Replace webassembly folder with master

src/webassembly/instance.rs

@@ -20,6 +20,7 @@ use std::iter::FromIterator;
 use std::iter::Iterator;
 use std::mem::size_of;
 use std::ptr::write_unaligned;
+use std::sync::Arc;
 use std::{fmt, mem, slice};
 
 use super::super::common::slice::{BoundedSlice, UncheckedSlice};
@@ -29,8 +30,6 @@ use super::libcalls;
 use super::memory::LinearMemory;
 use super::module::{Export, ImportableExportable, Module};
 use super::relocation::{Reloc, RelocSink, RelocationType};
-use super::vm;
-use super::backing::{LocalBacking, ImportsBacking};
 
 type TablesSlice = UncheckedSlice<BoundedSlice<usize>>;
 // TODO: this should be `type MemoriesSlice = UncheckedSlice<UncheckedSlice<u8>>;`, but that crashes for some reason.
@@ -75,6 +74,81 @@ pub struct EmscriptenData {
     pub stack_alloc: extern "C" fn(u32, &Instance) -> u32,
 }
 
+impl EmscriptenData {
+    pub fn new(module: &Module, instance: &Instance) -> Self {
+        unsafe {
+            debug!("emscripten::new");
+            let malloc_export = module.info.exports.get("_malloc");
+            let free_export = module.info.exports.get("_free");
+            let memalign_export = module.info.exports.get("_memalign");
+            let memset_export = module.info.exports.get("_memset");
+            let stack_alloc_export = module.info.exports.get("stackAlloc");
+
+            let mut malloc_addr = 0 as *const u8;
+            let mut free_addr = 0 as *const u8;
+            let mut memalign_addr = 0 as *const u8;
+            let mut memset_addr = 0 as *const u8;
+            let mut stack_alloc_addr = 0 as _;
+
+            if let Some(Export::Function(malloc_index)) = malloc_export {
+                malloc_addr = instance.get_function_pointer(*malloc_index);
+            }
+
+            if let Some(Export::Function(free_index)) = free_export {
+                free_addr = instance.get_function_pointer(*free_index);
+            }
+
+            if let Some(Export::Function(memalign_index)) = memalign_export {
+                memalign_addr = instance.get_function_pointer(*memalign_index);
+            }
+
+            if let Some(Export::Function(memset_index)) = memset_export {
+                memset_addr = instance.get_function_pointer(*memset_index);
+            }
+
+            if let Some(Export::Function(stack_alloc_index)) = stack_alloc_export {
+                stack_alloc_addr = instance.get_function_pointer(*stack_alloc_index);
+            }
+
+            EmscriptenData {
+                malloc: mem::transmute(malloc_addr),
+                free: mem::transmute(free_addr),
+                memalign: mem::transmute(memalign_addr),
+                memset: mem::transmute(memset_addr),
+                stack_alloc: mem::transmute(stack_alloc_addr),
+            }
+        }
+    }
+
+    // Emscripten __ATINIT__
+    pub fn atinit(&self, module: &Module, instance: &Instance) -> Result<(), String> {
+        debug!("emscripten::atinit");
+        if let Some(&Export::Function(environ_constructor_index)) =
+            module.info.exports.get("___emscripten_environ_constructor")
+        {
+            debug!("emscripten::___emscripten_environ_constructor");
+            let ___emscripten_environ_constructor: extern "C" fn(&Instance) =
+                get_instance_function!(instance, environ_constructor_index);
+            call_protected!(___emscripten_environ_constructor(&instance))
+                .map_err(|err| format!("{}", err))?;
+        };
+        // TODO: We also need to handle TTY.init() and SOCKFS.root = FS.mount(SOCKFS, {}, null)
+        Ok(())
+    }
+
+    // Emscripten __ATEXIT__
+    pub fn atexit(&self, _module: &Module, _instance: &Instance) -> Result<(), String> {
+        debug!("emscripten::atexit");
+        use libc::fflush;
+        use std::ptr;
+        // Flush all open streams
+        unsafe {
+            fflush(ptr::null_mut());
+        };
+        Ok(())
+    }
+}
+
 impl fmt::Debug for EmscriptenData {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_struct("EmscriptenData")
@@ -96,16 +170,25 @@ pub enum InstanceABI {
 #[derive(Debug)]
 #[repr(C)]
 pub struct Instance {
-    pub vmctx: vm::Ctx,
     // C-like pointers to data (heaps, globals, tables)
     pub data_pointers: DataPointers,
 
