Improve panics/traps from imported functions (#857)

* Improve panics/traps from imported functions

This commit performs a few refactorings and fixes a bug as well. The
changes here are:

* The `thread_local!` in the `wasmtime` crate for trap information is
  removed. The thread local in the `wasmtime_runtime` crate is now
  leveraged to transmit trap information.

* Panics in user-provided functions are now caught explicitly to be
  carried across JIT code manually. Getting Rust panics unwinding
  through JIT code is pretty likely to be super tricky and difficult to
  do, so in the meantime we can get by with catching panics and resuming
  the panic once we've resumed in Rust code.

* Various take/record trap apis have all been removed in favor of
  working directly with `Trap` objects, where the internal trap object
  has been expanded slightly to encompass user-provided errors as well.

This borrows a bit #839 and otherwise will...

Closes #848

* Rename `r#return` to `ret`

crates/api/src/callable.rs

@@ -1,5 +1,5 @@
 use crate::runtime::Store;
-use crate::trampoline::{generate_func_export, take_api_trap};
+use crate::trampoline::generate_func_export;
 use crate::trap::Trap;
 use crate::types::FuncType;
 use crate::values::Val;
@@ -157,8 +157,7 @@ impl WrappedCallable for WasmtimeFn {
                 )
             })
         } {
-            let trap = take_api_trap().unwrap_or_else(|| Trap::from_jit(error));
+            return Err(Trap::from_jit(error));
-            return Err(trap);
         }
 
         // Load the return values out of `values_vec`.

crates/api/src/instance.rs

@@ -1,7 +1,6 @@
 use crate::externals::Extern;
 use crate::module::Module;
 use crate::runtime::Store;
-use crate::trampoline::take_api_trap;
 use crate::trap::Trap;
 use crate::types::{ExportType, ExternType};
 use anyhow::{Error, Result};
@@ -29,12 +28,9 @@ fn instantiate(
         let instance = compiled_module
             .instantiate(&mut resolver)
             .map_err(|e| -> Error {
-                if let Some(trap) = take_api_trap() {
+                match e {
-                    trap.into()
+                    InstantiationError::StartTrap(trap) => Trap::from_jit(trap).into(),
-                } else if let InstantiationError::StartTrap(trap) = e {
+                    other => other.into(),
-                    Trap::from_jit(trap).into()
-                } else {
-                    e.into()
                 }
             })?;
         Ok(instance)

crates/api/src/trampoline/func.rs

@@ -1,11 +1,12 @@
 //! Support for a calling of an imported function.
 
 use super::create_handle::create_handle;
-use super::trap::{record_api_trap, TrapSink, API_TRAP_CODE};
+use super::trap::TrapSink;
-use crate::{Callable, FuncType, Store, Val};
+use crate::{Callable, FuncType, Store, Trap, Val};
 use anyhow::{bail, Result};
 use std::cmp;
 use std::convert::TryFrom;
+use std::panic::{self, AssertUnwindSafe};
 use std::rc::Rc;
 use wasmtime_environ::entity::{EntityRef, PrimaryMap};
 use wasmtime_environ::ir::types;
@@ -69,42 +70,70 @@ unsafe extern "C" fn stub_fn(
     _caller_vmctx: *mut VMContext,
     call_id: u32,
     values_vec: *mut i128,
-) -> u32 {
+) {
-    let instance = InstanceHandle::from_vmctx(vmctx);
+    // Here we are careful to use `catch_unwind` to ensure Rust panics don't
+    // unwind past us. The primary reason for this is that Rust considers it UB
+    // to unwind past an `extern "C"` function. Here we are in an `extern "C"`
+    // function and the cross into wasm was through an `extern "C"` function at
+    // the base of the stack as well. We'll need to wait for assorted RFCs and
+    // language features to enable this to be done in a sound and stable fashion
+    // before avoiding catching the panic here.
+    //
+    // Also note that there are intentionally no local variables on this stack
+    // frame. The reason for that is that some of the "raise" functions we have
+    // below will trigger a longjmp, which won't run local destructors if we
+    // have any. To prevent leaks we avoid having any local destructors by
+    // avoiding local variables.
+    let result = panic::catch_unwind(AssertUnwindSafe(|| call_stub(vmctx, call_id, values_vec)));
 
