Fix and clean up SEH unwinding

Author: Amanieu

src/libpanic_unwind/seh.rs

@@ -50,9 +50,16 @@
 use alloc::boxed::Box;
 use core::any::Any;
 use core::mem;
-use core::raw;
 use libc::{c_int, c_uint, c_void};
 
+struct Exception {
+    // This needs to be an Option because we catch the exception by reference
+    // and its destructor is executed by the C++ runtime. When we take the Box
+    // out of the exception, we need to leave the exception in a valid state
+    // for its destructor to run without double-dropping the Box.
+    data: Option<Box<dyn Any + Send>>,
+}
+
 // First up, a whole bunch of type definitions. There's a few platform-specific
 // oddities here, and a lot that's just blatantly copied from LLVM. The purpose
 // of all this is to implement the `panic` function below through a call to
@@ -186,7 +193,7 @@ static mut CATCHABLE_TYPE: _CatchableType = _CatchableType {
     properties: 0,
     pType: ptr!(0),
     thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0 },
-    sizeOrOffset: mem::size_of::<[u64; 2]>() as c_int,
+    sizeOrOffset: mem::size_of::<Exception>() as c_int,
     copyFunction: ptr!(0),
 };
 
@@ -229,16 +236,16 @@ static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor {
 // because Box<dyn Any> isn't clonable.
 macro_rules! define_cleanup {
     ($abi:tt) => {
-        unsafe extern $abi fn exception_cleanup(e: *mut [u64; 2]) {
-            if (*e)[0] != 0 {
-                cleanup(*e);
+        unsafe extern $abi fn exception_cleanup(e: *mut Exception) {
+            if let Some(b) = e.read().data {
+                drop(b);
                 super::__rust_drop_panic();
             }
         }
         #[unwind(allowed)]
-        unsafe extern $abi fn exception_copy(_dest: *mut [u64; 2],
-                                             _src: *mut [u64; 2])
-                                             -> *mut [u64; 2] {
+        unsafe extern $abi fn exception_copy(_dest: *mut Exception,
+                                             _src: *mut Exception)
+                                             -> *mut Exception {
             panic!("Rust panics cannot be copied");
         }
     }
@@ -257,13 +264,8 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     // _CxxThrowException executes entirely on this stack frame, so there's no
     // need to otherwise transfer `data` to the heap. We just pass a stack
     // pointer to this function.
-    //
-    // The first argument is the payload being thrown (our two pointers), and
-    // the second argument is the type information object describing the
-    // exception (constructed above).
-    let ptrs = mem::transmute::<_, raw::TraitObject>(data);
-    let mut ptrs = [ptrs.data as u64, ptrs.vtable as u64];
-    let throw_ptr = ptrs.as_mut_ptr() as *mut _;
+    let mut exception = Exception { data: Some(data) };
+    let throw_ptr = &mut exception as *mut _ as *mut _;
 
     // This... may seems surprising, and justifiably so. On 32-bit MSVC the
     // pointers between these structure are just that, pointers. On 64-bit MSVC,
@@ -312,14 +314,8 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
 }
 
 pub unsafe fn cleanup(payload: *mut u8) -> Box<dyn Any + Send> {
-    let payload = *&mut (payload as *mut [u64; 2]);
-
-    // Clear the first word of the exception so avoid double-dropping it.
-    // This will be read by the destructor which is implicitly called at the
-    // end of the catch block by the runtime.
-    payload[0] = 0;
-
-    mem::transmute(raw::TraitObject { data: payload[0] as *mut _, vtable: payload[1] as *mut _ })
+    let exception = &mut *(payload as *mut Exception);
+    exception.data.take().unwrap()
 }
 
 // This is required by the compiler to exist (e.g., it's a lang item), but

src/libstd/panicking.rs

@@ -297,9 +297,12 @@ pub unsafe fn r#try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<dyn Any + Send>>
             let payload_ptr = payload.as_mut_ptr() as *mut u8;
             let r = intrinsics::r#try(try_fn, data, payload_ptr);
             if r != 0 {
-                if cfg!(target_env = "msvc") {
+                #[cfg(target_env = "msvc")]
+                {
                     catch_fn(data, payload_ptr)
-                } else {
+                }
+                #[cfg(not(target_env = "msvc"))]
+                {
                     catch_fn(data, payload.assume_init())
                 }
             }