0)
475    }
476
477    pub(crate) fn internal_data_size(&self, store: &StoreOpaque) -> usize {
478        store[self.instance].memory(self.index).current_length()
479    }
480
481    /// Returns the size, in units of pages, of this Wasm memory.
482    ///
483    /// WebAssembly memories are made up of a whole number of pages, so the byte
484    /// size returned will always be a multiple of this memory's page size. Note
485    /// that different Wasm memories may have different page sizes. You can get
486    /// a memory's page size via the [`Memory::page_size`] method.
487    ///
488    /// By default the page size is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or
489    /// `65536`) but [the custom-page-sizes proposal] allows a memory to opt
490    /// into a page size of `1`. Future extensions might allow any power of two
491    /// as a page size.
492    ///
493    /// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
494    ///
495    /// # Panics
496    ///
497    /// Panics if this memory doesn't belong to `store`.
498    pub fn size(&self, store: impl AsContext) -> u64 {
499        self.internal_size(store.as_context().0)
500    }
501
502    pub(crate) fn internal_size(&self, store: &StoreOpaque) -> u64 {
503        let byte_size = self.internal_data_size(store);
504        let page_size = usize::try_from(self._page_size(store)).unwrap();
505        u64::try_from(byte_size / page_size).unwrap()
506    }
507
508    /// Returns the size of a page, in bytes, for this memory.
509    ///
510    /// WebAssembly memories are made up of a whole number of pages, so the byte
511    /// size (as returned by [`Memory::data_size`]) will always be a multiple of
512    /// their page size. Different Wasm memories may have different page sizes.
513    ///
514    /// By default this is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or `65536`)
515    /// but [the custom-page-sizes proposal] allows opting into a page size of
516    /// `1`. Future extensions might allow any power of two as a page size.
517    ///
518    /// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
519    pub fn page_size(&self, store: impl AsContext) -> u64 {
520        self._page_size(store.as_context().0)
521    }
522
523    pub(crate) fn _page_size(&self, store: &StoreOpaque) -> u64 {
524        self.wasmtime_ty(store).page_size()
525    }
526
527    /// Returns the log2 of this memory's page size, in bytes.
528    ///
529    /// WebAssembly memories are made up of a whole number of pages, so the byte
530    /// size (as returned by [`Memory::data_size`]) will always be a multiple of
531    /// their page size. Different Wasm memories may have different page sizes.
532    ///
533    /// By default the page size is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or
534    /// `65536`) but [the custom-page-sizes proposal] allows opting into a page
535    /// size of `1`. Future extensions might allow any power of two as a page
536    /// size.
537    ///
538    /// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
539    pub fn page_size_log2(&self, store: impl AsContext) -> u8 {
540        self._page_size_log2(store.as_context().0)
541    }
542
543    pub(crate) fn _page_size_log2(&self, store: &StoreOpaque) -> u8 {
544        self.wasmtime_ty(store).page_size_log2
545    }
546
547    /// Grows this WebAssembly memory by `delta` pages.
548    ///
549    /// This will attempt to add `delta` more pages of memory on to the end of
550    /// this `Memory` instance. If successful this may relocate the memory and
551    /// cause [`Memory::data_ptr`] to return a new value. Additionally any
552    /// unsafely constructed slices into this memory may no longer be valid.
553    ///
554    /// On success returns the number of pages this memory previously had
555    /// before the growth succeeded.
556    ///
557    /// Note that, by default, a WebAssembly memory's page size is 64KiB (aka
558    /// 65536 or 2<sup>16</sup>). The [custom-page-sizes proposal] allows Wasm
559    /// memories to opt into a page size of one byte (and this may be further
560    /// relaxed to any power of two in a future extension).
561    ///
562    /// [custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
563    ///
564    /// # Errors
565    ///
566    /// Returns an error if memory could not be grown, for example if it exceeds
567    /// the maximum limits of this memory. A
568    /// [`ResourceLimiter`](crate::ResourceLimiter) is another example of
569    /// preventing a memory to grow.
