fix: remove _start fallback for instantiation

Signed-off-by: Henry <mail@henrygressmann.de>

Author: explodingcamera

.github/workflows/test.yaml

@@ -106,7 +106,7 @@ jobs:
         if: matrix.target == ''
 
       - name: Run tests (${{ matrix.target }})
-        uses: houseabsolute/actions-rust-cross@fe6ede73c7f0a50412ec05994e2aa5a7bf635eac # v1.0.7
+        uses: houseabsolute/actions-rust-cross@21b0f18dc621b25bfae556ff2791fca4173121e8 # v1.0.8
         with:
           command: test
           target: ${{ matrix.target }}

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Changed
+
+- `instantiate` / `start` / `start_func` no longer fall back to `_start` if the module has no start section. This is technically a breaking change, but it is more consistent with the WebAssembly spec and prevents accidental execution of `_start` in wasi modules that don't have a start section. Call the `_start` function explicitly if you want to run it.
+
 ## [0.9.1] - 2026-06-29
 
 ### Added

crates/cli/src/cmd/run.rs

@@ -1,4 +1,4 @@
-use eyre::{Result, bail};
+use eyre::Result;
 use tinywasm::types::ExportType;
 use tinywasm::{ModuleInstance, Store};
 
@@ -34,13 +34,17 @@ pub fn run(args: RunArgs) -> Result<()> {
             Ok(())
         }
         None => {
-            if instance.start_func(&store)?.is_none() {
-                bail!(
-                    "module has no start function or `_start` export; use `tinywasm inspect {module_path}` or `tinywasm run --invoke <export> {module_path}`"
-                )
+            if instance.start_func(&store)?.is_some() {
+                let _ = instance.start(&mut store)?;
+                return Ok(());
             }
 
-            let _ = instance.start(&mut store)?;
+            let start = instance.func_untyped(&store, "_start").map_err(|_| {
+                eyre::eyre!(
+                    "module has no start function or `_start` export. Use `tinywasm inspect {module_path}` or `tinywasm run --invoke <export> {module_path}`"
+                )
+            })?;
+            start.call(&mut store, &[])?;
             Ok(())
         }
     }

crates/tinywasm/src/instance.rs

@@ -604,7 +604,8 @@ impl ModuleInstance {
     /// # use tinywasm::{ModuleInstance, Store};
     /// # let wasm = wat::parse_str(r#"
     /// #     (module
-    /// #       (func (export "_start"))
+    /// #       (func $start)
+    /// #       (start $start)
     /// #     )
     /// # "#).expect("valid wat");
     /// # let module = tinywasm::parse_bytes(&wasm)?;
@@ -619,21 +620,8 @@ impl ModuleInstance {
     pub fn start_func(&self, store: &Store) -> Result<Option<Function>> {
         self.validate_store(store)?;
 
-        let func_addr = match self.0.func_start {
-            Some(func_index) => func_index,
-            None => {
-                // Alternatively, check for a _start function in the exports.
-                let Some(ExternVal::Func(func_addr)) = self.export_addr("_start") else {
-                    return Ok(None);
-                };
-
-                return Ok(Some(Function {
-                    item: crate::StoreItem::new(self.0.store_id, func_addr),
-                    module_addr: self.id(),
-                    addr: func_addr,
-                    ty: store.state.get_func(func_addr).ty().clone(),
-                }));
-            }
+        let Some(func_addr) = self.0.func_start else {
+            return Ok(None);
         };
 
         let func_addr = self.resolve_func_addr(func_addr);
@@ -656,9 +644,10 @@ impl ModuleInstance {
     /// # let wasm = wat::parse_str(r#"
     /// #     (module
     /// #       (global $g (mut i32) (i32.const 0))
-    /// #       (func (export "_start")
+    /// #       (func $start
     /// #         i32.const 7
     /// #         global.set $g)
+    /// #       (start $start)
     /// #       (export "g" (global $g)))
     /// # "#).expect("valid wat");
     /// # let module = tinywasm::parse_bytes(&wasm)?;

crates/tinywasm/src/interpreter/executor.rs

@@ -49,6 +49,7 @@ impl<'store, const BUDGETED: bool> Executor<'store, BUDGETED> {
         }
     }
 
+    #[allow(clippy::redundant_closure_call)] // we wrap these in closures to get better perf traces
     #[inline(always)]
     fn exec(&mut self) -> Result<Option<()>, Trap> {
         macro_rules! stack_op {
@@ -264,7 +265,7 @@ impl<'store, const BUDGETED: bool> Executor<'store, BUDGETED> {
             }};
         }
 
-        let next = match self.func.instructions.get(self.cf.instr_ptr as usize) {
+        let next = match self.func.instructions.get(self.cf.instr_ptr) {
             Some(instr) => instr,
             None => {
                 cold_path();
@@ -1654,12 +1655,6 @@ impl<'store> Executor<'store, true> {
     }
 }
 
-#[cold]
-#[inline(never)]
-fn instruction_pointer_out_of_bounds(instr_ptr: usize, instruction_count: usize) -> ! {
-    panic!("Instruction pointer out of bounds: {instr_ptr} ({instruction_count} instructions)")
-}
-
 #[inline(always)]
 fn cmp_i32(lhs: i32, rhs: i32, op: CmpOp) -> bool {
     match op {

crates/tinywasm/tests/internal_refs.rs

@@ -169,14 +169,15 @@ fn export_func_type_index_mismatch_fixture_would_break_old_lookup() -> Result<()
 }
 
 #[test]
-fn start_prefers_exported_start_without_re_resolving_store_addr() -> Result<()> {
+fn start_resolves_module_func_index_to_store_addr() -> Result<()> {
     let wasm = wat::parse_str(
         r#"
         (module
           (global (export "g") (mut i32) (i32.const 0))
-          (func (export "_start")
+          (func $start
             i32.const 1
             global.set 0)
+          (start $start)
         )
         "#,
     )?;

crates/tinywasm/tests/start_function.rs

@@ -0,0 +1,23 @@
+use eyre::Result;
+use tinywasm::{ModuleInstance, Store};
+
+#[test]
+fn exported_wasi_start_is_not_run_during_instantiation() -> Result<()> {
+    let wasm = wat::parse_str(
+        r#"
+        (module
+          (table 1 funcref)
+          (func $_start (export "_start")
+            unreachable)
+          (elem (i32.const 0) func $_start))
+        "#,
+    )?;
+    let module = tinywasm::parse_bytes(&wasm)?;
+    let mut store = Store::default();
+
+    let instance = ModuleInstance::instantiate(&mut store, &module, None)?;
+
+    assert!(instance.start_func(&store)?.is_none());
+    assert!(instance.func::<(), ()>(&store, "_start")?.call(&mut store, ()).is_err());
+    Ok(())
+}