Make Emscripten unwinding use a valid type_info

This allows catch_panic to ignore C++ exceptions.

Author: Amanieu

src/doc/unstable-book/src/language-features/lang-items.md

@@ -252,7 +252,8 @@ the source code.
   - `eh_personality`: `libpanic_unwind/gcc.rs` (GNU)
   - `eh_personality`: `libpanic_unwind/seh.rs` (SEH)
   - `eh_unwind_resume`: `libpanic_unwind/gcc.rs` (GCC)
-  - `msvc_try_filter`: `libpanic_unwind/seh.rs` (SEH)
+  - `eh_catch_typeinfo`: `libpanic_unwind/seh.rs` (SEH)
+  - `eh_catch_typeinfo`: `libpanic_unwind/emcc.rs` (EMCC)
   - `panic`: `libcore/panicking.rs`
   - `panic_bounds_check`: `libcore/panicking.rs`
   - `panic_impl`: `libcore/panicking.rs`

src/libpanic_unwind/emcc.rs

@@ -15,14 +15,48 @@ use alloc::boxed::Box;
 use libc::{self, c_int};
 use unwind as uw;
 
+// This matches the layout of std::type_info in C++
+#[repr(C)]
+struct TypeInfo {
+    vtable: *const usize,
+    name: *const u8,
+}
+unsafe impl Sync for TypeInfo {}
+
+extern "C" {
+    // The leading `\x01` byte here is actually a magical signal to LLVM to
+    // *not* apply any other mangling like prefixing with a `_` character.
+    //
+    // This symbol is the vtable used by C++'s `std::type_info`. Objects of type
+    // `std::type_info`, type descriptors, have a pointer to this table. Type
+    // descriptors are referenced by the C++ EH structures defined above and
+    // that we construct below.
+    //
+    // Note that the real size is larger than 3 usize, but we only need our
+    // vtable to point to the third element.
+    #[link_name = "\x01_ZTVN10__cxxabiv117__class_type_infoE"]
+    static CLASS_TYPE_INFO_VTABLE: [usize; 3];
+}
+
+// std::type_info for a rust_panic class
+#[lang = "eh_catch_typeinfo"]
+static EXCEPTION_TYPE_INFO: TypeInfo = TypeInfo {
+    // Normally we would use .as_ptr().add(2) but this doesn't work in a const context.
+    vtable: unsafe { &CLASS_TYPE_INFO_VTABLE[2] },
+    // This intentionally doesn't use the normal name mangling scheme because
+    // we don't want C++ to be able to produce or catch Rust panics.
+    name: b"rust_panic\0".as_ptr(),
+};
+
 pub fn payload() -> *mut u8 {
     ptr::null_mut()
 }
 
 pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {
     assert!(!ptr.is_null());
-    let ex = ptr::read(ptr as *mut _);
-    __cxa_free_exception(ptr as *mut _);
+    let adjusted_ptr = __cxa_begin_catch(ptr as *mut libc::c_void);
+    let ex = ptr::read(adjusted_ptr as *mut _);
+    __cxa_end_catch();
     ex
 }
 
@@ -32,11 +66,8 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     if exception == ptr::null_mut() {
         return uw::_URC_FATAL_PHASE1_ERROR as u32;
     }
-    let exception = exception as *mut Box<dyn Any + Send>;
-    ptr::write(exception, data);
-    __cxa_throw(exception as *mut _, ptr::null_mut(), ptr::null_mut());
-
-    unreachable!()
+    ptr::write(exception as *mut _, data);
+    __cxa_throw(exception as *mut _, &EXCEPTION_TYPE_INFO, ptr::null_mut());
 }
 
