2a6969d2bd
* Shrink the size of the anyfunc table in `VMContext` This commit shrinks the size of the `VMCallerCheckedAnyfunc` table allocated into a `VMContext` to be the size of the number of "escaped" functions in a module rather than the number of functions in a module. Escaped functions include exports, table elements, etc, and are typically an order of magnitude smaller than the number of functions in general. This should greatly shrink the `VMContext` for some modules which while we aren't necessarily having any problems with that today shouldn't cause any problems in the future. The original motivation for this was that this came up during the recent lazy-table-initialization work and while it no longer has a direct performance benefit since tables aren't initialized at all on instantiation it should still improve long-running instances theoretically with smaller `VMContext` allocations as well as better locality between anyfuncs. * Fix some tests * Remove redundant hash set * Use a helper for pushing function type information * Use a more descriptive `is_escaping` method * Clarify a comment * Fix condition
1110 lines
42 KiB
Rust
1110 lines
42 KiB
Rust
//! # `VMExternRef`
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//!
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//! `VMExternRef` is a reference-counted box for any kind of data that is
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//! external and opaque to running Wasm. Sometimes it might hold a Wasmtime
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//! thing, other times it might hold something from a Wasmtime embedder and is
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//! opaque even to us. It is morally equivalent to `Rc<dyn Any>` in Rust, but
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//! additionally always fits in a pointer-sized word. `VMExternRef` is
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//! non-nullable, but `Option<VMExternRef>` is a null pointer.
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//!
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//! The one part of `VMExternRef` that can't ever be opaque to us is the
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//! reference count. Even when we don't know what's inside an `VMExternRef`, we
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//! need to be able to manipulate its reference count as we add and remove
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//! references to it. And we need to do this from compiled Wasm code, so it must
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//! be `repr(C)`!
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//!
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//! ## Memory Layout
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//!
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//! `VMExternRef` itself is just a pointer to an `VMExternData`, which holds the
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//! opaque, boxed value, its reference count, and its vtable pointer.
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//!
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//! The `VMExternData` struct is *preceded* by the dynamically-sized value boxed
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//! up and referenced by one or more `VMExternRef`s:
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//!
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//! ```text
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//! ,-------------------------------------------------------.
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//! | |
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//! V |
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//! +----------------------------+-----------+-----------+ |
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//! | dynamically-sized value... | ref_count | value_ptr |---'
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//! +----------------------------+-----------+-----------+
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//! | VMExternData |
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//! +-----------------------+
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//! ^
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//! +-------------+ |
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//! | VMExternRef |-------------------+
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//! +-------------+ |
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//! |
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//! +-------------+ |
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//! | VMExternRef |-------------------+
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//! +-------------+ |
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//! |
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//! ... ===
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//! |
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//! +-------------+ |
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//! | VMExternRef |-------------------'
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//! +-------------+
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//! ```
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//!
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//! The `value_ptr` member always points backwards to the start of the
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//! dynamically-sized value (which is also the start of the heap allocation for
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//! this value-and-`VMExternData` pair). Because it is a `dyn` pointer, it is
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//! fat, and also points to the value's `Any` vtable.
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//!
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//! The boxed value and the `VMExternRef` footer are held a single heap
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//! allocation. The layout described above is used to make satisfying the
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//! value's alignment easy: we just need to ensure that the heap allocation used
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//! to hold everything satisfies its alignment. It also ensures that we don't
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//! need a ton of excess padding between the `VMExternData` and the value for
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//! values with large alignment.
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//!
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//! ## Reference Counting, Wasm Functions, and Garbage Collection
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//!
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//! For host VM code, we use plain reference counting, where cloning increments
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//! the reference count, and dropping decrements it. We can avoid many of the
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//! on-stack increment/decrement operations that typically plague the
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//! performance of reference counting via Rust's ownership and borrowing system.
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//! Moving a `VMExternRef` avoids mutating its reference count, and borrowing it
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//! either avoids the reference count increment or delays it until if/when the
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//! `VMExternRef` is cloned.
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//!
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//! When passing a `VMExternRef` into compiled Wasm code, we don't want to do
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//! reference count mutations for every compiled `local.{get,set}`, nor for
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//! every function call. Therefore, we use a variation of **deferred reference
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//! counting**, where we only mutate reference counts when storing
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//! `VMExternRef`s somewhere that outlives the activation: into a global or
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//! table. Simultaneously, we over-approximate the set of `VMExternRef`s that
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//! are inside Wasm function activations. Periodically, we walk the stack at GC
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//! safe points, and use stack map information to precisely identify the set of
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//! `VMExternRef`s inside Wasm activations. Then we take the difference between
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//! this precise set and our over-approximation, and decrement the reference
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//! count for each of the `VMExternRef`s that are in our over-approximation but
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//! not in the precise set. Finally, the over-approximation is replaced with the
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//! precise set.
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//!
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//! The `VMExternRefActivationsTable` implements the over-approximized set of
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//! `VMExternRef`s referenced by Wasm activations. Calling a Wasm function and
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//! passing it a `VMExternRef` moves the `VMExternRef` into the table, and the
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//! compiled Wasm function logically "borrows" the `VMExternRef` from the
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//! table. Similarly, `global.get` and `table.get` operations clone the gotten
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//! `VMExternRef` into the `VMExternRefActivationsTable` and then "borrow" the
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//! reference out of the table.
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//!
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//! When a `VMExternRef` is returned to host code from a Wasm function, the host
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//! increments the reference count (because the reference is logically
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//! "borrowed" from the `VMExternRefActivationsTable` and the reference count
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//! from the table will be dropped at the next GC).
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//!