570    ///
571    /// # Panics
572    ///
573    /// Panics if this memory doesn't belong to `store`.
574    ///
575    /// This function will panic if the [`Store`](`crate::Store`) has a
576    /// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`) (see also:
577    /// [`Store::limiter_async`](`crate::Store::limiter_async`). When using an
578    /// async resource limiter, use [`Memory::grow_async`] instead.
579    ///
580    /// # Examples
581    ///
582    /// ```
583    /// # use wasmtime::*;
584    /// # fn main() -> anyhow::Result<()> {
585    /// let engine = Engine::default();
586    /// let mut store = Store::new(&engine, ());
587    /// let module = Module::new(&engine, "(module (memory (export \"mem\") 1 2))")?;
588    /// let instance = Instance::new(&mut store, &module, &[])?;
589    /// let memory = instance.get_memory(&mut store, "mem").unwrap();
590    ///
591    /// assert_eq!(memory.size(&store), 1);
592    /// assert_eq!(memory.grow(&mut store, 1)?, 1);
593    /// assert_eq!(memory.size(&store), 2);
594    /// assert!(memory.grow(&mut store, 1).is_err());
595    /// assert_eq!(memory.size(&store), 2);
596    /// assert_eq!(memory.grow(&mut store, 0)?, 2);
597    /// # Ok(())
598    /// # }
599    /// ```
600    pub fn grow(&self, mut store: impl AsContextMut, delta: u64) -> Result<u64> {
601        let store = store.as_context_mut().0;
602        let (mut limiter, store) = store.resource_limiter_and_store_opaque();
603        vm::one_poll(self._grow(store, limiter.as_mut(), delta))
604            .expect("must use `grow_async` if an async resource limiter is used")
605    }
606
607    /// Async variant of [`Memory::grow`]. Required when using a
608    /// [`ResourceLimiterAsync`](`crate::ResourceLimiterAsync`).
609    ///
610    /// # Panics
611    ///
612    /// This function will panic when used with a non-async
613    /// [`Store`](`crate::Store`).
614    #[cfg(feature = "async")]
615    pub async fn grow_async(&self, mut store: impl AsContextMut, delta: u64) -> Result<u64> {
616        let store = store.as_context_mut();
617        let (mut limiter, store) = store.0.resource_limiter_and_store_opaque();
618        self._grow(store, limiter.as_mut(), delta).await
619    }
620
621    async fn _grow(
622        &self,
623        store: &mut StoreOpaque,
624        limiter: Option<&mut StoreResourceLimiter<'_>>,
625        delta: u64,
626    ) -> Result<u64> {
627        let result = self
628            .instance
629            .get_mut(store)
630            .memory_grow(limiter, self.index, delta)
631            .await?;
632        match result {
633            Some(size) => {
634                let page_size = self.wasmtime_ty(store).page_size();
635                Ok(u64::try_from(size).unwrap() / page_size)
636            }
637            None => bail!("failed to grow memory by `{}`", delta),
638        }
639    }
640
641    pub(crate) fn from_raw(instance: StoreInstanceId, index: DefinedMemoryIndex) -> Memory {
642        Memory { instance, index }
643    }
644
645    pub(crate) fn wasmtime_ty<'a>(&self, store: &'a StoreOpaque) -> &'a wasmtime_environ::Memory {
646        let module = store[self.instance].env_module();
647        let index = module.memory_index(self.index);
648        &module.memories[index]
649    }
650
651    pub(crate) fn vmimport(&self, store: &StoreOpaque) -> crate::runtime::vm::VMMemoryImport {
652        let instance = &store[self.instance];
653        crate::runtime::vm::VMMemoryImport {
654            from: instance.memory_ptr(self.index).into(),
655            vmctx: instance.vmctx().into(),
656            index: self.index,
657        }
658    }
659
660    pub(crate) fn comes_from_same_store(&self, store: &StoreOpaque) -> bool {
661        store.id() == self.instance.store_id()
662    }
663
664    /// Get a stable hash key for this memory.