-    let (args, returns_len) = {
+    match result {
-        let module = instance.module_ref();
+        Ok(Ok(())) => {}
-        let signature = &module.signatures[module.functions[FuncIndex::new(call_id as usize)]];
 
-        let mut args = Vec::new();
+        // If a trap was raised (an error returned from the imported function)
-        for i in 2..signature.params.len() {
+        // then we smuggle the trap through `Box<dyn Error>` through to the
-            args.push(Val::read_value_from(
+        // call-site, which gets unwrapped in `Trap::from_jit` later on as we
-                values_vec.offset(i as isize - 2),
+        // convert from the internal `Trap` type to our own `Trap` type in this
-                signature.params[i].value_type,
+        // crate.
-            ))
+        Ok(Err(trap)) => wasmtime_runtime::raise_user_trap(Box::new(trap)),
-        }
-        (args, signature.returns.len())
-    };
 
-    let mut returns = vec![Val::null(); returns_len];
+        // And finally if the imported function panicked, then we trigger the
-    let func = &instance
+        // form of unwinding that's safe to jump over wasm code on all
-        .host_state()
+        // platforms.
-        .downcast_ref::<TrampolineState>()
+        Err(panic) => wasmtime_runtime::resume_panic(panic),
-        .expect("state")
+    }
-        .func;
 
-    match func.call(&args, &mut returns) {
+    unsafe fn call_stub(
-        Ok(()) => {
+        vmctx: *mut VMContext,
-            for (i, r#return) in returns.iter_mut().enumerate() {
+        call_id: u32,
-                // TODO check signature.returns[i].value_type ?
+        values_vec: *mut i128,
-                r#return.write_value_to(values_vec.add(i));
+    ) -> Result<(), Trap> {
+        let instance = InstanceHandle::from_vmctx(vmctx);
+
+        let (args, returns_len) = {
+            let module = instance.module_ref();
+            let signature = &module.signatures[module.functions[FuncIndex::new(call_id as usize)]];
+
+            let mut args = Vec::new();
+            for i in 2..signature.params.len() {
+                args.push(Val::read_value_from(
+                    values_vec.offset(i as isize - 2),
+                    signature.params[i].value_type,
+                ))
             }
-            0
+            (args, signature.returns.len())
-        }
+        };
-        Err(trap) => {
+
-            record_api_trap(trap);
+        let mut returns = vec![Val::null(); returns_len];
-            1
+        let state = &instance
+            .host_state()
+            .downcast_ref::<TrampolineState>()
+            .expect("state");
+        state.func.call(&args, &mut returns)?;
+        for (i, ret) in returns.iter_mut().enumerate() {
+            // TODO check signature.returns[i].value_type ?
+            ret.write_value_to(values_vec.add(i));
         }
+        Ok(())
     }
 }
 
@@ -136,9 +165,6 @@ fn make_trampoline(
     // Add the `values_vec` parameter.
     stub_sig.params.push(ir::AbiParam::new(pointer_type));
 
-    // Add error/trap return.
-    stub_sig.returns.push(ir::AbiParam::new(types::I32));
-
     // Compute the size of the values vector. The vmctx and caller vmctx are passed separately.
     let value_size = 16;
     let values_vec_len = ((value_size as usize)
@@ -195,13 +221,10 @@ fn make_trampoline(
         let callee_value = builder
             .ins()
             .iconst(pointer_type, stub_fn as *const VMFunctionBody as i64);
-        let call = builder
+        builder
             .ins()
             .call_indirect(new_sig, callee_value, &callee_args);
 
-        let call_result = builder.func.dfg.inst_results(call)[0];
-        builder.ins().trapnz(call_result, API_TRAP_CODE);
-
         let mflags = MemFlags::trusted();
         let mut results = Vec::new();
         for (i, r) in signature.returns.iter().enumerate() {

crates/api/src/trampoline/mod.rs

@@ -16,7 +16,6 @@ use anyhow::Result;
 use std::rc::Rc;
 
 pub use self::global::GlobalState;
-pub use self::trap::take_api_trap;
 
 pub fn generate_func_export(
     ft: &FuncType,

crates/api/src/trampoline/trap.rs

@@ -1,32 +1,7 @@
-use std::cell::Cell;
-
-use crate::Trap;
 use wasmtime_environ::ir::{SourceLoc, TrapCode};
 use wasmtime_environ::TrapInformation;
 use wasmtime_jit::trampoline::binemit;
 