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//! For more general information on deferred reference counting, see *An
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//! Examination of Deferred Reference Counting and Cycle Detection* by Quinane:
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//! <https://openresearch-repository.anu.edu.au/bitstream/1885/42030/2/hon-thesis.pdf>
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use std::any::Any;
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use std::cell::UnsafeCell;
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use std::cmp;
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use std::collections::HashSet;
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use std::hash::{Hash, Hasher};
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use std::mem;
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use std::ops::Deref;
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use std::ptr::{self, NonNull};
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use std::sync::atomic::{self, AtomicUsize, Ordering};
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use std::{alloc::Layout, sync::Arc};
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use wasmtime_environ::StackMap;
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/// An external reference to some opaque data.
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///
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/// `VMExternRef`s dereference to their underlying opaque data as `dyn Any`.
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///
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/// Unlike the `externref` in the Wasm spec, `VMExternRef`s are non-nullable,
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/// and always point to a valid value. You may use `Option<VMExternRef>` to
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/// represent nullable references, and `Option<VMExternRef>` is guaranteed to
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/// have the same size and alignment as a raw pointer, with `None` represented
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/// with the null pointer.
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///
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/// `VMExternRef`s are reference counted, so cloning is a cheap, shallow
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/// operation. It also means they are inherently shared, so you may not get a
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/// mutable, exclusive reference to their inner contents, only a shared,
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/// immutable reference. You may use interior mutability with `RefCell` or
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/// `Mutex` to work around this restriction, if necessary.
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///
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/// `VMExternRef`s have pointer-equality semantics, not structural-equality
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/// semantics. Given two `VMExternRef`s `a` and `b`, `a == b` only if `a` and
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/// `b` point to the same allocation. `a` and `b` are considered not equal, even
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/// if `a` and `b` are two different identical copies of the same data, if they
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/// are in two different allocations. The hashing and ordering implementations
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/// also only operate on the pointer.
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///
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/// # Example
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///
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/// ```
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/// # fn foo() -> Result<(), Box<dyn std::error::Error>> {
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/// use std::cell::RefCell;
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/// use wasmtime_runtime::VMExternRef;
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///
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/// // Open a file. Wasm doesn't know about files, but we can let Wasm instances
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/// // work with files via opaque `externref` handles.
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/// let file = std::fs::File::create("some/file/path")?;
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///
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/// // Wrap the file up as an `VMExternRef` that can be passed to Wasm.
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/// let extern_ref_to_file = VMExternRef::new(file);
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///
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/// // `VMExternRef`s dereference to `dyn Any`, so you can use `Any` methods to
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/// // perform runtime type checks and downcasts.
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///
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/// assert!(extern_ref_to_file.is::<std::fs::File>());
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/// assert!(!extern_ref_to_file.is::<String>());
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///
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/// if let Some(mut file) = extern_ref_to_file.downcast_ref::<std::fs::File>() {
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/// use std::io::Write;
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/// writeln!(&mut file, "Hello, `VMExternRef`!")?;
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/// }
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/// # Ok(())
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/// # }
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/// ```
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#[derive(Debug)]
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#[repr(transparent)]
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pub struct VMExternRef(NonNull<VMExternData>);
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// Data contained is always Send+Sync so these should be safe.
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unsafe impl Send for VMExternRef {}
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unsafe impl Sync for VMExternRef {}
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#[repr(C)]
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pub(crate) struct VMExternData {
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// Implicit, dynamically-sized member that always preceded an
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// `VMExternData`.
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//
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// value: [u8],
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//
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/// The reference count for this `VMExternData` and value. When it reaches
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/// zero, we can safely destroy the value and free this heap
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/// allocation. This is an `UnsafeCell`, rather than plain `Cell`, because
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/// it can be modified by compiled Wasm code.
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///
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/// Note: this field's offset must be kept in sync with
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/// `wasmtime_environ::VMOffsets::vm_extern_data_ref_count()` which is
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/// currently always zero.
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ref_count: AtomicUsize,
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/// Always points to the implicit, dynamically-sized `value` member that
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/// precedes this `VMExternData`.
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value_ptr: NonNull<dyn Any + Send + Sync>,
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}
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impl Clone for VMExternRef {
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#[inline]
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fn clone(&self) -> VMExternRef {
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self.extern_data().increment_ref_count();
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VMExternRef(self.0)
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}
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}
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impl Drop for VMExternRef {
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#[inline]
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fn drop(&mut self) {
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let data = self.extern_data();
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// Note that the memory orderings here also match the standard library
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// itself. Documentation is more available in the implementation of
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// `Arc`, but the general idea is that this is a special pattern allowed
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// by the C standard with atomic orderings where we "release" for all
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// the decrements and only the final decrementer performs an acquire
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// fence. This properly ensures that the final thread, which actually
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// destroys the data, sees all the updates from all other threads.
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if data.ref_count.fetch_sub(1, Ordering::Release) != 1 {
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return;
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}
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atomic::fence(Ordering::Acquire);
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// Drop our live reference to `data` before we drop it itself.
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drop(data);
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unsafe {
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VMExternData::drop_and_dealloc(self.0);
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}
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}
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}
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impl VMExternData {
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/// Get the `Layout` for a value with the given size and alignment, and the
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/// offset within that layout where the `VMExternData` footer resides.
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///
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/// This doesn't take a `value: &T` because `VMExternRef::new_with` hasn't
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/// constructed a `T` value yet, and it isn't generic over `T` because
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/// `VMExternData::drop_and_dealloc` doesn't know what `T` to use, and has
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/// to use `std::mem::{size,align}_of_val` instead.
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unsafe fn layout_for(value_size: usize, value_align: usize) -> (Layout, usize) {
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let extern_data_size = mem::size_of::<VMExternData>();
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let extern_data_align = mem::align_of::<VMExternData>();
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let value_and_padding_size = round_up_to_align(value_size, extern_data_align).unwrap();
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let alloc_align = std::cmp::max(value_align, extern_data_align);
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let alloc_size = value_and_padding_size + extern_data_size;
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debug_assert!(Layout::from_size_align(alloc_size, alloc_align).is_ok());
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(
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Layout::from_size_align_unchecked(alloc_size, alloc_align),
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value_and_padding_size,
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)
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}
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/// Drop the inner value and then free this `VMExternData` heap allocation.