 #[lang = "eh_personality"]
@@ -52,10 +83,11 @@ unsafe extern "C" fn rust_eh_personality(version: c_int,
 
 extern "C" {
     fn __cxa_allocate_exception(thrown_size: libc::size_t) -> *mut libc::c_void;
-    fn __cxa_free_exception(thrown_exception: *mut libc::c_void);
+    fn __cxa_begin_catch(thrown_exception: *mut libc::c_void) -> *mut libc::c_void;
+    fn __cxa_end_catch();
     fn __cxa_throw(thrown_exception: *mut libc::c_void,
-                   tinfo: *mut libc::c_void,
-                   dest: *mut libc::c_void);
+                   tinfo: *const TypeInfo,
+                   dest: *mut libc::c_void) -> !;
     fn __gxx_personality_v0(version: c_int,
                             actions: uw::_Unwind_Action,
                             exception_class: uw::_Unwind_Exception_Class,

src/libpanic_unwind/seh.rs

@@ -213,12 +213,13 @@ extern "C" {
     static TYPE_INFO_VTABLE: *const u8;
 }
 
-// We use #[lang = "msvc_try_filter"] here as this is the type descriptor which
+// We use #[lang = "eh_catch_typeinfo"] here as this is the type descriptor which
 // we'll use in LLVM's `catchpad` instruction which ends up also being passed as
 // an argument to the C++ personality function.
 //
 // Again, I'm not entirely sure what this is describing, it just seems to work.
-#[cfg_attr(not(test), lang = "msvc_try_filter")]
+#[cfg_attr(bootstrap(msvc_try_filter))]
+#[cfg_attr(not(any(test, bootstrap)), lang = "eh_catch_typeinfo")]
 static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor {
     pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ as *const _,
     spare: core::ptr::null_mut(),

src/librustc/middle/lang_items.rs

@@ -385,7 +385,7 @@ language_item_table! {
 
     EhPersonalityLangItem,       "eh_personality",     eh_personality,          Target::Fn;
     EhUnwindResumeLangItem,      "eh_unwind_resume",   eh_unwind_resume,        Target::Fn;
-    MSVCTryFilterLangItem,       "msvc_try_filter",    msvc_try_filter,         Target::Static;
+    EhCatchTypeinfoLangItem,     "eh_catch_typeinfo",  eh_catch_typeinfo,       Target::Static;
 
     OwnedBoxLangItem,            "owned_box",          owned_box,               Target::Struct;
 

src/librustc_codegen_llvm/intrinsic.rs

@@ -881,9 +881,9 @@ fn codegen_msvc_try(
         let cs = catchswitch.catch_switch(None, None, 1);
         catchswitch.add_handler(cs, catchpad.llbb());
 
-        let tydesc = match bx.tcx().lang_items().msvc_try_filter() {
+        let tydesc = match bx.tcx().lang_items().eh_catch_typeinfo() {
             Some(did) => bx.get_static(did),
-            None => bug!("msvc_try_filter not defined"),
+            None => bug!("eh_catch_typeinfo not defined, but needed for SEH unwinding"),
         };
         let funclet = catchpad.catch_pad(cs, &[tydesc, bx.const_i32(0), slot]);
 
@@ -957,7 +957,14 @@ fn codegen_gnu_try(
         // rust_try ignores the selector.
         let lpad_ty = bx.type_struct(&[bx.type_i8p(), bx.type_i32()], false);
         let vals = catch.landing_pad(lpad_ty, bx.eh_personality(), 1);
-        catch.add_clause(vals, bx.const_null(bx.type_i8p()));
+        let tydesc = match bx.tcx().lang_items().eh_catch_typeinfo() {
+            Some(tydesc) => {
+                let tydesc = bx.get_static(tydesc);
+                bx.bitcast(tydesc, bx.type_i8p())
+            }
+            None => bx.const_null(bx.type_i8p()),
+        };
+        catch.add_clause(vals, tydesc);
         let ptr = catch.extract_value(vals, 0);
         let ptr_align = bx.tcx().data_layout.pointer_align.abi;
         let bitcast = catch.bitcast(local_ptr, bx.type_ptr_to(bx.type_i8p()));