-// Randomly selected user TrapCode magic number 13.
-pub const API_TRAP_CODE: TrapCode = TrapCode::User(13);
-
-thread_local! {
-    static RECORDED_API_TRAP: Cell<Option<Trap>> = Cell::new(None);
-}
-
-pub fn record_api_trap(trap: Trap) {
-    RECORDED_API_TRAP.with(|data| {
-        let trap = Cell::new(Some(trap));
-        data.swap(&trap);
-        assert!(
-            trap.take().is_none(),
-            "Only one API trap per thread can be recorded at a moment!"
-        );
-    });
-}
-
-pub fn take_api_trap() -> Option<Trap> {
-    RECORDED_API_TRAP.with(|data| data.take())
-}
-
 pub(crate) struct TrapSink {
     pub traps: Vec<TrapInformation>,
 }

crates/api/src/trap.rs

@@ -33,7 +33,24 @@ impl Trap {
     }
 
     pub(crate) fn from_jit(jit: wasmtime_runtime::Trap) -> Self {
-        Trap::new_with_trace(jit.to_string(), jit.backtrace)
+        match jit {
+            wasmtime_runtime::Trap::User(error) => {
+                // Since we're the only one using the wasmtime internals (in
+                // theory) we should only see user errors which were originally
+                // created from our own `Trap` type (see the trampoline module
+                // with functions).
+                //
+                // If this unwrap trips for someone we'll need to tweak the
+                // return type of this function to probably be `anyhow::Error`
+                // or something like that.
+                *error
+                    .downcast()
+                    .expect("only `Trap` user errors are supported")
+            }
+            wasmtime_runtime::Trap::Wasm { desc, backtrace } => {
+                Trap::new_with_trace(desc.to_string(), backtrace)
+            }
+        }
     }
 
     fn new_with_trace(message: String, native_trace: Backtrace) -> Self {

crates/api/tests/traps.rs

@@ -1,4 +1,5 @@
 use anyhow::Result;
+use std::panic::{self, AssertUnwindSafe};
 use std::rc::Rc;
 use wasmtime::*;
 
@@ -215,3 +216,95 @@ wasm backtrace:
     );
     Ok(())
 }
+
+#[test]
+fn trap_start_function_import() -> Result<()> {
+    struct ReturnTrap;
+
+    impl Callable for ReturnTrap {
+        fn call(&self, _params: &[Val], _results: &mut [Val]) -> Result<(), Trap> {
+            Err(Trap::new("user trap"))
+        }
+    }
+
+    let store = Store::default();
+    let binary = wat::parse_str(
+        r#"
+            (module $a
+                (import "" "" (func $foo))
+                (start $foo)
+            )
+        "#,
+    )?;
+
+    let module = Module::new(&store, &binary)?;
+    let sig = FuncType::new(Box::new([]), Box::new([]));
+    let func = Func::new(&store, sig, Rc::new(ReturnTrap));
+    let err = Instance::new(&module, &[func.into()]).err().unwrap();
+    assert_eq!(err.downcast_ref::<Trap>().unwrap().message(), "user trap");
+    Ok(())
+}
+
+#[test]
+fn rust_panic_import() -> Result<()> {
+    struct Panic;
+
+    impl Callable for Panic {
+        fn call(&self, _params: &[Val], _results: &mut [Val]) -> Result<(), Trap> {
+            panic!("this is a panic");
+        }
+    }
+
+    let store = Store::default();
+    let binary = wat::parse_str(
+        r#"
+            (module $a
+                (import "" "" (func $foo))
+                (func (export "foo") call $foo)
+            )
+        "#,
+    )?;
+
+    let module = Module::new(&store, &binary)?;
+    let sig = FuncType::new(Box::new([]), Box::new([]));
+    let func = Func::new(&store, sig, Rc::new(Panic));
+    let instance = Instance::new(&module, &[func.into()])?;
+    let func = instance.exports()[0].func().unwrap().clone();
+    let err = panic::catch_unwind(AssertUnwindSafe(|| {
+        drop(func.call(&[]));
+    }))
+    .unwrap_err();
+    assert_eq!(err.downcast_ref::<&'static str>(), Some(&"this is a panic"));
+    Ok(())
+}
+
+#[test]
+fn rust_panic_start_function() -> Result<()> {
+    struct Panic;
+
+    impl Callable for Panic {
+        fn call(&self, _params: &[Val], _results: &mut [Val]) -> Result<(), Trap> {
+            panic!("this is a panic");
+        }
+    }
+
+    let store = Store::default();
+    let binary = wat::parse_str(
+        r#"
+            (module $a
+                (import "" "" (func $foo))
+                (start $foo)
+            )
+        "#,
+    )?;
+
+    let module = Module::new(&store, &binary)?;
+    let sig = FuncType::new(Box::new([]), Box::new([]));
+    let func = Func::new(&store, sig, Rc::new(Panic));
+    let err = panic::catch_unwind(AssertUnwindSafe(|| {
+        drop(Instance::new(&module, &[func.into()]));
+    }))
+    .unwrap_err();
+    assert_eq!(err.downcast_ref::<&'static str>(), Some(&"this is a panic"));
+    Ok(())
+}