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pub(crate) unsafe fn drop_and_dealloc(mut data: NonNull<VMExternData>) {
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// Note: we introduce a block scope so that we drop the live
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// reference to the data before we free the heap allocation it
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// resides within after this block.
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let (alloc_ptr, layout) = {
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let data = data.as_mut();
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debug_assert_eq!(data.ref_count.load(Ordering::SeqCst), 0);
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// Same thing, but for the dropping the reference to `value` before
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// we drop it itself.
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let (layout, _) = {
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let value = data.value_ptr.as_ref();
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Self::layout_for(mem::size_of_val(value), mem::align_of_val(value))
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};
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ptr::drop_in_place(data.value_ptr.as_ptr());
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let alloc_ptr = data.value_ptr.cast::<u8>();
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(alloc_ptr, layout)
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};
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ptr::drop_in_place(data.as_ptr());
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std::alloc::dealloc(alloc_ptr.as_ptr(), layout);
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}
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#[inline]
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fn increment_ref_count(&self) {
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// This is only using during cloning operations, and like the standard
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// library we use `Relaxed` here. The rationale is better documented in
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// libstd's implementation of `Arc`, but the general gist is that we're
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// creating a new pointer for our own thread, so there's no need to have
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// any synchronization with orderings. The synchronization with other
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// threads with respect to orderings happens when the pointer is sent to
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// another thread.
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self.ref_count.fetch_add(1, Ordering::Relaxed);
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}
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}
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#[inline]
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fn round_up_to_align(n: usize, align: usize) -> Option<usize> {
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debug_assert!(align.is_power_of_two());
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let align_minus_one = align - 1;
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Some(n.checked_add(align_minus_one)? & !align_minus_one)
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}
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impl VMExternRef {
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/// Wrap the given value inside an `VMExternRef`.
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pub fn new<T>(value: T) -> VMExternRef
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where
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T: 'static + Any + Send + Sync,
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{
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VMExternRef::new_with(|| value)
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}
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/// Construct a new `VMExternRef` in place by invoking `make_value`.
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pub fn new_with<T>(make_value: impl FnOnce() -> T) -> VMExternRef
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where
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T: 'static + Any + Send + Sync,
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{
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unsafe {
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let (layout, footer_offset) =
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VMExternData::layout_for(mem::size_of::<T>(), mem::align_of::<T>());
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let alloc_ptr = std::alloc::alloc(layout);
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let alloc_ptr = NonNull::new(alloc_ptr).unwrap_or_else(|| {
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std::alloc::handle_alloc_error(layout);
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});
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let value_ptr = alloc_ptr.cast::<T>();
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ptr::write(value_ptr.as_ptr(), make_value());
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let extern_data_ptr =
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alloc_ptr.cast::<u8>().as_ptr().add(footer_offset) as *mut VMExternData;
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ptr::write(
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extern_data_ptr,
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VMExternData {
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ref_count: AtomicUsize::new(1),
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// Cast from `*mut T` to `*mut dyn Any` here.
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value_ptr: NonNull::new_unchecked(value_ptr.as_ptr()),
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},
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);
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VMExternRef(NonNull::new_unchecked(extern_data_ptr))
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}
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}
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/// Turn this `VMExternRef` into a raw, untyped pointer.
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///
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/// Unlike `into_raw`, this does not consume and forget `self`. It is *not*
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/// safe to use `from_raw` on pointers returned from this method; only use
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/// `clone_from_raw`!
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///
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/// Nor does this method increment the reference count. You must ensure
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/// that `self` (or some other clone of `self`) stays alive until
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/// `clone_from_raw` is called.
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#[inline]
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pub fn as_raw(&self) -> *mut u8 {
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let ptr = self.0.cast::<u8>().as_ptr();
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ptr
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}
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/// Consume this `VMExternRef` into a raw, untyped pointer.
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///
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/// # Safety
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///
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/// This method forgets self, so it is possible to create a leak of the
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/// underlying reference counted data if not used carefully.
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///
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/// Use `from_raw` to recreate the `VMExternRef`.
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pub unsafe fn into_raw(self) -> *mut u8 {
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let ptr = self.0.cast::<u8>().as_ptr();
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std::mem::forget(self);
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ptr
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}
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|
|
/// Recreate a `VMExternRef` from a pointer returned from a previous call to
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/// `as_raw`.
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///
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/// # Safety
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|
///
|
|
/// Unlike `clone_from_raw`, this does not increment the reference count of the
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/// underlying data. It is not safe to continue to use the pointer passed to this
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/// function.
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pub unsafe fn from_raw(ptr: *mut u8) -> Self {
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debug_assert!(!ptr.is_null());
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VMExternRef(NonNull::new_unchecked(ptr).cast())
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}
|
|
|
|
/// Recreate a `VMExternRef` from a pointer returned from a previous call to
|
|
/// `as_raw`.
|
|
///
|
|
/// # Safety
|
|
///
|
|
/// Wildly unsafe to use with anything other than the result of a previous
|
|
/// `as_raw` call!
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///
|
|
/// Additionally, it is your responsibility to ensure that this raw
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|
/// `VMExternRef`'s reference count has not dropped to zero. Failure to do
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|
/// so will result in use after free!
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pub unsafe fn clone_from_raw(ptr: *mut u8) -> Self {
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debug_assert!(!ptr.is_null());
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let x = VMExternRef(NonNull::new_unchecked(ptr).cast());
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x.extern_data().increment_ref_count();
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x
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}
|
|
|
|
/// Get the strong reference count for this `VMExternRef`.
|
|
///
|
|
/// Note that this loads with a `SeqCst` ordering to synchronize with other
|
|
/// threads.