+    /// WebAssembly table data
+    // pub tables: Arc<Vec<RwLock<Vec<usize>>>>,
+    pub tables: Arc<Vec<Vec<usize>>>,
+
+    /// WebAssembly linear memory data
+    pub memories: Arc<Vec<LinearMemory>>,
+
+    /// WebAssembly global variable data
+    pub globals: Vec<u8>,
+
     /// Webassembly functions
-    finalized_funcs: Box<[*const vm::Func]>,
+    // functions: Vec<usize>,
+    functions: Vec<Vec<u8>>,
 
-    backing: LocalBacking,
-
-    imports: ImportsBacking,
+    /// Imported functions
+    import_functions: Vec<*const u8>,
 
     /// The module start function
     pub start_func: Option<FuncIndex>,
@@ -114,14 +197,6 @@ pub struct Instance {
     pub emscripten_data: Option<EmscriptenData>,
 }
 
-impl Instance {
-    /// Shortcut for converting from a `vm::Ctx` pointer to a reference to the `Instance`.
-    /// This works because of the `vm::Ctx` is the first field of the `Instance`.
-    pub unsafe fn from_vmctx<'a>(ctx: *mut vm::Ctx) -> &'a mut Instance {
-        &mut *(ctx as *mut Instance)
-    }
-}
-
 /// Contains pointers to data (heaps, globals, tables) needed
 /// by Cranelift.
 /// NOTE: Rearranging the fields will break the memory arrangement model
@@ -421,13 +496,114 @@ impl Instance {
         debug!("Instance - Instantiating tables");
         // Instantiate tables
         {
-            
+            // Reserve space for tables
+            tables.reserve_exact(module.info.tables.len());
+
+            // Get tables in module
+            for table in &module.info.tables {
+                let table: Vec<usize> = match table.import_name.as_ref() {
+                    Some((module_name, field_name)) => {
+                        let imported =
+                            import_object.get(&module_name.as_str(), &field_name.as_str());
+                        match imported {
+                            Some(ImportValue::Table(t)) => t.to_vec(),
+                            None => {
+                                if options.mock_missing_tables {
+                                    debug!(
+                                        "The Imported table {}.{} is not provided, therefore will be mocked.",
+                                        module_name, field_name
+                                    );
+                                    let len = table.entity.minimum as usize;
+                                    let mut v = Vec::with_capacity(len);
+                                    v.resize(len, 0);
+                                    v
+                                } else {
+                                    panic!(
+                                        "Imported table value was not provided ({}.{})",
+                                        module_name, field_name
+                                    )
+                                }
+                            }
+                            _ => panic!(
+                                "Expected global table, but received {:?} ({}.{})",
+                                imported, module_name, field_name
+                            ),
+                        }
+                    }
+                    None => {
+                        let len = table.entity.minimum as usize;
+                        let mut v = Vec::with_capacity(len);
+                        v.resize(len, 0);
+                        v
+                    }
+                };
+                tables.push(table);
+            }
+
+            // instantiate tables
+            for table_element in &module.info.table_elements {
+                let base = match table_element.base {
+                    Some(global_index) => globals_data[global_index.index()] as usize,
+                    None => 0,
+                };
+
+                let table = &mut tables[table_element.table_index.index()];
+                for (i, func_index) in table_element.elements.iter().enumerate() {
+                    // since the table just contains functions in the MVP
+                    // we get the address of the specified function indexes
+                    // to populate the table.
+
+                    // let func_index = *elem_index - module.info.imported_funcs.len() as u32;
+                    // let func_addr = functions[func_index.index()].as_ptr();
+                    let func_addr = get_function_addr(&func_index, &import_functions, &functions);
+                    table[base + table_element.offset + i] = func_addr as _;
+                }
+            }
         }
 