crates/c-api/src/lib.rs

@@ -6,6 +6,7 @@
 // TODO complete the C API
 
 use std::cell::RefCell;
+use std::panic::{self, AssertUnwindSafe};
 use std::rc::Rc;
 use std::{mem, ptr, slice};
 use wasmtime::{
@@ -488,15 +489,34 @@ pub unsafe extern "C" fn wasm_func_call(
         let val = &(*args.add(i));
         params.push(val.val());
     }
-    match func.call(&params) {
+
-        Ok(out) => {
+    // We're calling arbitrary code here most of the time, and we in general
+    // want to try to insulate callers against bugs in wasmtime/wasi/etc if we
+    // can. As a result we catch panics here and transform them to traps to
+    // allow the caller to have any insulation possible against Rust panics.
+    let result = panic::catch_unwind(AssertUnwindSafe(|| func.call(&params)));
+    match result {
+        Ok(Ok(out)) => {
             for i in 0..func.result_arity() {
                 let val = &mut (*results.add(i));
                 *val = wasm_val_t::from_val(&out[i]);
             }
             ptr::null_mut()
         }
-        Err(trap) => {
+        Ok(Err(trap)) => {
+            let trap = Box::new(wasm_trap_t {
+                trap: HostRef::new(trap),
+            });
+            Box::into_raw(trap)
+        }
+        Err(panic) => {
+            let trap = if let Some(msg) = panic.downcast_ref::<String>() {
+                Trap::new(msg)
+            } else if let Some(msg) = panic.downcast_ref::<&'static str>() {
+                Trap::new(*msg)
+            } else {
+                Trap::new("rust panic happened")
+            };
             let trap = Box::new(wasm_trap_t {
                 trap: HostRef::new(trap),
             });

crates/runtime/src/lib.rs

@@ -44,7 +44,8 @@ pub use crate::jit_int::GdbJitImageRegistration;
 pub use crate::mmap::Mmap;
 pub use crate::sig_registry::SignatureRegistry;
 pub use crate::trap_registry::{get_mut_trap_registry, get_trap_registry, TrapRegistrationGuard};
-pub use crate::traphandlers::{wasmtime_call, wasmtime_call_trampoline, Trap};
+pub use crate::traphandlers::resume_panic;
+pub use crate::traphandlers::{raise_user_trap, wasmtime_call, wasmtime_call_trampoline, Trap};
 pub use crate::vmcontext::{
     VMCallerCheckedAnyfunc, VMContext, VMFunctionBody, VMFunctionImport, VMGlobalDefinition,
     VMGlobalImport, VMInvokeArgument, VMMemoryDefinition, VMMemoryImport, VMSharedSignatureIndex,

crates/runtime/src/trap_registry.rs

@@ -1,5 +1,6 @@
 use lazy_static::lazy_static;
 use std::collections::HashMap;
+use std::fmt;
 use std::sync::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 use wasmtime_environ::ir;
 
@@ -22,6 +23,35 @@ pub struct TrapDescription {
     pub trap_code: ir::TrapCode,
 }
 