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|
pub fn strong_count(&self) -> usize {
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self.extern_data().ref_count.load(Ordering::SeqCst)
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|
}
|
|
|
|
#[inline]
|
|
fn extern_data(&self) -> &VMExternData {
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|
unsafe { self.0.as_ref() }
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|
}
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|
}
|
|
|
|
/// Methods that would normally be trait implementations, but aren't to avoid
|
|
/// potential footguns around `VMExternRef`'s pointer-equality semantics.
|
|
///
|
|
/// Note that none of these methods are on `&self`, they all require a
|
|
/// fully-qualified `VMExternRef::foo(my_ref)` invocation.
|
|
impl VMExternRef {
|
|
/// Check whether two `VMExternRef`s point to the same inner allocation.
|
|
///
|
|
/// Note that this uses pointer-equality semantics, not structural-equality
|
|
/// semantics, and so only pointers are compared, and doesn't use any `Eq`
|
|
/// or `PartialEq` implementation of the pointed-to values.
|
|
#[inline]
|
|
pub fn eq(a: &Self, b: &Self) -> bool {
|
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ptr::eq(a.0.as_ptr(), b.0.as_ptr())
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}
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|
|
|
/// Hash a given `VMExternRef`.
|
|
///
|
|
/// Note that this just hashes the pointer to the inner value, it does *not*
|
|
/// use the inner value's `Hash` implementation (if any).
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|
#[inline]
|
|
pub fn hash<H>(externref: &Self, hasher: &mut H)
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|
where
|
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H: Hasher,
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{
|
|
ptr::hash(externref.0.as_ptr(), hasher);
|
|
}
|
|
|
|
/// Compare two `VMExternRef`s.
|
|
///
|
|
/// Note that this uses pointer-equality semantics, not structural-equality
|
|
/// semantics, and so only pointers are compared, and doesn't use any `Cmp`
|
|
/// or `PartialCmp` implementation of the pointed-to values.
|
|
#[inline]
|
|
pub fn cmp(a: &Self, b: &Self) -> cmp::Ordering {
|
|
let a = a.0.as_ptr() as usize;
|
|
let b = b.0.as_ptr() as usize;
|
|
a.cmp(&b)
|
|
}
|
|
}
|
|
|
|
impl Deref for VMExternRef {
|
|
type Target = dyn Any;
|
|
|
|
fn deref(&self) -> &dyn Any {
|
|
unsafe { self.extern_data().value_ptr.as_ref() }
|
|
}
|
|
}
|
|
|
|
/// A wrapper around a `VMExternRef` that implements `Eq` and `Hash` with
|
|
/// pointer semantics.
|
|
///
|
|
/// We use this so that we can morally put `VMExternRef`s inside of `HashSet`s
|
|
/// even though they don't implement `Eq` and `Hash` to avoid foot guns.
|
|
#[derive(Clone, Debug)]
|
|
struct VMExternRefWithTraits(VMExternRef);
|
|
|
|
impl Hash for VMExternRefWithTraits {
|
|
fn hash<H>(&self, hasher: &mut H)
|
|
where
|
|
H: Hasher,
|
|
{
|
|
VMExternRef::hash(&self.0, hasher)
|
|
}
|
|
}
|
|
|
|
impl PartialEq for VMExternRefWithTraits {
|
|
fn eq(&self, other: &Self) -> bool {
|
|
VMExternRef::eq(&self.0, &other.0)
|
|
}
|
|
}
|
|
|
|
impl Eq for VMExternRefWithTraits {}
|
|
|
|
type TableElem = UnsafeCell<Option<VMExternRef>>;
|
|
|
|
/// A table that over-approximizes the set of `VMExternRef`s that any Wasm
|
|
/// activation on this thread is currently using.
|
|
///
|
|
/// Under the covers, this is a simple bump allocator that allows duplicate
|
|
/// entries. Deduplication happens at GC time.
|
|
#[repr(C)] // `alloc` must be the first member, it's accessed from JIT code.
|
|
pub struct VMExternRefActivationsTable {
|
|
/// Structures used to perform fast bump allocation of storage of externref
|
|
/// values.
|
|
///
|
|
/// This is the only member of this structure accessed from JIT code.
|
|
alloc: VMExternRefTableAlloc,
|
|
|
|
/// When unioned with `chunk`, this is an over-approximation of the GC roots
|
|
/// on the stack, inside Wasm frames.
|
|
///
|
|
/// This is used by slow-path insertion, and when a GC cycle finishes, is
|
|
/// re-initialized to the just-discovered precise set of stack roots (which
|
|
/// immediately becomes an over-approximation again as soon as Wasm runs and
|
|
/// potentially drops references).
|
|
over_approximated_stack_roots: HashSet<VMExternRefWithTraits>,
|
|
|
|
/// The precise set of on-stack, inside-Wasm GC roots that we discover via
|
|
/// walking the stack and interpreting stack maps.
|
|
///
|
|
/// This is *only* used inside the `gc` function, and is empty otherwise. It
|
|
/// is just part of this struct so that we can reuse the allocation, rather
|
|
/// than create a new hash set every GC.
|
|
precise_stack_roots: HashSet<VMExternRefWithTraits>,
|
|
|
|
/// A pointer to the youngest host stack frame before we called
|
|
/// into Wasm for the first time. When walking the stack in garbage
|
|
/// collection, if we don't find this frame, then we failed to walk every
|
|
/// Wasm stack frame, which means we failed to find all on-stack,
|
|
/// inside-a-Wasm-frame roots, and doing a GC could lead to freeing one of
|
|
/// those missed roots, and use after free.
|
|
stack_canary: Option<usize>,
|
|
|
|
/// A debug-only field for asserting that we are in a region of code where
|
|
/// GC is okay to preform.
|
|
#[cfg(debug_assertions)]
|
|
gc_okay: bool,
|
|
}
|
|
|
|
#[repr(C)] // This is accessed from JIT code.
|
|
struct VMExternRefTableAlloc {
|
|
/// Bump-allocation finger within the `chunk`.
|
|
///
|
|
/// NB: this is an `UnsafeCell` because it is written to by compiled Wasm
|
|
/// code.
|
|
next: UnsafeCell<NonNull<TableElem>>,
|
|
|
|
/// Pointer to just after the `chunk`.