665    ///
666    /// Even if the same underlying memory definition is added to the
667    /// `StoreData` multiple times and becomes multiple `wasmtime::Memory`s,
668    /// this hash key will be consistent across all of these memories.
669    #[cfg(feature = "coredump")]
670    pub(crate) fn hash_key(&self, store: &StoreOpaque) -> impl core::hash::Hash + Eq + use<> {
671        store[self.instance].memory_ptr(self.index).as_ptr().addr()
672    }
673}
674
675/// A linear memory. This trait provides an interface for raw memory buffers
676/// which are used by wasmtime, e.g. inside ['Memory']. Such buffers are in
677/// principle not thread safe. By implementing this trait together with
678/// MemoryCreator, one can supply wasmtime with custom allocated host managed
679/// memory.
680///
681/// # Safety
682///
683/// The memory should be page aligned and a multiple of page size.
684/// To prevent possible silent overflows, the memory should be protected by a
685/// guard page.  Additionally the safety concerns explained in ['Memory'], for
686/// accessing the memory apply here as well.
687///
688/// Note that this is a relatively advanced feature and it is recommended to be
689/// familiar with wasmtime runtime code to use it.
690pub unsafe trait LinearMemory: Send + Sync + 'static {
691    /// Returns the number of allocated bytes which are accessible at this time.
692    fn byte_size(&self) -> usize;
693
694    /// Returns byte capacity of this linear memory's current allocation.
695    ///
696    /// Growth up to this value should not relocate the linear memory base
697    /// pointer.
698    fn byte_capacity(&self) -> usize;
699
700    /// Grows this memory to have the `new_size`, in bytes, specified.
701    ///
702    /// Returns `Err` if memory can't be grown by the specified amount
703    /// of bytes. The error may be downcastable to `std::io::Error`.
704    /// Returns `Ok` if memory was grown successfully.
705    fn grow_to(&mut self, new_size: usize) -> Result<()>;
706
707    /// Return the allocated memory as a mutable pointer to u8.
708    fn as_ptr(&self) -> *mut u8;
709}
710
711/// A memory creator. Can be used to provide a memory creator
712/// to wasmtime which supplies host managed memory.
713///
714/// # Safety
715///
716/// This trait is unsafe, as the memory safety depends on proper implementation
717/// of memory management. Memories created by the MemoryCreator should always be
718/// treated as owned by wasmtime instance, and any modification of them outside
719/// of wasmtime invoked routines is unsafe and may lead to corruption.
720///
721/// Note that this is a relatively advanced feature and it is recommended to be
722/// familiar with Wasmtime runtime code to use it.
723pub unsafe trait MemoryCreator: Send + Sync {
724    /// Create a new `LinearMemory` object from the specified parameters.
725    ///
726    /// The type of memory being created is specified by `ty` which indicates
727    /// both the minimum and maximum size, in wasm pages. The minimum and
728    /// maximum sizes, in bytes, are also specified as parameters to avoid
729    /// integer conversion if desired.
730    ///
731    /// The `reserved_size_in_bytes` value indicates the expected size of the
732    /// reservation that is to be made for this memory. If this value is `None`
733    /// than the implementation is free to allocate memory as it sees fit. If
734    /// the value is `Some`, however, then the implementation is expected to
735    /// reserve that many bytes for the memory's allocation, plus the guard
736    /// size at the end. Note that this reservation need only be a virtual
737    /// memory reservation, physical memory does not need to be allocated
738    /// immediately. In this case `grow` should never move the base pointer and
739    /// the maximum size of `ty` is guaranteed to fit within
740    /// `reserved_size_in_bytes`.
