Fix incorrect range in ininitialize_instance.

This commit fixes a bug where the wrong destination range was used when copying
data from the module's memory initialization upon instance initialization.

This affects the on-demand allocator only when using the `uffd` feature on
Linux and when the Wasm page being initialized is not the last in the module's
initial pages.

Fixes #2784.

crates/runtime/src/instance/allocator.rs

@@ -379,7 +379,9 @@ fn initialize_instance(
                 for (page_index, page) in pages.iter().enumerate() {
                     if let Some(data) = page {
                         debug_assert_eq!(data.len(), WASM_PAGE_SIZE as usize);
-                        slice[page_index * WASM_PAGE_SIZE as usize..].copy_from_slice(data);
+                        let start = page_index * WASM_PAGE_SIZE as usize;
+                        let end = start + WASM_PAGE_SIZE as usize;
+                        slice[start..end].copy_from_slice(data);
                     }
                 }
             }

tests/all/instance.rs

@@ -11,3 +11,23 @@ fn wrong_import_numbers() -> Result<()> {
     assert!(Instance::new(&store, &module, &[func.clone().into(), func.into()]).is_err());
     Ok(())
 }
+
+#[test]
+fn initializes_linear_memory() -> Result<()> {
+    // Test for https://github.com/bytecodealliance/wasmtime/issues/2784
+    let wat = r#"
+        (module
+            (memory (export "memory") 2)
+            (data (i32.const 0) "Hello World!")
+        )"#;
+    let module = Module::new(&Engine::default(), wat)?;
+
+    let store = Store::new(module.engine());
+    let instance = Instance::new(&store, &module, &[])?;
+    let memory = instance.get_memory("memory").unwrap();
+
+    let mut bytes = [0; 12];
+    memory.read(0, &mut bytes)?;
+    assert_eq!(bytes, "Hello World!".as_bytes());
+    Ok(())
+}