|
|
///
|
|
/// This is *not* within the current chunk and therefore is not a valid
|
|
/// place to insert a reference!
|
|
end: NonNull<TableElem>,
|
|
|
|
/// Bump allocation chunk that stores fast-path insertions.
|
|
///
|
|
/// This is not accessed from JIT code.
|
|
chunk: Box<[TableElem]>,
|
|
}
|
|
|
|
// This gets around the usage of `UnsafeCell` throughout the internals of this
|
|
// allocator, but the storage should all be Send/Sync and synchronization isn't
|
|
// necessary since operations require `&mut self`.
|
|
unsafe impl Send for VMExternRefTableAlloc {}
|
|
unsafe impl Sync for VMExternRefTableAlloc {}
|
|
|
|
fn _assert_send_sync() {
|
|
fn _assert<T: Send + Sync>() {}
|
|
_assert::<VMExternRefActivationsTable>();
|
|
_assert::<VMExternRef>();
|
|
}
|
|
|
|
impl VMExternRefActivationsTable {
|
|
const CHUNK_SIZE: usize = 4096 / mem::size_of::<usize>();
|
|
|
|
/// Create a new `VMExternRefActivationsTable`.
|
|
pub fn new() -> Self {
|
|
// Start with an empty chunk in case this activations table isn't used.
|
|
// This means that there's no space in the bump-allocation area which
|
|
// will force any path trying to use this to the slow gc path. The first
|
|
// time this happens, though, the slow gc path will allocate a new chunk
|
|
// for actual fast-bumping.
|
|
let mut chunk: Box<[TableElem]> = Box::new([]);
|
|
let next = chunk.as_mut_ptr();
|
|
let end = unsafe { next.add(chunk.len()) };
|
|
|
|
VMExternRefActivationsTable {
|
|
alloc: VMExternRefTableAlloc {
|
|
next: UnsafeCell::new(NonNull::new(next).unwrap()),
|
|
end: NonNull::new(end).unwrap(),
|
|
chunk,
|
|
},
|
|
over_approximated_stack_roots: HashSet::new(),
|
|
precise_stack_roots: HashSet::new(),
|
|
stack_canary: None,
|
|
#[cfg(debug_assertions)]
|
|
gc_okay: true,
|
|
}
|
|
}
|
|
|
|
fn new_chunk(size: usize) -> Box<[UnsafeCell<Option<VMExternRef>>]> {
|
|
assert!(size >= Self::CHUNK_SIZE);
|
|
(0..size).map(|_| UnsafeCell::new(None)).collect()
|
|
}
|
|
|
|
/// Get the available capacity in the bump allocation chunk.
|
|
#[inline]
|
|
pub fn bump_capacity_remaining(&self) -> usize {
|
|
let end = self.alloc.end.as_ptr() as usize;
|
|
let next = unsafe { *self.alloc.next.get() };
|
|
end - next.as_ptr() as usize
|
|
}
|
|
|
|
/// Try and insert a `VMExternRef` into this table.
|
|
///
|
|
/// This is a fast path that only succeeds when the bump chunk has the
|
|
/// capacity for the requested insertion.
|
|
///
|
|
/// If the insertion fails, then the `VMExternRef` is given back. Callers
|
|
/// may attempt a GC to free up space and try again, or may call
|
|
/// `insert_slow_path` to infallibly insert the reference (potentially
|
|
/// allocating additional space in the table to hold it).
|
|
#[inline]
|
|
pub fn try_insert(&mut self, externref: VMExternRef) -> Result<(), VMExternRef> {
|
|
unsafe {
|
|
let next = *self.alloc.next.get();
|
|
if next == self.alloc.end {
|
|
return Err(externref);
|
|
}
|
|
|
|
debug_assert!(
|
|
(*next.as_ref().get()).is_none(),
|
|
"slots >= the `next` bump finger are always `None`"
|
|
);
|
|
ptr::write(next.as_ptr(), UnsafeCell::new(Some(externref)));
|
|
|
|
let next = NonNull::new_unchecked(next.as_ptr().add(1));
|
|
debug_assert!(next <= self.alloc.end);
|
|
*self.alloc.next.get() = next;
|
|
|
|
Ok(())
|
|
}
|
|
}
|
|
|
|
/// Insert a reference into the table, falling back on a GC to clear up
|
|
/// space if the table is already full.
|
|
///
|
|
/// # Unsafety
|
|
///
|
|
/// The same as `gc`.
|
|
#[inline]
|
|
pub unsafe fn insert_with_gc(
|
|
&mut self,
|
|
externref: VMExternRef,
|
|
module_info_lookup: &dyn ModuleInfoLookup,
|
|
) {
|
|
#[cfg(debug_assertions)]
|
|
assert!(self.gc_okay);
|
|
|
|
if let Err(externref) = self.try_insert(externref) {
|
|
self.gc_and_insert_slow(externref, module_info_lookup);
|
|
}
|
|
}
|
|
|
|
#[inline(never)]
|
|
unsafe fn gc_and_insert_slow(
|
|
&mut self,
|
|
externref: VMExternRef,
|
|
module_info_lookup: &dyn ModuleInfoLookup,
|
|
) {
|
|
gc(module_info_lookup, self);
|
|
|
|
// Might as well insert right into the hash set, rather than the bump
|
|
// chunk, since we are already on a slow path and we get de-duplication
|
|
// this way.
|
|
self.over_approximated_stack_roots
|
|
.insert(VMExternRefWithTraits(externref));
|
|
}
|
|
|
|
/// Insert a reference into the table, without ever performing GC.