741    ///
742    /// The `guard_size_in_bytes` parameter indicates how many bytes of space,
743    /// after the memory allocation, is expected to be unmapped. JIT code will
744    /// elide bounds checks based on the `guard_size_in_bytes` provided, so for
745    /// JIT code to work correctly the memory returned will need to be properly
746    /// guarded with `guard_size_in_bytes` bytes left unmapped after the base
747    /// allocation.
748    ///
749    /// Note that the `reserved_size_in_bytes` and `guard_size_in_bytes` options
750    /// are tuned from the various [`Config`](crate::Config) methods about
751    /// memory sizes/guards. Additionally these two values are guaranteed to be
752    /// multiples of the system page size.
753    ///
754    /// Memory created from this method should be zero filled.
755    fn new_memory(
756        &self,
757        ty: MemoryType,
758        minimum: usize,
759        maximum: Option<usize>,
760        reserved_size_in_bytes: Option<usize>,
761        guard_size_in_bytes: usize,
762    ) -> Result<Box<dyn LinearMemory>, String>;
763}
764
765/// A constructor for externally-created shared memory.
766///
767/// The [threads proposal] adds the concept of "shared memory" to WebAssembly.
768/// This is much the same as a Wasm linear memory (i.e., [`Memory`]), but can be
769/// used concurrently by multiple agents. Because these agents may execute in
770/// different threads, [`SharedMemory`] must be thread-safe.
771///
772/// When the threads proposal is enabled, there are multiple ways to construct
773/// shared memory:
774///  1. for imported shared memory, e.g., `(import "env" "memory" (memory 1 1
775///     shared))`, the user must supply a [`SharedMemory`] with the
776///     externally-created memory as an import to the instance--e.g.,
777///     `shared_memory.into()`.
778///  2. for private or exported shared memory, e.g., `(export "env" "memory"
779///     (memory 1 1 shared))`, Wasmtime will create the memory internally during
780///     instantiation--access using `Instance::get_shared_memory()`.
781///
782/// [threads proposal]:
783///     https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md
784///
785/// # Examples
786///
787/// ```
788/// # use wasmtime::*;
789/// # fn main() -> anyhow::Result<()> {
790/// let mut config = Config::new();
791/// config.wasm_threads(true);
792/// # if Engine::new(&config).is_err() { return Ok(()); }
793/// let engine = Engine::new(&config)?;
794/// let mut store = Store::new(&engine, ());
795///
796/// let shared_memory = SharedMemory::new(&engine, MemoryType::shared(1, 2))?;
797/// let module = Module::new(&engine, r#"(module (memory (import "" "") 1 2 shared))"#)?;
798/// let instance = Instance::new(&mut store, &module, &[shared_memory.into()])?;
799/// // ...
800/// # Ok(())
801/// # }
802/// ```
803#[derive(Clone)]
804pub struct SharedMemory {
805    vm: crate::runtime::vm::SharedMemory,
806    engine: Engine,
807    page_size_log2: u8,
808}
809
810impl SharedMemory {
811    /// Construct a [`SharedMemory`] by providing both the `minimum` and
812    /// `maximum` number of 64K-sized pages. This call allocates the necessary
813    /// pages on the system.
814    #[cfg(feature = "threads")]
815    pub fn new(engine: &Engine, ty: MemoryType) -> Result<Self> {
816        if !ty.is_shared() {
817            bail!("shared memory must have the `shared` flag enabled on its memory type")
818        }
819        debug_assert!(ty.maximum().is_some());
820
821        let tunables = engine.tunables();
822        let ty = ty.wasmtime_memory();
823        let page_size_log2 = ty.page_size_log2;
824        let memory = crate::runtime::vm::SharedMemory::new(ty, tunables)?;
825
826        Ok(Self {
827            vm: memory,
828            engine: engine.clone(),
829            page_size_log2,
830        })
831    }
832
833    /// Return the type of the shared memory.