         debug!("Instance - Instantiating memories");
         // Instantiate memories
         {
-            
+            // Reserve space for memories
+            memories.reserve_exact(module.info.memories.len());
+
+            // Get memories in module
+            for memory in &module.info.memories {
+                let memory = memory.entity;
+                // If we use emscripten, we set a fixed initial and maximum
+                debug!(
+                    "Instance - init memory ({}, {:?})",
+                    memory.minimum, memory.maximum
+                );
+                let memory = if options.abi == InstanceABI::Emscripten {
+                    // We use MAX_PAGES, so at the end the result is:
+                    // (initial * LinearMemory::PAGE_SIZE) == LinearMemory::DEFAULT_HEAP_SIZE
+                    // However, it should be: (initial * LinearMemory::PAGE_SIZE) == 16777216
+                    LinearMemory::new(LinearMemory::MAX_PAGES, None)
+                } else {
+                    LinearMemory::new(memory.minimum, memory.maximum.map(|m| m as u32))
+                };
+                memories.push(memory);
+            }
+
+            for init in &module.info.data_initializers {
+                debug_assert!(init.base.is_none(), "globalvar base not supported yet");
+                let offset = init.offset;
+                let mem = &mut memories[init.memory_index.index()];
+                let end_of_init = offset + init.data.len();
+                if end_of_init > mem.current_size() {
+                    let grow_pages = (end_of_init / LinearMemory::PAGE_SIZE as usize) + 1;
+                    mem.grow(grow_pages as u32)
+                        .expect("failed to grow memory for data initializers");
+                }
+                let to_init = &mut mem[offset..offset + init.data.len()];
+                to_init.copy_from_slice(&init.data);
+            }
+            if options.abi == InstanceABI::Emscripten {
+                debug!("emscripten::setup memory");
+                crate::apis::emscripten::emscripten_set_up_memory(&mut memories[0]);
+                debug!("emscripten::finish setup memory");
+            }
         }
 
         let start_func: Option<FuncIndex> =
@@ -453,83 +629,37 @@ impl Instance {
             tables: tables_pointer[..].into(),
         };
 
-        let emscripten_data = if options.abi == InstanceABI::Emscripten {
-            unsafe {
-                debug!("emscripten::initiating data");
-                let malloc_export = module.info.exports.get("_malloc");
-                let free_export = module.info.exports.get("_free");
-                let memalign_export = module.info.exports.get("_memalign");
-                let memset_export = module.info.exports.get("_memset");
-                let stack_alloc_export = module.info.exports.get("stackAlloc");
-
-                let mut malloc_addr = 0 as *const u8;
-                let mut free_addr = 0 as *const u8;
-                let mut memalign_addr = 0 as *const u8;
-                let mut memset_addr = 0 as *const u8;
-                let mut stack_alloc_addr = 0 as _;
-
-                if malloc_export.is_none()
-                    && free_export.is_none()
-                    && memalign_export.is_none()
-                    && memset_export.is_none()
-                {
-                    None
-                } else {
-                    if let Some(Export::Function(malloc_index)) = malloc_export {
-                        malloc_addr =
-                            get_function_addr(&malloc_index, &import_functions, &functions);
-                    }
-
-                    if let Some(Export::Function(free_index)) = free_export {
-                        free_addr = get_function_addr(&free_index, &import_functions, &functions);
-                    }
-
-                    if let Some(Export::Function(memalign_index)) = memalign_export {
-                        memalign_addr =
-                            get_function_addr(&memalign_index, &import_functions, &functions);
-                    }
-
-                    if let Some(Export::Function(memset_index)) = memset_export {
-                        memset_addr =
-                            get_function_addr(&memset_index, &import_functions, &functions);
-                    }
-
-                    if let Some(Export::Function(stack_alloc_index)) = stack_alloc_export {
-                        stack_alloc_addr =
-                            get_function_addr(&stack_alloc_index, &import_functions, &functions);
-                    }
-
-                    Some(EmscriptenData {
-                        malloc: mem::transmute(malloc_addr),
-                        free: mem::transmute(free_addr),
-                        memalign: mem::transmute(memalign_addr),
-                        memset: mem::transmute(memset_addr),
-                        stack_alloc: mem::transmute(stack_alloc_addr),
-                    })
-                }
-            }
-        } else {
-            None
-        };
-
-        Ok(Instance {
+        let mut instance = Instance {
             data_pointers,
-            tables: tables.into_iter().collect(),
-            memories: memories.into_iter().collect(),
+            tables: Arc::new(tables.into_iter().collect()), // tables.into_iter().map(|table| RwLock::new(table)).collect()),
+            memories: Arc::new(memories.into_iter().collect()),
             globals,
             functions,
             import_functions,
             start_func,
-            emscripten_data,
-        })
+            emscripten_data: None,
+        };
+
+        if options.abi == InstanceABI::Emscripten {
+            instance.emscripten_data = Some(EmscriptenData::new(module, &instance));
+        }
+
+        Ok(instance)
     }
 
     pub fn memory_mut(&mut self, memory_index: usize) -> &mut LinearMemory {
-        self.memories
+        let memories = Arc::get_mut(&mut self.memories).unwrap_or_else(|| {
+            panic!("Can't get memories as a mutable pointer (there might exist more mutable pointers to the memories)")
+        });
+        memories
             .get_mut(memory_index)
             .unwrap_or_else(|| panic!("no memory for index {}", memory_index))
     }
 