|
|
#[inline]
|
|
pub fn insert_without_gc(&mut self, externref: VMExternRef) {
|
|
if let Err(externref) = self.try_insert(externref) {
|
|
self.insert_slow_without_gc(externref);
|
|
}
|
|
}
|
|
|
|
#[inline(never)]
|
|
fn insert_slow_without_gc(&mut self, externref: VMExternRef) {
|
|
self.over_approximated_stack_roots
|
|
.insert(VMExternRefWithTraits(externref));
|
|
}
|
|
|
|
fn num_filled_in_bump_chunk(&self) -> usize {
|
|
let next = unsafe { *self.alloc.next.get() };
|
|
let bytes_unused = (self.alloc.end.as_ptr() as usize) - (next.as_ptr() as usize);
|
|
let slots_unused = bytes_unused / mem::size_of::<TableElem>();
|
|
self.alloc.chunk.len().saturating_sub(slots_unused)
|
|
}
|
|
|
|
fn elements(&self, mut f: impl FnMut(&VMExternRef)) {
|
|
for elem in self.over_approximated_stack_roots.iter() {
|
|
f(&elem.0);
|
|
}
|
|
|
|
// The bump chunk is not all the way full, so we only iterate over its
|
|
// filled-in slots.
|
|
let num_filled = self.num_filled_in_bump_chunk();
|
|
for slot in self.alloc.chunk.iter().take(num_filled) {
|
|
if let Some(elem) = unsafe { &*slot.get() } {
|
|
f(elem);
|
|
}
|
|
}
|
|
}
|
|
|
|
fn insert_precise_stack_root(
|
|
precise_stack_roots: &mut HashSet<VMExternRefWithTraits>,
|
|
root: NonNull<VMExternData>,
|
|
) {
|
|
let root = unsafe { VMExternRef::clone_from_raw(root.as_ptr().cast()) };
|
|
precise_stack_roots.insert(VMExternRefWithTraits(root));
|
|
}
|
|
|
|
/// Sweep the bump allocation table after we've discovered our precise stack
|
|
/// roots.
|
|
fn sweep(&mut self) {
|
|
// Sweep our bump chunk.
|
|
let num_filled = self.num_filled_in_bump_chunk();
|
|
unsafe {
|
|
*self.alloc.next.get() = self.alloc.end;
|
|
}
|
|
for slot in self.alloc.chunk.iter().take(num_filled) {
|
|
unsafe {
|
|
*slot.get() = None;
|
|
}
|
|
}
|
|
debug_assert!(
|
|
self.alloc
|
|
.chunk
|
|
.iter()
|
|
.all(|slot| unsafe { (*slot.get()).as_ref().is_none() }),
|
|
"after sweeping the bump chunk, all slots should be `None`"
|
|
);
|
|
|
|
// If this is the first instance of gc then the initial chunk is empty,
|
|
// so we lazily allocate space for fast bump-allocation in the future.
|
|
if self.alloc.chunk.is_empty() {
|
|
self.alloc.chunk = Self::new_chunk(Self::CHUNK_SIZE);
|
|
self.alloc.end =
|
|
NonNull::new(unsafe { self.alloc.chunk.as_mut_ptr().add(self.alloc.chunk.len()) })
|
|
.unwrap();
|
|
}
|
|
|
|
// Reset our `next` finger to the start of the bump allocation chunk.
|
|
unsafe {
|
|
let next = self.alloc.chunk.as_mut_ptr();
|
|
debug_assert!(!next.is_null());
|
|
*self.alloc.next.get() = NonNull::new_unchecked(next);
|
|
}
|
|
|
|
// The current `precise_stack_roots` becomes our new over-appoximated
|
|
// set for the next GC cycle.
|
|
mem::swap(
|
|
&mut self.precise_stack_roots,
|
|
&mut self.over_approximated_stack_roots,
|
|
);
|
|
|
|
// And finally, the new `precise_stack_roots` should be cleared and
|
|
// remain empty until the next GC cycle.
|
|
//
|
|
// Note that this may run arbitrary code as we run externref
|
|
// destructors. Because of our `&mut` borrow above on this table,
|
|
// though, we're guaranteed that nothing will touch this table.
|
|
self.precise_stack_roots.clear();
|
|
}
|
|
|
|
/// Fetches the current value of this table's stack canary.
|
|
///
|
|
/// This should only be used in conjunction with setting the stack canary
|
|
/// below if the return value is `None` typically. This is called from RAII
|
|
/// guards in `wasmtime::func::invoke_wasm_and_catch_traps`.
|
|
///
|
|
/// For more information on canaries see the gc functions below.
|
|
#[inline]
|
|
pub fn stack_canary(&self) -> Option<usize> {
|
|
self.stack_canary
|
|
}
|
|
|
|
/// Sets the current value of the stack canary.
|
|
///
|
|
/// This is called from RAII guards in
|
|
/// `wasmtime::func::invoke_wasm_and_catch_traps`. This is used to update
|
|
/// the stack canary to a concrete value and then reset it back to `None`
|
|
/// when wasm is finished.
|
|
///
|
|
/// For more information on canaries see the gc functions below.
|
|
#[inline]
|
|
pub fn set_stack_canary(&mut self, canary: Option<usize>) {
|
|
self.stack_canary = canary;
|
|
}
|
|
|
|
/// Set whether it is okay to GC or not right now.
|
|
///
|
|
/// This is provided as a helper for enabling various debug-only assertions
|
|
/// and checking places where the `wasmtime-runtime` user expects there not
|
|
/// to be any GCs.
|
|
#[inline]
|
|
pub fn set_gc_okay(&mut self, okay: bool) -> bool {
|
|
#[cfg(debug_assertions)]
|
|
{
|
|
return std::mem::replace(&mut self.gc_okay, okay);
|
|
}
|
|
#[cfg(not(debug_assertions))]
|
|
{
|
|
let _ = okay;
|
|
return true;
|
|
}
|
|
}
|
|
}
|
|
|
|
/// Used by the runtime to lookup information about a module given a
|
|
/// program counter value.
|
|
pub trait ModuleInfoLookup {
|
|
/// Lookup the module information from a program counter value.
|
|
fn lookup(&self, pc: usize) -> Option<Arc<dyn ModuleInfo>>;
|
|
}
|
|
|
|
/// Used by the runtime to query module information.
