Struct wasmtime_runtime::VMMemoryDefinition

#[repr(C)]
pub struct VMMemoryDefinition {
    pub base: *mut u8,
    pub current_length: AtomicUsize,
}

The fields compiled code needs to access to utilize a WebAssembly linear memory defined within the instance, namely the start address and the size in bytes.

Fields

base: *mut u8

The start address.

current_length: AtomicUsize

The current logical size of this linear memory in bytes.

This is atomic because shared memories must be able to grow their length atomically. For relaxed access, see VMMemoryDefinition::current_length().

Implementations

impl VMMemoryDefinition

pub fn current_length(&self) -> usize

Return the current length of the VMMemoryDefinition by performing a relaxed load; do not use this function for situations in which a precise length is needed. Owned memories (i.e., non-shared) will always return a precise result (since no concurrent modification is possible) but shared memories may see an imprecise value–a current_length potentially smaller than what some other thread observes. Since Wasm memory only grows, this under-estimation may be acceptable in certain cases.

Examples found in repository

src/vmcontext.rs (line 248)

    pub unsafe fn load(ptr: *mut Self) -> Self {
        let other = &*ptr;
        VMMemoryDefinition {
            base: other.base,
            current_length: other.current_length().into(),
        }
    }

src/memory.rs (line 894)

fn validate_atomic_addr(
    def: &VMMemoryDefinition,
    addr: u64,
    access_size: u64,
    access_alignment: u64,
) -> Result<*mut u8, Trap> {
    debug_assert!(access_alignment.is_power_of_two());
    if !(addr % access_alignment == 0) {
        return Err(Trap::HeapMisaligned);
    }

    let length = u64::try_from(def.current_length()).unwrap();
    if !(addr.saturating_add(access_size) < length) {
        return Err(Trap::MemoryOutOfBounds);
    }

    Ok(def.base.wrapping_add(addr as usize))
}

src/instance/allocator.rs (line 256)

fn check_memory_init_bounds(instance: &Instance, initializers: &[MemoryInitializer]) -> Result<()> {
    for init in initializers {
        let memory = instance.get_memory(init.memory_index);
        let start = get_memory_init_start(init, instance)?;
        let end = usize::try_from(start)
            .ok()
            .and_then(|start| start.checked_add(init.data.len()));

        match end {
            Some(end) if end <= memory.current_length() => {
                // Initializer is in bounds
            }
            _ => {
                bail!("memory out of bounds: data segment does not fit")
            }
        }
    }

    Ok(())
}

fn initialize_memories(instance: &mut Instance, module: &Module) -> Result<()> {
    let memory_size_in_pages =
        &|memory| (instance.get_memory(memory).current_length() as u64) / u64::from(WASM_PAGE_SIZE);

    // Loads the `global` value and returns it as a `u64`, but sign-extends
    // 32-bit globals which can be used as the base for 32-bit memories.
    let get_global_as_u64 = &|global| unsafe {
        let def = if let Some(def_index) = instance.module().defined_global_index(global) {
            instance.global(def_index)
        } else {
            &*instance.imported_global(global).from
        };
        if module.globals[global].wasm_ty == WasmType::I64 {
            *def.as_u64()
        } else {
            u64::from(*def.as_u32())
        }
    };

    // Delegates to the `init_memory` method which is sort of a duplicate of
    // `instance.memory_init_segment` but is used at compile-time in other
    // contexts so is shared here to have only one method of memory
    // initialization.
    //
    // This call to `init_memory` notably implements all the bells and whistles
    // so errors only happen if an out-of-bounds segment is found, in which case
    // a trap is returned.
    let ok = module.memory_initialization.init_memory(
        InitMemory::Runtime {
            memory_size_in_pages,
            get_global_as_u64,
        },
        &mut |memory_index, init| {
            // If this initializer applies to a defined memory but that memory
            // doesn't need initialization, due to something like copy-on-write
            // pre-initializing it via mmap magic, then this initializer can be
            // skipped entirely.
            if let Some(memory_index) = module.defined_memory_index(memory_index) {
                if !instance.memories[memory_index].needs_init() {
                    return true;
                }
            }
            let memory = instance.get_memory(memory_index);

            unsafe {
                let src = instance.wasm_data(init.data.clone());
                let dst = memory.base.add(usize::try_from(init.offset).unwrap());
                // FIXME audit whether this is safe in the presence of shared
                // memory
                // (https://github.com/bytecodealliance/wasmtime/issues/4203).
                ptr::copy_nonoverlapping(src.as_ptr(), dst, src.len())
            }
            true
        },
    );
    if !ok {
        return Err(Trap::MemoryOutOfBounds.into());
    }

    Ok(())
}

src/instance.rs (line 684)

    pub(crate) fn memory_copy(
        &mut self,
        dst_index: MemoryIndex,
        dst: u64,
        src_index: MemoryIndex,
        src: u64,
        len: u64,
    ) -> Result<(), Trap> {
        // https://webassembly.github.io/reference-types/core/exec/instructions.html#exec-memory-copy

        let src_mem = self.get_memory(src_index);
        let dst_mem = self.get_memory(dst_index);

        let src = self.validate_inbounds(src_mem.current_length(), src, len)?;
        let dst = self.validate_inbounds(dst_mem.current_length(), dst, len)?;