834    pub fn ty(&self) -> MemoryType {
835        MemoryType::from_wasmtime_memory(&self.vm.ty())
836    }
837
838    /// Returns the size, in WebAssembly pages, of this wasm memory.
839    pub fn size(&self) -> u64 {
840        let byte_size = u64::try_from(self.data_size()).unwrap();
841        let page_size = u64::from(self.page_size());
842        byte_size / page_size
843    }
844
845    /// Returns the size of a page, in bytes, for this memory.
846    ///
847    /// By default this is 64KiB (aka `0x10000`, `2**16`, `1<<16`, or `65536`)
848    /// but [the custom-page-sizes proposal] allows opting into a page size of
849    /// `1`. Future extensions might allow any power of two as a page size.
850    ///
851    /// [the custom-page-sizes proposal]: https://github.com/WebAssembly/custom-page-sizes
852    pub fn page_size(&self) -> u32 {
853        debug_assert!(self.page_size_log2 == 0 || self.page_size_log2 == 16);
854        1 << self.page_size_log2
855    }
856
857    /// Returns the byte length of this memory.
858    ///
859    /// The returned value will be a multiple of the wasm page size, 64k.
860    ///
861    /// For more information and examples see the documentation on the
862    /// [`Memory`] type.
863    pub fn data_size(&self) -> usize {
864        self.vm.byte_size()
865    }
866
867    /// Return access to the available portion of the shared memory.
868    ///
869    /// The slice returned represents the region of accessible memory at the
870    /// time that this function was called. The contents of the returned slice
871    /// will reflect concurrent modifications happening on other threads.
872    ///
873    /// # Safety
874    ///
875    /// The returned slice is valid for the entire duration of the lifetime of
876    /// this instance of [`SharedMemory`]. The base pointer of a shared memory
877    /// does not change. This [`SharedMemory`] may grow further after this
878    /// function has been called, but the slice returned will not grow.
879    ///
880    /// Concurrent modifications may be happening to the data returned on other
881    /// threads. The `UnsafeCell<u8>` represents that safe access to the
882    /// contents of the slice is not possible through normal loads and stores.
883    ///
884    /// The memory returned must be accessed safely through the `Atomic*` types
885    /// in the [`std::sync::atomic`] module. Casting to those types must
886    /// currently be done unsafely.
887    pub fn data(&self) -> &[UnsafeCell<u8>] {
888        unsafe {
889            let definition = self.vm.vmmemory_ptr().as_ref();
890            slice::from_raw_parts(definition.base.as_ptr().cast(), definition.current_length())
891        }
892    }
893
894    /// Grows this WebAssembly memory by `delta` pages.
895    ///
896    /// This will attempt to add `delta` more pages of memory on to the end of
897    /// this `Memory` instance. If successful this may relocate the memory and
898    /// cause [`Memory::data_ptr`] to return a new value. Additionally any
899    /// unsafely constructed slices into this memory may no longer be valid.
900    ///
901    /// On success returns the number of pages this memory previously had
902    /// before the growth succeeded.
903    ///
904    /// # Errors
905    ///
906    /// Returns an error if memory could not be grown, for example if it exceeds
907    /// the maximum limits of this memory. A
908    /// [`ResourceLimiter`](crate::ResourceLimiter) is another example of
909    /// preventing a memory to grow.
910    pub fn grow(&self, delta: u64) -> Result<u64> {
911        match self.vm.grow(delta)? {
912            Some((old_size, _new_size)) => {
913                // For shared memory, the `VMMemoryDefinition` is updated inside
914                // the locked region.
915                Ok(u64::try_from(old_size).unwrap() / u64::from(self.page_size()))
916            }
917            None => bail!("failed to grow memory by `{}`", delta),
918        }
919    }
920
921    /// Equivalent of the WebAssembly `memory.atomic.notify` instruction for
922    /// this shared memory.