|
|
pub trait ModuleInfo {
|
|
/// Lookup the stack map at a program counter value.
|
|
fn lookup_stack_map(&self, pc: usize) -> Option<&StackMap>;
|
|
}
|
|
|
|
#[derive(Debug, Default)]
|
|
struct DebugOnly<T> {
|
|
inner: T,
|
|
}
|
|
|
|
impl<T> std::ops::Deref for DebugOnly<T> {
|
|
type Target = T;
|
|
|
|
fn deref(&self) -> &T {
|
|
if cfg!(debug_assertions) {
|
|
&self.inner
|
|
} else {
|
|
panic!(
|
|
"only deref `DebugOnly` when `cfg(debug_assertions)` or \
|
|
inside a `debug_assert!(..)`"
|
|
)
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<T> std::ops::DerefMut for DebugOnly<T> {
|
|
fn deref_mut(&mut self) -> &mut T {
|
|
if cfg!(debug_assertions) {
|
|
&mut self.inner
|
|
} else {
|
|
panic!(
|
|
"only deref `DebugOnly` when `cfg(debug_assertions)` or \
|
|
inside a `debug_assert!(..)`"
|
|
)
|
|
}
|
|
}
|
|
}
|
|
|
|
/// Perform garbage collection of `VMExternRef`s.
|
|
///
|
|
/// # Unsafety
|
|
///
|
|
/// You must have called `VMExternRefActivationsTable::set_stack_canary` for at
|
|
/// least the oldest host-->Wasm stack frame transition on this thread's stack
|
|
/// (it is idempotent to call it more than once) and keep its return value alive
|
|
/// across the duration of that host-->Wasm call.
|
|
///
|
|
/// Additionally, you must have registered the stack maps for every Wasm module
|
|
/// that has frames on the stack with the given `stack_maps_registry`.
|
|
pub unsafe fn gc(
|
|
module_info_lookup: &dyn ModuleInfoLookup,
|
|
externref_activations_table: &mut VMExternRefActivationsTable,
|
|
) {
|
|
log::debug!("start GC");
|
|
|
|
#[cfg(debug_assertions)]
|
|
assert!(externref_activations_table.gc_okay);
|
|
|
|
debug_assert!({
|
|
// This set is only non-empty within this function. It is built up when
|
|
// walking the stack and interpreting stack maps, and then drained back
|
|
// into the activations table's bump-allocated space at the
|
|
// end. Therefore, it should always be empty upon entering this
|
|
// function.
|
|
externref_activations_table.precise_stack_roots.is_empty()
|
|
});
|
|
|
|
// Whenever we call into Wasm from host code for the first time, we set a
|
|
// stack canary. When we return to that host code, we unset the stack
|
|
// canary. If there is *not* a stack canary, then there must be zero Wasm
|
|
// frames on the stack. Therefore, we can simply reset the table without
|
|
// walking the stack.
|
|
let stack_canary = match externref_activations_table.stack_canary {
|
|
None => {
|
|
if cfg!(debug_assertions) {
|
|
// Assert that there aren't any Wasm frames on the stack.
|
|
backtrace::trace(|frame| {
|
|
assert!(module_info_lookup.lookup(frame.ip() as usize).is_none());
|
|
true
|
|
});
|
|
}
|
|
externref_activations_table.sweep();
|
|
log::debug!("end GC");
|
|
return;
|
|
}
|
|
Some(canary) => canary,
|
|
};
|
|
|
|
// There is a stack canary, so there must be Wasm frames on the stack. The
|
|
// rest of this function consists of:
|
|
//
|
|
// * walking the stack,
|
|
//
|
|
// * finding the precise set of roots inside Wasm frames via our stack maps,
|
|
// and
|
|
//
|
|
// * resetting our bump-allocated table's over-approximation to the
|
|
// newly-discovered precise set.
|
|
|
|
// The SP of the previous (younger) frame we processed.
|
|
let mut last_sp = None;
|
|
|
|
// Whether we have found our stack canary or not yet.
|
|
let mut found_canary = false;
|
|
|
|
// The `activations_table_set` is used for `debug_assert!`s checking that
|
|
// every reference we read out from the stack via stack maps is actually in
|
|
// the table. If that weren't true, than either we forgot to insert a
|
|
// reference in the table when passing it into Wasm (a bug) or we are
|
|
// reading invalid references from the stack (another bug).
|
|
let mut activations_table_set: DebugOnly<HashSet<_>> = Default::default();
|
|
if cfg!(debug_assertions) {
|
|
externref_activations_table.elements(|elem| {
|
|
activations_table_set.insert(elem.as_raw() as *mut VMExternData);
|
|
});
|
|
}
|
|
|
|
backtrace::trace(|frame| {
|
|
let pc = frame.ip() as usize;
|
|
let sp = frame.sp() as usize;
|
|
|
|
if let Some(module_info) = module_info_lookup.lookup(pc) {
|
|
if let Some(stack_map) = module_info.lookup_stack_map(pc) {
|
|
debug_assert!(sp != 0, "we should always get a valid SP for Wasm frames");
|
|
|
|
for i in 0..(stack_map.mapped_words() as usize) {
|
|
if stack_map.get_bit(i) {
|
|
// Stack maps have one bit per word in the frame, and the
|
|
// zero^th bit is the *lowest* addressed word in the frame,
|
|
// i.e. the closest to the SP. So to get the `i`^th word in
|
|
// this frame, we add `i * sizeof(word)` to the SP.
|
|
let ptr_to_ref = sp + i * mem::size_of::<usize>();
|
|
|
|
let r = std::ptr::read(ptr_to_ref as *const *mut VMExternData);
|
|
debug_assert!(
|
|
r.is_null() || activations_table_set.contains(&r),
|
|
"every on-stack externref inside a Wasm frame should \
|
|
have an entry in the VMExternRefActivationsTable; \
|
|
{:?} is not in the table",
|
|
r
|
|
);
|
|
if let Some(r) = NonNull::new(r) {
|
|
VMExternRefActivationsTable::insert_precise_stack_root(
|
|
&mut externref_activations_table.precise_stack_roots,
|
|
r,
|
|
);
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
if let Some(last_sp) = last_sp {
|
|
// We've found the stack canary when we walk over the frame that it
|
|
// is contained within.
