Fix access to VMMemoryDefinition::current_length on big-endian (#3013)

The current_length member is defined as "usize" in Rust code,
but generated wasm code refers to it as if it were "u32".
While this happens to mostly work on little-endian machines
(as long as the length is < 4GB), it will always fail on
big-endian machines.

Fixed by making current_length "u32" in Rust as well, and
ensuring the actual memory size is always less than 4GB.

crates/wasmtime/src/memory.rs

@@ -318,7 +318,7 @@ impl Memory {
         unsafe {
             let store = store.into();
             let definition = *store[self.0].definition;
-            slice::from_raw_parts(definition.base, definition.current_length)
+            slice::from_raw_parts(definition.base, definition.current_length as usize)
         }
     }
 
@@ -334,7 +334,7 @@ impl Memory {
         unsafe {
             let store = store.into();
             let definition = *store[self.0].definition;
-            slice::from_raw_parts_mut(definition.base, definition.current_length)
+            slice::from_raw_parts_mut(definition.base, definition.current_length as usize)
         }
     }
 
@@ -395,7 +395,7 @@ impl Memory {
     ///
     /// Panics if this memory doesn't belong to `store`.
     pub fn data_size(&self, store: impl AsContext) -> usize {
-        unsafe { (*store.as_context()[self.0].definition).current_length }
+        unsafe { (*store.as_context()[self.0].definition).current_length as usize }
     }
 
     /// Returns the size, in WebAssembly pages, of this wasm memory.

crates/wasmtime/src/trampoline/memory.rs

@@ -7,6 +7,7 @@ use wasmtime_environ::entity::PrimaryMap;
 use wasmtime_environ::{wasm, MemoryPlan, MemoryStyle, Module, WASM_PAGE_SIZE};
 use wasmtime_runtime::{RuntimeLinearMemory, RuntimeMemoryCreator, VMMemoryDefinition};
 
+use std::convert::TryFrom;
 use std::sync::Arc;
 
 pub fn create_memory(store: &mut StoreOpaque<'_>, memory: &MemoryType) -> Result<InstanceId> {
@@ -48,7 +49,8 @@ impl RuntimeLinearMemory for LinearMemoryProxy {
     fn vmmemory(&self) -> VMMemoryDefinition {
         VMMemoryDefinition {
             base: self.mem.as_ptr(),
-            current_length: self.mem.size() as usize * WASM_PAGE_SIZE as usize,
+            current_length: u32::try_from(self.mem.size() as usize * WASM_PAGE_SIZE as usize)
+                .unwrap(),
         }
     }
 }