Implement on-demand memory initialization for the uffd feature.

This commit implements copying paged initialization data upon a fault of a
linear memory page.

If the initialization data is "paged", then the appropriate pages are copied
into the Wasm page (or zeroed if the page is not present in the
initialization data).

If the initialization data is not "paged", the Wasm page is zeroed so that
module instantiation can initialize the pages.

crates/environ/src/module_environ.rs

@@ -1,6 +1,6 @@
 use crate::module::{
     Initializer, InstanceSignature, MemoryPlan, Module, ModuleSignature, ModuleType, ModuleUpvar,
-    TableElements, TablePlan, TypeTables,
+    TableInitializer, TablePlan, TypeTables,
 };
 use crate::tunables::Tunables;
 use cranelift_codegen::ir;
@@ -13,7 +13,6 @@ use cranelift_wasm::{
     ModuleIndex, ModuleTypeIndex, SignatureIndex, Table, TableIndex, TargetEnvironment, TypeIndex,
     WasmError, WasmFuncType, WasmResult,
 };
-use serde::{Deserialize, Serialize};
 use std::collections::{hash_map::Entry, HashMap};
 use std::convert::TryFrom;
 use std::mem;
@@ -684,7 +683,7 @@ impl<'data> cranelift_wasm::ModuleEnvironment<'data> for ModuleEnvironment<'data
     fn reserve_table_elements(&mut self, num: u32) -> WasmResult<()> {
         self.result
             .module
-            .table_elements
+            .table_initializers
             .reserve_exact(usize::try_from(num).unwrap());
         Ok(())
     }
@@ -696,12 +695,15 @@ impl<'data> cranelift_wasm::ModuleEnvironment<'data> for ModuleEnvironment<'data
         offset: usize,
         elements: Box<[FuncIndex]>,
     ) -> WasmResult<()> {
-        self.result.module.table_elements.push(TableElements {
-            table_index,
-            base,
-            offset,
-            elements,
-        });
+        self.result
+            .module
+            .table_initializers
+            .push(TableInitializer {
+                table_index,
+                base,
+                offset,
+                elements,
+            });
         Ok(())
     }
 
@@ -774,11 +776,9 @@ impl<'data> cranelift_wasm::ModuleEnvironment<'data> for ModuleEnvironment<'data
         data: &'data [u8],
     ) -> WasmResult<()> {
         self.result.data_initializers.push(DataInitializer {
-            location: DataInitializerLocation {
-                memory_index,
-                base,
-                offset,
-            },
+            memory_index,
+            base,
+            offset,
             data,
         });
         Ok(())
@@ -1072,10 +1072,8 @@ pub fn translate_signature(mut sig: ir::Signature, pointer_type: ir::Type) -> ir
     sig
 }
 
-/// A memory index and offset within that memory where a data initialization
-/// should is to be performed.
-#[derive(Clone, Serialize, Deserialize)]
-pub struct DataInitializerLocation {
+/// A data initializer for linear memory.
+pub struct DataInitializer<'data> {
     /// The index of the memory to initialize.
     pub memory_index: MemoryIndex,
 
@@ -1084,34 +1082,7 @@ pub struct DataInitializerLocation {
 
     /// A constant offset to initialize at.
     pub offset: usize,
-}
-
-/// A data initializer for linear memory.
-pub struct DataInitializer<'data> {
-    /// The location where the initialization is to be performed.
-    pub location: DataInitializerLocation,
 
     /// The initialization data.
     pub data: &'data [u8],
 }
-
-/// Similar to `DataInitializer`, but owns its own copy of the data rather
-/// than holding a slice of the original module.
-#[derive(Serialize, Deserialize)]
-pub struct OwnedDataInitializer {
-    /// The location where the initialization is to be performed.
-    pub location: DataInitializerLocation,
-
-    /// The initialization data.
-    pub data: Box<[u8]>,
-}
-
-impl OwnedDataInitializer {
-    /// Creates a new owned data initializer from a borrowed data initializer.
-    pub fn new(borrowed: DataInitializer<'_>) -> Self {
-        Self {
-            location: borrowed.location.clone(),
-            data: borrowed.data.into(),
-        }
-    }
-}