+impl fmt::Display for TrapDescription {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "wasm trap: {}, source location: {}",
+            trap_code_to_expected_string(self.trap_code),
+            self.source_loc
+        )
+    }
+}
+
+fn trap_code_to_expected_string(trap_code: ir::TrapCode) -> String {
+    use ir::TrapCode::*;
+    match trap_code {
+        StackOverflow => "call stack exhausted".to_string(),
+        HeapOutOfBounds => "out of bounds memory access".to_string(),
+        TableOutOfBounds => "undefined element".to_string(),
+        OutOfBounds => "out of bounds".to_string(), // Note: not covered by the test suite
+        IndirectCallToNull => "uninitialized element".to_string(),
+        BadSignature => "indirect call type mismatch".to_string(),
+        IntegerOverflow => "integer overflow".to_string(),
+        IntegerDivisionByZero => "integer divide by zero".to_string(),
+        BadConversionToInteger => "invalid conversion to integer".to_string(),
+        UnreachableCodeReached => "unreachable".to_string(),
+        Interrupt => "interrupt".to_string(), // Note: not covered by the test suite
+        User(x) => format!("user trap {}", x), // Note: not covered by the test suite
+    }
+}
+
 /// RAII guard for deregistering traps
 pub struct TrapRegistrationGuard(usize);
 

crates/runtime/src/traphandlers.rs

@@ -5,7 +5,9 @@ use crate::trap_registry::get_trap_registry;
 use crate::trap_registry::TrapDescription;
 use crate::vmcontext::{VMContext, VMFunctionBody};
 use backtrace::Backtrace;
+use std::any::Any;
 use std::cell::Cell;
+use std::error::Error;
 use std::fmt;
 use std::ptr;
 use wasmtime_environ::ir;
@@ -24,6 +26,7 @@ extern "C" {
         caller_vmctx: *mut u8,
         callee: *const VMFunctionBody,
     ) -> i32;
+    fn Unwind(jmp_buf: *const u8) -> !;
 }
 
 /// Record the Trap code and wasm bytecode offset in TLS somewhere
@@ -44,7 +47,7 @@ pub extern "C" fn RecordTrap(pc: *const u8, reset_guard_page: bool) -> *const u8
         }
 
         let registry = get_trap_registry();
-        let trap = Trap {
+        let trap = Trap::Wasm {
             desc: registry
                 .get_trap(pc as usize)
                 .unwrap_or_else(|| TrapDescription {
@@ -58,16 +61,38 @@ pub extern "C" fn RecordTrap(pc: *const u8, reset_guard_page: bool) -> *const u8
             info.reset_guard_page.set(true);
         }
 
-        let prev = info.trap.replace(Some(trap));
+        info.unwind.replace(UnwindReason::Trap(trap));
-        assert!(
-            prev.is_none(),
-            "Only one trap per thread can be recorded at a moment!"
-        );
-
         info.jmp_buf.get()
     })
 }
 
+/// Raises a user-defined trap immediately.
+///
+/// This function performs as-if a wasm trap was just executed, only the trap
+/// has a dynamic payload associated with it which is user-provided. This trap
+/// payload is then returned from `wasmtime_call` an `wasmtime_call_trampoline`
+/// below.
+///
+/// # Safety
+///
+/// Only safe to call when wasm code is on the stack, aka `wasmtime_call` or
+/// `wasmtime_call_trampoline` must have been previously called.
+pub unsafe fn raise_user_trap(data: Box<dyn Error + Send + Sync>) -> ! {
+    let trap = Trap::User(data);
+    tls::with(|info| info.unwind_with(UnwindReason::Trap(trap)))
+}
+
+/// Carries a Rust panic across wasm code and resumes the panic on the other
+/// side.
+///
+/// # Safety
+///
+/// Only safe to call when wasm code is on the stack, aka `wasmtime_call` or
+/// `wasmtime_call_trampoline` must have been previously called.
+pub unsafe fn resume_panic(payload: Box<dyn Any + Send>) -> ! {
+    tls::with(|info| info.unwind_with(UnwindReason::Panic(payload)))
+}
+
 #[cfg(target_os = "windows")]
 fn reset_guard_page() {
     extern "C" {
@@ -86,45 +111,30 @@ fn reset_guard_page() {}
 