+    pub fn memories(&self) -> Arc<Vec<LinearMemory>> {
+        self.memories.clone()
+    }
+
     pub fn get_function_pointer(&self, func_index: FuncIndex) -> *const u8 {
         get_function_addr(&func_index, &self.import_functions, &self.functions)
     }

src/webassembly/memory.rs

@@ -0,0 +1,207 @@
+//! The webassembly::Memory() constructor creates a new Memory object which is
+//! a structure that holds the raw bytes of memory accessed by a
+//! webassembly::Instance.
+//! A memory created by Rust or in WebAssembly code will be accessible and
+//! mutable from both Rust and WebAssembly.
+use region;
+use std::ops::{Deref, DerefMut};
+use std::slice;
+
+use crate::common::mmap::Mmap;
+
+/// A linear memory instance.
+#[derive(Debug)]
+pub struct LinearMemory {
+    // The mmap allocation
+    mmap: Mmap,
+
+    // current number of wasm pages
+    current: u32,
+
+    // The maximum size the WebAssembly Memory is allowed to grow
+    // to, in units of WebAssembly pages.  When present, the maximum
+    // parameter acts as a hint to the engine to reserve memory up
+    // front.  However, the engine may ignore or clamp this reservation
+    // request.  In general, most WebAssembly modules shouldn't need
+    // to set a maximum.
+    maximum: Option<u32>,
+
+    // The size of the extra guard pages after the end.
+    // Is used to optimize loads and stores with constant offsets.
+    offset_guard_size: usize,
+}
+
+/// It holds the raw bytes of memory accessed by a WebAssembly Instance
+impl LinearMemory {
+    pub const PAGE_SIZE: u32 = 65536;
+    pub const MAX_PAGES: u32 = 65536;
+    pub const DEFAULT_HEAP_SIZE: usize = 1 << 32; // 4 GiB
+    pub const DEFAULT_GUARD_SIZE: usize = 1 << 31; // 2 GiB
+    pub const DEFAULT_SIZE: usize = Self::DEFAULT_HEAP_SIZE + Self::DEFAULT_GUARD_SIZE; // 6 GiB
+
+    /// Create a new linear memory instance with specified initial and maximum number of pages.
+    ///
+    /// `maximum` cannot be set to more than `65536` pages.
+    pub fn new(initial: u32, maximum: Option<u32>) -> Self {
+        assert!(initial <= Self::MAX_PAGES);
+        assert!(maximum.is_none() || maximum.unwrap() <= Self::MAX_PAGES);
+        debug!(
+            "Instantiate LinearMemory(initial={:?}, maximum={:?})",
+            initial, maximum
+        );
+
+        let mut mmap = Mmap::with_size(Self::DEFAULT_SIZE).expect("Can't create mmap");
+
+        let base = mmap.as_mut_ptr();
+
+        // map initial pages as readwrite since the inital mmap is mapped as not accessible.
+        if initial != 0 {
+            unsafe {
+                region::protect(
+                    base,
+                    initial as usize * Self::PAGE_SIZE as usize,
+                    region::Protection::ReadWrite,
+                )
+            }
+            .expect("unable to make memory inaccessible");
+        }
+
+        debug!("LinearMemory instantiated");
+        debug!(
+            "  - usable: {:#x}..{:#x}",
+            base as usize,
+            (base as usize) + LinearMemory::DEFAULT_HEAP_SIZE
+        );
+        debug!(
+            "  - guard: {:#x}..{:#x}",
+            (base as usize) + LinearMemory::DEFAULT_HEAP_SIZE,
+            (base as usize) + LinearMemory::DEFAULT_SIZE
+        );
+        Self {
+            mmap,
+            current: initial,
+            offset_guard_size: LinearMemory::DEFAULT_GUARD_SIZE,
+            maximum,
+        }
+    }
+
+    /// Returns an base address of this linear memory.
+    pub fn base(&mut self) -> *mut u8 {
+        self.mmap.as_mut_ptr() as _
+    }
+
+    /// Returns a number of allocated wasm pages.
+    pub fn current_size(&self) -> usize {
+        self.current as usize * Self::PAGE_SIZE as usize
+    }
+
+    pub fn current_pages(&self) -> u32 {
+        self.current
+    }
+
+    /// Returns the maximum number of wasm pages allowed.
+    pub fn maximum_size(&self) -> u32 {
+        self.maximum.unwrap_or(Self::MAX_PAGES)