923    ///
924    /// This method allows embedders to notify threads blocked on the specified
925    /// `addr`, an index into wasm linear memory. Threads could include
926    /// wasm threads blocked on a `memory.atomic.wait*` instruction or embedder
927    /// threads blocked on [`SharedMemory::atomic_wait32`], for example.
928    ///
929    /// The `count` argument is the number of threads to wake up.
930    ///
931    /// This function returns the number of threads awoken.
932    ///
933    /// # Errors
934    ///
935    /// This function will return an error if `addr` is not within bounds or
936    /// not aligned to a 4-byte boundary.
937    pub fn atomic_notify(&self, addr: u64, count: u32) -> Result<u32, Trap> {
938        self.vm.atomic_notify(addr, count)
939    }
940
941    /// Equivalent of the WebAssembly `memory.atomic.wait32` instruction for
942    /// this shared memory.
943    ///
944    /// This method allows embedders to block the current thread until notified
945    /// via the `memory.atomic.notify` instruction or the
946    /// [`SharedMemory::atomic_notify`] method, enabling synchronization with
947    /// the wasm guest as desired.
948    ///
949    /// The `expected` argument is the expected 32-bit value to be stored at
950    /// the byte address `addr` specified. The `addr` specified is an index
951    /// into this linear memory.
952    ///
953    /// The optional `timeout` argument is the maximum amount of time to block
954    /// the current thread. If not specified the thread may sleep indefinitely.
955    ///
956    /// This function returns one of three possible values:
957    ///
958    /// * `WaitResult::Ok` - this function, loaded the value at `addr`, found
959    ///   it was equal to `expected`, and then blocked (all as one atomic
960    ///   operation). The thread was then awoken with a `memory.atomic.notify`
961    ///   instruction or the [`SharedMemory::atomic_notify`] method.
962    /// * `WaitResult::Mismatch` - the value at `addr` was loaded but was not
963    ///   equal to `expected` so the thread did not block and immediately
964    ///   returned.
965    /// * `WaitResult::TimedOut` - all the steps of `Ok` happened, except this
966    ///   thread was woken up due to a timeout.
967    ///
968    /// This function will not return due to spurious wakeups.
969    ///
970    /// # Errors
971    ///
972    /// This function will return an error if `addr` is not within bounds or
973    /// not aligned to a 4-byte boundary.
974    pub fn atomic_wait32(
975        &self,
976        addr: u64,
977        expected: u32,
978        timeout: Option<Duration>,
979    ) -> Result<WaitResult, Trap> {
980        self.vm.atomic_wait32(addr, expected, timeout)
981    }
982
983    /// Equivalent of the WebAssembly `memory.atomic.wait64` instruction for
984    /// this shared memory.
985    ///
986    /// For more information see [`SharedMemory::atomic_wait32`].
987    ///
988    /// # Errors
989    ///
990    /// Returns the same error as [`SharedMemory::atomic_wait32`] except that
991    /// the specified address must be 8-byte aligned instead of 4-byte aligned.
992    pub fn atomic_wait64(
993        &self,
994        addr: u64,
995        expected: u64,
996        timeout: Option<Duration>,
997    ) -> Result<WaitResult, Trap> {
998        self.vm.atomic_wait64(addr, expected, timeout)
999    }
1000
1001    /// Return a reference to the [`Engine`] used to configure the shared
1002    /// memory.
1003    pub(crate) fn engine(&self) -> &Engine {
1004        &self.engine
1005    }
1006
1007    /// Construct a single-memory instance to provide a way to import
1008    /// [`SharedMemory`] into other modules.
1009    pub(crate) fn vmimport(&self, store: &mut StoreOpaque) -> crate::runtime::vm::VMMemoryImport {
1010        // Note `vm::assert_ready` shouldn't panic here because this isn't
1011        // actually allocating any new memory (also no limiter), so resource
1012        // limiting shouldn't kick in.