        // Bounds and casts are checked above, by this point we know that
        // everything is safe.
        unsafe {
            let dst = dst_mem.base.add(dst);
            let src = src_mem.base.add(src);
            // FIXME audit whether this is safe in the presence of shared memory
            // (https://github.com/bytecodealliance/wasmtime/issues/4203).
            ptr::copy(src, dst, len as usize);
        }

        Ok(())
    }

    fn validate_inbounds(&self, max: usize, ptr: u64, len: u64) -> Result<usize, Trap> {
        let oob = || Trap::MemoryOutOfBounds;
        let end = ptr
            .checked_add(len)
            .and_then(|i| usize::try_from(i).ok())
            .ok_or_else(oob)?;
        if end > max {
            Err(oob())
        } else {
            Ok(ptr as usize)
        }
    }

    /// Perform the `memory.fill` operation on a locally defined memory.
    ///
    /// # Errors
    ///
    /// Returns a `Trap` error if the memory range is out of bounds.
    pub(crate) fn memory_fill(
        &mut self,
        memory_index: MemoryIndex,
        dst: u64,
        val: u8,
        len: u64,
    ) -> Result<(), Trap> {
        let memory = self.get_memory(memory_index);
        let dst = self.validate_inbounds(memory.current_length(), dst, len)?;

        // Bounds and casts are checked above, by this point we know that
        // everything is safe.
        unsafe {
            let dst = memory.base.add(dst);
            // FIXME audit whether this is safe in the presence of shared memory
            // (https://github.com/bytecodealliance/wasmtime/issues/4203).
            ptr::write_bytes(dst, val, len as usize);
        }

        Ok(())
    }

    /// Performs the `memory.init` operation.
    ///
    /// # Errors
    ///
    /// Returns a `Trap` error if the destination range is out of this module's
    /// memory's bounds or if the source range is outside the data segment's
    /// bounds.
    pub(crate) fn memory_init(
        &mut self,
        memory_index: MemoryIndex,
        data_index: DataIndex,
        dst: u64,
        src: u32,
        len: u32,
    ) -> Result<(), Trap> {
        let range = match self.module().passive_data_map.get(&data_index).cloned() {
            Some(range) if !self.dropped_data.contains(data_index) => range,
            _ => 0..0,
        };
        self.memory_init_segment(memory_index, range, dst, src, len)
    }

    pub(crate) fn wasm_data(&self, range: Range<u32>) -> &[u8] {
        &self.runtime_info.wasm_data()[range.start as usize..range.end as usize]
    }

    pub(crate) fn memory_init_segment(
        &mut self,
        memory_index: MemoryIndex,
        range: Range<u32>,
        dst: u64,
        src: u32,
        len: u32,
    ) -> Result<(), Trap> {
        // https://webassembly.github.io/bulk-memory-operations/core/exec/instructions.html#exec-memory-init

        let memory = self.get_memory(memory_index);
        let data = self.wasm_data(range);
        let dst = self.validate_inbounds(memory.current_length(), dst, len.into())?;
        let src = self.validate_inbounds(data.len(), src.into(), len.into())?;
        let len = len as usize;

        unsafe {
            let src_start = data.as_ptr().add(src);
            let dst_start = memory.base.add(dst);
            // FIXME audit whether this is safe in the presence of shared memory
            // (https://github.com/bytecodealliance/wasmtime/issues/4203).
            ptr::copy_nonoverlapping(src_start, dst_start, len);
        }

        Ok(())
    }

pub unsafe fn load(ptr: *mut Self) -> Self

Return a copy of the VMMemoryDefinition using the relaxed value of current_length; see VMMemoryDefinition::current_length().

Examples found in repository

src/instance.rs (line 194)

    pub(crate) fn get_memory(&self, index: MemoryIndex) -> VMMemoryDefinition {
        if let Some(defined_index) = self.module().defined_memory_index(index) {
            self.memory(defined_index)
        } else {
            let import = self.imported_memory(index);
            unsafe { VMMemoryDefinition::load(import.from) }
        }
    }

    /// Get a locally defined or imported memory.
    pub(crate) fn get_runtime_memory(&mut self, index: MemoryIndex) -> &mut Memory {
        if let Some(defined_index) = self.module().defined_memory_index(index) {
            unsafe { &mut *self.get_defined_memory(defined_index) }
        } else {
            let import = self.imported_memory(index);
            let ctx = unsafe { &mut *import.vmctx };
            unsafe { &mut *ctx.instance_mut().get_defined_memory(import.index) }
        }
    }

    /// Return the indexed `VMMemoryDefinition`.
    fn memory(&self, index: DefinedMemoryIndex) -> VMMemoryDefinition {
        unsafe { VMMemoryDefinition::load(self.memory_ptr(index)) }
    }

Trait Implementations

impl Debug for VMMemoryDefinition

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.