+    }
+
+    /// Grow memory by the specified amount of pages.
+    ///
+    /// Returns `None` if memory can't be grown by the specified amount
+    /// of pages.
+    pub fn grow(&mut self, add_pages: u32) -> Option<i32> {
+        debug!("grow_memory called!");
+        if add_pages == 0 {
+            return Some(self.current as _);
+        }
+
+        let prev_pages = self.current;
+
+        let new_pages = match self.current.checked_add(add_pages) {
+            Some(new_pages) => new_pages,
+            None => return None,
+        };
+
+        if let Some(val) = self.maximum {
+            if new_pages > val {
+                return None;
+            }
+        // Wasm linear memories are never allowed to grow beyond what is
+        // indexable. If the memory has no maximum, enforce the greatest
+        // limit here.
+        } else if new_pages >= Self::MAX_PAGES {
+            return None;
+        }
+
+        let prev_bytes = (prev_pages * Self::PAGE_SIZE) as usize;
+        let new_bytes = (new_pages * Self::PAGE_SIZE) as usize;
+
+        // if new_bytes > self.mmap.len() - self.offset_guard_size {
+        unsafe {
+            region::protect(
+                self.mmap.as_ptr().add(prev_bytes) as _,
+                new_bytes - prev_bytes,
+                region::Protection::ReadWrite,
+            )
+        }
+        .expect("unable to make memory inaccessible");
+        // };
+        // if new_bytes > self.mmap.len() - self.offset_guard_size {
+        //     // If we have no maximum, this is a "dynamic" heap, and it's allowed to move.
+        //     assert!(self.maximum.is_none());
+        //     let guard_bytes = self.offset_guard_size;
+        //     let request_bytes = new_bytes.checked_add(guard_bytes)?;
+
+        //     let mut new_mmap = Mmap::with_size(request_bytes).ok()?;
+
+        //     // Make the offset-guard pages inaccessible.
+        //     unsafe {
+        //         region::protect(
+        //             new_mmap.as_ptr().add(new_bytes),
+        //             guard_bytes,
+        //             region::Protection::Read | region::Protection::Write,
+        //             // region::Protection::None,
+        //         )
+        //     }
+        //     .expect("unable to make memory inaccessible");
+
+        //     let copy_len = self.mmap.len() - self.offset_guard_size;
+        //     new_mmap.as_mut_slice()[..copy_len].copy_from_slice(&self.mmap.as_slice()[..copy_len]);
+
+        //     self.mmap = new_mmap;
+        // }
+
+        self.current = new_pages;
+
+        Some(prev_pages as i32)
+    }
+}
+
+// Not comparing based on memory content. That would be inefficient.
+impl PartialEq for LinearMemory {
+    fn eq(&self, other: &LinearMemory) -> bool {
+        self.current == other.current && self.maximum == other.maximum
+    }
+}
+
+impl Deref for LinearMemory {
+    type Target = [u8];
+    fn deref(&self) -> &[u8] {
+        unsafe {
+            slice::from_raw_parts(
+                self.mmap.as_ptr() as _,
+                self.current as usize * Self::PAGE_SIZE as usize,
+            )
+        }
+    }
+}
+
+impl DerefMut for LinearMemory {
+    fn deref_mut(&mut self) -> &mut [u8] {
+        unsafe {
+            slice::from_raw_parts_mut(
+                self.mmap.as_mut_ptr() as _,
+                self.current as usize * Self::PAGE_SIZE as usize,
+            )
+        }
+    }
+}

src/webassembly/mod.rs

@@ -6,15 +6,15 @@ pub mod memory;
 pub mod module;
 pub mod relocation;
 pub mod utils;
-pub mod vm;
-pub mod table;
-pub mod backing;
+pub mod vmcontext;
+pub mod vmoffsets;
 
 use cranelift_codegen::{
     isa,
     settings::{self, Configurable},
 };
 use cranelift_wasm::ModuleEnvironment;
+use std::io::{self, Write};
 use std::panic;
 use std::str::FromStr;
 use target_lexicon;
@@ -154,7 +154,7 @@ pub fn get_isa() -> Box<isa::TargetIsa> {
     isa::lookup(triple!("x86_64")).unwrap().finish(flags)
 }
 