 /// Stores trace message with backtrace.
 #[derive(Debug)]
-pub struct Trap {
+pub enum Trap {
-    /// What sort of trap happened, as well as where in the original wasm module
+    /// A user-raised trap through `raise_user_trap`.
-    /// it happened.
+    User(Box<dyn Error + Send + Sync>),
-    pub desc: TrapDescription,
+    /// A wasm-originating trap from wasm code itself.
-    /// Native stack backtrace at the time the trap occurred
+    Wasm {
-    pub backtrace: Backtrace,
+        /// What sort of trap happened, as well as where in the original wasm module
+        /// it happened.
+        desc: TrapDescription,
+        /// Native stack backtrace at the time the trap occurred
+        backtrace: Backtrace,
+    },
 }
 
 impl fmt::Display for Trap {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(
+        match self {
-            f,
+            Trap::User(user) => user.fmt(f),
-            "wasm trap: {}, source location: {}",
+            Trap::Wasm { desc, .. } => desc.fmt(f),
-            trap_code_to_expected_string(self.desc.trap_code),
+        }
-            self.desc.source_loc
-        )
     }
 }
 
 impl std::error::Error for Trap {}
 
-fn trap_code_to_expected_string(trap_code: ir::TrapCode) -> String {
-    use ir::TrapCode::*;
-    match trap_code {
-        StackOverflow => "call stack exhausted".to_string(),
-        HeapOutOfBounds => "out of bounds memory access".to_string(),
-        TableOutOfBounds => "undefined element".to_string(),
-        OutOfBounds => "out of bounds".to_string(), // Note: not covered by the test suite
-        IndirectCallToNull => "uninitialized element".to_string(),
-        BadSignature => "indirect call type mismatch".to_string(),
-        IntegerOverflow => "integer overflow".to_string(),
-        IntegerDivisionByZero => "integer divide by zero".to_string(),
-        BadConversionToInteger => "invalid conversion to integer".to_string(),
-        UnreachableCodeReached => "unreachable".to_string(),
-        Interrupt => "interrupt".to_string(), // Note: not covered by the test suite
-        User(x) => format!("user trap {}", x), // Note: not covered by the test suite
-    }
-}
-
 /// Call the wasm function pointed to by `callee`. `values_vec` points to
 /// a buffer which holds the incoming arguments, and to which the outgoing
 /// return values will be written.
@@ -145,12 +155,7 @@ pub unsafe extern "C" fn wasmtime_call_trampoline(
             values_vec,
         )
     });
-
+    cx.into_result(ret)
-    if ret == 0 {
-        Err(cx.unwrap_trap())
-    } else {
-        Ok(())
-    }
 }
 
 /// Call the wasm function pointed to by `callee`, which has no arguments or
@@ -170,34 +175,54 @@ pub unsafe extern "C" fn wasmtime_call(
             callee,
         )
     });
-    if ret == 0 {
+    cx.into_result(ret)
-        Err(cx.unwrap_trap())
-    } else {
-        Ok(())
-    }
 }
 
 /// Temporary state stored on the stack which is registered in the `tls` module
 /// below for calls into wasm.
 pub struct CallThreadState {
-    trap: Cell<Option<Trap>>,
+    unwind: Cell<UnwindReason>,
     jmp_buf: Cell<*const u8>,
     reset_guard_page: Cell<bool>,
 }
 
+enum UnwindReason {
+    None,
+    Panic(Box<dyn Any + Send>),
+    Trap(Trap),
+}
+
 impl CallThreadState {
     fn new() -> CallThreadState {
         CallThreadState {
-            trap: Cell::new(None),
+            unwind: Cell::new(UnwindReason::None),
             jmp_buf: Cell::new(ptr::null()),
             reset_guard_page: Cell::new(false),
         }
     }
 
-    fn unwrap_trap(self) -> Trap {
+    fn into_result(self, ret: i32) -> Result<(), Trap> {
-        self.trap
+        match self.unwind.replace(UnwindReason::None) {
-            .replace(None)
+            UnwindReason::None => {
-            .expect("unwrap_trap must be called after trap occurred")
+                debug_assert_eq!(ret, 1);
+                Ok(())
+            }
+            UnwindReason::Trap(trap) => {
+                debug_assert_eq!(ret, 0);
+                Err(trap)
+            }
+            UnwindReason::Panic(panic) => {
+                debug_assert_eq!(ret, 0);
+                std::panic::resume_unwind(panic)
+            }
+        }
+    }
+
+    fn unwind_with(&self, reason: UnwindReason) -> ! {
+        self.unwind.replace(reason);
+        unsafe {
+            Unwind(self.jmp_buf.get());
+        }
     }
 }