|
|
found_canary |= last_sp <= stack_canary && stack_canary <= sp;
|
|
}
|
|
last_sp = Some(sp);
|
|
|
|
// Keep walking the stack until we've found the canary, which is the
|
|
// oldest frame before we ever called into Wasm. We can stop once we've
|
|
// found it because there won't be any more Wasm frames, and therefore
|
|
// there won't be anymore on-stack, inside-a-Wasm-frame roots.
|
|
!found_canary
|
|
});
|
|
|
|
// Only sweep and reset the table if we found the stack canary, and
|
|
// therefore know that we discovered all the on-stack, inside-a-Wasm-frame
|
|
// roots. If we did *not* find the stack canary, then `libunwind` failed to
|
|
// walk the whole stack, and we might be missing roots. Reseting the table
|
|
// would free those missing roots while they are still in use, leading to
|
|
// use-after-free.
|
|
if found_canary {
|
|
externref_activations_table.sweep();
|
|
} else {
|
|
log::warn!("did not find stack canary; skipping GC sweep");
|
|
externref_activations_table.precise_stack_roots.clear();
|
|
}
|
|
|
|
log::debug!("end GC");
|
|
}
|
|
|
|
#[cfg(test)]
|
|
mod tests {
|
|
use super::*;
|
|
use std::convert::TryInto;
|
|
|
|
#[test]
|
|
fn extern_ref_is_pointer_sized_and_aligned() {
|
|
assert_eq!(mem::size_of::<VMExternRef>(), mem::size_of::<*mut ()>());
|
|
assert_eq!(mem::align_of::<VMExternRef>(), mem::align_of::<*mut ()>());
|
|
assert_eq!(
|
|
mem::size_of::<Option<VMExternRef>>(),
|
|
mem::size_of::<*mut ()>()
|
|
);
|
|
assert_eq!(
|
|
mem::align_of::<Option<VMExternRef>>(),
|
|
mem::align_of::<*mut ()>()
|
|
);
|
|
}
|
|
|
|
#[test]
|
|
fn ref_count_is_at_correct_offset() {
|
|
let s = "hi";
|
|
let s: &(dyn Any + Send + Sync) = &s as _;
|
|
let s: *const (dyn Any + Send + Sync) = s as _;
|
|
let s: *mut (dyn Any + Send + Sync) = s as _;
|
|
|
|
let extern_data = VMExternData {
|
|
ref_count: AtomicUsize::new(0),
|
|
value_ptr: NonNull::new(s).unwrap(),
|
|
};
|
|
|
|
let extern_data_ptr = &extern_data as *const _;
|
|
let ref_count_ptr = &extern_data.ref_count as *const _;
|
|
|
|
let actual_offset = (ref_count_ptr as usize) - (extern_data_ptr as usize);
|
|
|
|
let offsets = wasmtime_environ::VMOffsets::from(wasmtime_environ::VMOffsetsFields {
|
|
ptr: 8,
|
|
num_imported_functions: 0,
|
|
num_imported_tables: 0,
|
|
num_imported_memories: 0,
|
|
num_imported_globals: 0,
|
|
num_defined_functions: 0,
|
|
num_defined_tables: 0,
|
|
num_defined_memories: 0,
|
|
num_defined_globals: 0,
|
|
num_escaped_funcs: 0,
|
|
});
|
|
assert_eq!(
|
|
offsets.vm_extern_data_ref_count(),
|
|
actual_offset.try_into().unwrap(),
|
|
);
|
|
}
|
|
|
|
#[test]
|
|
fn table_next_is_at_correct_offset() {
|
|
let table = VMExternRefActivationsTable::new();
|
|
|
|
let table_ptr = &table as *const _;
|
|
let next_ptr = &table.alloc.next as *const _;
|
|
|
|
let actual_offset = (next_ptr as usize) - (table_ptr as usize);
|
|
|
|
let offsets = wasmtime_environ::VMOffsets::from(wasmtime_environ::VMOffsetsFields {
|
|
ptr: 8,
|
|
num_imported_functions: 0,
|
|
num_imported_tables: 0,
|
|
num_imported_memories: 0,
|
|
num_imported_globals: 0,
|
|
num_defined_functions: 0,
|
|
num_defined_tables: 0,
|
|
num_defined_memories: 0,
|
|
num_defined_globals: 0,
|
|
num_escaped_funcs: 0,
|
|
});
|
|
assert_eq!(
|
|
offsets.vm_extern_ref_activation_table_next() as usize,
|
|
actual_offset
|
|
);
|
|
}
|
|
|
|
#[test]
|
|
fn table_end_is_at_correct_offset() {
|
|
let table = VMExternRefActivationsTable::new();
|
|
|
|
let table_ptr = &table as *const _;
|
|
let end_ptr = &table.alloc.end as *const _;
|
|
|
|
let actual_offset = (end_ptr as usize) - (table_ptr as usize);
|
|
|
|
let offsets = wasmtime_environ::VMOffsets::from(wasmtime_environ::VMOffsetsFields {
|
|
ptr: 8,
|
|
num_imported_functions: 0,
|
|
num_imported_tables: 0,
|
|
num_imported_memories: 0,
|
|
num_imported_globals: 0,
|
|
num_defined_functions: 0,
|
|
num_defined_tables: 0,
|
|
num_defined_memories: 0,
|
|
num_defined_globals: 0,
|
|
num_escaped_funcs: 0,
|
|
});
|
|
assert_eq!(
|
|
offsets.vm_extern_ref_activation_table_end() as usize,
|
|
actual_offset
|
|
);
|
|
}
|
|
}
|