-fn store_module_arguments(path: &str, args: Vec<&str>, instance: &mut Instance) -> (u32, u32) {
+fn store_module_arguments(path: &str, args: Vec<&str>, instance: &Instance) -> (u32, u32) {
     let argc = args.len() + 1;
 
     let (argv_offset, argv_slice): (_, &mut [u32]) =
@@ -212,19 +212,9 @@ pub fn start_instance(
     path: &str,
     args: Vec<&str>,
 ) -> Result<(), String> {
-    if is_emscripten_module(&module) {
-        // Emscripten __ATINIT__
-        if let Some(&Export::Function(environ_constructor_index)) =
-            module.info.exports.get("___emscripten_environ_constructor")
-        {
-            debug!("emscripten::___emscripten_environ_constructor");
-            let ___emscripten_environ_constructor: extern "C" fn(&Instance) =
-                get_instance_function!(instance, environ_constructor_index);
-            call_protected!(___emscripten_environ_constructor(&instance))
-                .map_err(|err| format!("{}", err))?;
-        };
+    if let Some(ref emscripten_data) = &instance.emscripten_data {
+        emscripten_data.atinit(module, instance)?;
 
-        // TODO: We also need to handle TTY.init() and SOCKFS.root = FS.mount(SOCKFS, {}, null)
         let func_index = match module.info.exports.get("_main") {
             Some(&Export::Function(index)) => index,
             _ => panic!("_main emscripten function not found"),
@@ -233,7 +223,7 @@ pub fn start_instance(
         let sig_index = module.get_func_type(func_index);
         let signature = module.get_signature(sig_index);
         let num_params = signature.params.len();
-        match num_params {
+        let result = match num_params {
             2 => {
                 let main: extern "C" fn(u32, u32, &Instance) =
                     get_instance_function!(instance, func_index);
@@ -249,8 +239,11 @@ pub fn start_instance(
                 num_params
             ),
         }
-        .map_err(|err| format!("{}", err))
-    // TODO: We should implement emscripten __ATEXIT__
+        .map_err(|err| format!("{}", err));
+
+        emscripten_data.atexit(module, instance)?;
+
+        result
     } else {
         let func_index =
             instance

src/webassembly/module.rs

@@ -6,7 +6,7 @@ use std::string::String;
 use std::vec::Vec;
 
 use cranelift_codegen::cursor::FuncCursor;
-use cranelift_codegen::ir::immediates::{Offset32, Uimm64};
+use cranelift_codegen::ir::immediates::{Imm64, Offset32, Uimm64};
 use cranelift_codegen::ir::types::*;
 use cranelift_codegen::ir::{
     self, AbiParam, ArgumentPurpose, ExtFuncData, ExternalName, FuncRef, InstBuilder, Signature,
@@ -199,7 +199,7 @@ pub struct DataInitializer {
 #[derive(Clone, Debug)]
 pub enum TableElement {
     /// A element that, if called, produces a trap.
-    Trap,
+    Trap(),
     /// A function.
     Function(FuncIndex),
 }