1013        vm::assert_ready(generate_memory_export(
1014            store,
1015            None,
1016            &self.ty(),
1017            Some(&self.vm),
1018        ))
1019        .unwrap()
1020        .vmimport(store)
1021    }
1022
1023    /// Create a [`SharedMemory`] from an [`ExportMemory`] definition. This
1024    /// function is available to handle the case in which a Wasm module exports
1025    /// shared memory and the user wants host-side access to it.
1026    pub(crate) fn from_memory(mem: Memory, store: &StoreOpaque) -> Self {
1027        #![cfg_attr(
1028            not(feature = "threads"),
1029            expect(
1030                unused_variables,
1031                unreachable_code,
1032                reason = "definitions cfg'd to dummy",
1033            )
1034        )]
1035
1036        let instance = mem.instance.get(store);
1037        let memory = instance.get_defined_memory(mem.index);
1038        let module = instance.env_module();
1039        let page_size_log2 = module.memories[module.memory_index(mem.index)].page_size_log2;
1040        match memory.as_shared_memory() {
1041            Some(mem) => Self {
1042                vm: mem.clone(),
1043                engine: store.engine().clone(),
1044                page_size_log2,
1045            },
1046            None => panic!("unable to convert from a shared memory"),
1047        }
1048    }
1049}
1050
1051impl fmt::Debug for SharedMemory {
1052    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1053        f.debug_struct("SharedMemory").finish_non_exhaustive()
1054    }
1055}
1056
1057#[cfg(test)]
1058mod tests {
1059    use crate::*;
1060
1061    // Assert that creating a memory via `Memory::new` respects the limits/tunables
1062    // in `Config`.
1063    #[test]
1064    fn respect_tunables() {
1065        let mut cfg = Config::new();
1066        cfg.memory_reservation(0).memory_guard_size(0);
1067        let mut store = Store::new(&Engine::new(&cfg).unwrap(), ());
1068        let ty = MemoryType::new(1, None);
1069        let mem = Memory::new(&mut store, ty).unwrap();
1070        let store = store.as_context();
1071        let tunables = store.engine().tunables();
1072        assert_eq!(tunables.memory_guard_size, 0);
1073        assert!(
1074            !mem.wasmtime_ty(store.0)
1075                .can_elide_bounds_check(tunables, 12)
1076        );
1077    }
1078
1079    #[test]
1080    fn hash_key_is_stable_across_duplicate_store_data_entries() -> Result<()> {
1081        let mut store = Store::<()>::default();
1082        let module = Module::new(
1083            store.engine(),
1084            r#"
1085                (module
1086                    (memory (export "m") 1 1)
1087                )
1088            "#,
1089        )?;
1090        let instance = Instance::new(&mut store, &module, &[])?;
1091
1092        // Each time we `get_memory`, we call `Memory::from_wasmtime` which adds
1093        // a new entry to `StoreData`, so `g1` and `g2` will have different
1094        // indices into `StoreData`.
1095        let m1 = instance.get_memory(&mut store, "m").unwrap();
1096        let m2 = instance.get_memory(&mut store, "m").unwrap();
1097
1098        // That said, they really point to the same memory.
1099        assert_eq!(m1.data(&store)[0], 0);
1100        assert_eq!(m2.data(&store)[0], 0);
1101        m1.data_mut(&mut store)[0] = 42;
1102        assert_eq!(m1.data(&mut store)[0], 42);
1103        assert_eq!(m2.data(&mut store)[0], 42);
1104
1105        // And therefore their hash keys are the same.
1106        assert!(m1.hash_key(&store.as_context().0) == m2.hash_key(&store.as_context().0));
1107
1108        // But the hash keys are different from different memories.
1109        let instance2 = Instance::new(&mut store, &module, &[])?;
1110        let m3 = instance2.get_memory(&mut store, "m").unwrap();
1111        assert!(m1.hash_key(&store.as_context().0) != m3.hash_key(&store.as_context().0));
1112
1113        Ok(())
1114    }
1115}