src/webassembly/vmcontext.rs

@@ -0,0 +1,151 @@
+use crate::webassembly::vmoffsets::VMOffsets;
+use cranelift_wasm::{
+    DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex,
+    GlobalIndex, MemoryIndex, SignatureIndex, TableIndex,
+};
+
+#[repr(C)]
+pub struct VMContext {
+    /// A pointer to an array of imported functions, indexed by `FuncIndex`.
+    imported_functions: *const *const VMFunctionBody,
+
+    /// A pointer to an array of imported tables, indexed by `TableIndex`.
+    imported_tables: *mut VMTableImport,
+
+    /// A pointer to an array of imported memories, indexed by `MemoryIndex,
+    imported_memories: *mut VMMemoryImport,
+
+    /// A pointer to an array of imported globals, indexed by `GlobalIndex`.
+    imported_globals: *mut VMGlobalImport,
+
+    /// A pointer to an array of locally-defined tables, indexed by `DefinedTableIndex`.
+    tables: *mut VMTableDefinition,
+
+    /// A pointer to an array of locally-defined memories, indexed by `DefinedMemoryIndex`.
+    memories: *mut VMMemoryDefinition,
+
+    /// A pointer to an array of locally-defined globals, indexed by ``DefinedGlobalIndex`.
+    globals: *mut VMGlobalDefinition,
+
+    /// Signature identifiers for signature-checked indirect calls.
+    signature_ids: *mut VMSharedSigIndex,
+}
+
+/// Used to provide type safety for passing around function pointers.
+/// The typesystem ensures this cannot be dereferenced.
+pub enum VMFunctionBody {}
+
+/// Definition of a table used by the VM. (obviously)
+#[repr(C)]
+pub struct VMTableDefinition {
+    /// pointer to the elements in the table.
+    pub base: *mut u8,
+    /// Number of elements in the table (NOT necessarily the size of the table in bytes!).
+    pub current_elements: usize,
+}
+
+impl VMTableDefinition {
+    pub fn offset_base(offsets: &VMOffsets) -> u8 {
+        0 * offsets.ptr_size
+    }
+
+    pub fn offset_current_elements(offsets: &VMOffsets) -> u8 {
+        1 * offsets.ptr_size
+    }
+}
+
+#[repr(C)]
+pub struct VMTableImport {
+    /// A pointer to the table definition.
+    pub table: *mut VMTableDefinition,
+    /// A pointer to the vmcontext that owns this table definition.
+    pub vmctx: *mut VMContext,
+}
+
+impl VMTableImport {
+    pub fn offset_table(offsets: &VMOffsets) -> u8 {
+        0 * offsets.ptr_size
+    }
+
+    pub fn offset_vmctx(offsets: &VMOffsets) -> u8 {
+        1 * offsets.ptr_size
+    }
+}
+
+/// Definition of a memory used by the VM.
+#[repr(C)]
+pub struct VMMemoryDefinition {
+    /// Pointer to the bottom of linear memory.
+    pub base: *mut u8,
+    /// Current logical size of this linear memory in bytes.
+    pub size: usize,
+}
+
+impl VMMemoryDefinition {
+    pub fn offset_base(offsets: &VMOffsets) -> u8 {
+        0 * offsets.ptr_size
+    }
+
+    pub fn offset_size(offsets: &VMOffsets) -> u8 {
+        1 * offsets.ptr_size
+    }
+}
+
+#[repr(C)]
+pub struct VMMemoryImport {
+    /// A pointer to the memory definition.
+    pub memory: *mut VMMemoryDefinition,
+    /// A pointer to the vmcontext that owns this memory definition.
+    pub vmctx: *mut VMContext,
+}
+
+impl VMMemoryImport {
+    pub fn offset_memory(offsets: &VMOffsets) -> u8 {
+        0 * offsets.ptr_size
+    }
+
+    pub fn offset_vmctx(offsets: &VMOffsets) -> u8 {
+        1 * offsets.ptr_size
+    }
+}
+
+/// Definition of a global used by the VM.
+#[repr(C, align(8))]
+pub struct VMGlobalDefinition {
+    pub data: [u8; 8],
+}
+
+#[repr(C)]
+pub struct VMGlobalImport {
+    pub globals: *mut VMGlobalDefinition,
+}
+
+impl VMGlobalImport {
+    pub fn offset_globals(offsets: &VMOffsets) -> u8 {
+        0 * offsets.ptr_size
+    }
+}
+
+#[repr(C)]
+pub struct VMSharedSigIndex(u32);
+
+#[repr(C)]
+pub struct VMCallerCheckedAnyfunc {
+    pub func: *const VMFunctionBody,
+    pub type_index: VMSharedSigIndex,
+    pub vmctx: *mut VMContext,
+}
+
+impl VMCallerCheckedAnyfunc {
+    pub fn offset_func(offsets: &VMOffsets) -> u8 {
+        0 * offsets.ptr_size
+    }
+
+    pub fn offset_type_index(offsets: &VMOffsets) -> u8 {
+        1 * offsets.ptr_size
+    }
+
+    pub fn offset_vmctx(offsets: &VMOffsets) -> u8 {
+        2 * offsets.ptr_size
+    }
+}

src/webassembly/vmoffsets.rs

@@ -0,0 +1,9 @@
+pub struct VMOffsets {
+    pub(in crate::webassembly) ptr_size: u8,
+}
+
+impl VMOffsets {
+    pub fn new(ptr_size: u8) -> Self {
+        Self { ptr_size }
+    }
+}