3 //! On Windows (currently only on MSVC), the default exception handling
4 //! mechanism is Structured Exception Handling (SEH). This is quite different
5 //! than Dwarf-based exception handling (e.g., what other unix platforms use) in
6 //! terms of compiler internals, so LLVM is required to have a good deal of
7 //! extra support for SEH.
9 //! In a nutshell, what happens here is:
11 //! 1. The `panic` function calls the standard Windows function
12 //! `_CxxThrowException` to throw a C++-like exception, triggering the
13 //! unwinding process.
14 //! 2. All landing pads generated by the compiler use the personality function
15 //! `__CxxFrameHandler3`, a function in the CRT, and the unwinding code in
16 //! Windows will use this personality function to execute all cleanup code on
18 //! 3. All compiler-generated calls to `invoke` have a landing pad set as a
19 //! `cleanuppad` LLVM instruction, which indicates the start of the cleanup
20 //! routine. The personality (in step 2, defined in the CRT) is responsible
21 //! for running the cleanup routines.
22 //! 4. Eventually the "catch" code in the `try` intrinsic (generated by the
23 //! compiler) is executed and indicates that control should come back to
24 //! Rust. This is done via a `catchswitch` plus a `catchpad` instruction in
25 //! LLVM IR terms, finally returning normal control to the program with a
26 //! `catchret` instruction.
28 //! Some specific differences from the gcc-based exception handling are:
30 //! * Rust has no custom personality function, it is instead *always*
31 //! `__CxxFrameHandler3`. Additionally, no extra filtering is performed, so we
32 //! end up catching any C++ exceptions that happen to look like the kind we're
33 //! throwing. Note that throwing an exception into Rust is undefined behavior
34 //! anyway, so this should be fine.
35 //! * We've got some data to transmit across the unwinding boundary,
36 //! specifically a `Box<dyn Any + Send>`. Like with Dwarf exceptions
37 //! these two pointers are stored as a payload in the exception itself. On
38 //! MSVC, however, there's no need for an extra heap allocation because the
39 //! call stack is preserved while filter functions are being executed. This
40 //! means that the pointers are passed directly to `_CxxThrowException` which
41 //! are then recovered in the filter function to be written to the stack frame
42 //! of the `try` intrinsic.
44 //! [win64]: https://docs.microsoft.com/en-us/cpp/build/exception-handling-x64
45 //! [llvm]: http://llvm.org/docs/ExceptionHandling.html#background-on-windows-exceptions
47 #![allow(nonstandard_style)]
48 #![allow(private_no_mangle_fns)]
50 use alloc::boxed::Box;
54 use libc::{c_int, c_uint, c_void};
56 // First up, a whole bunch of type definitions. There's a few platform-specific
57 // oddities here, and a lot that's just blatantly copied from LLVM. The purpose
58 // of all this is to implement the `panic` function below through a call to
59 // `_CxxThrowException`.
61 // This function takes two arguments. The first is a pointer to the data we're
62 // passing in, which in this case is our trait object. Pretty easy to find! The
63 // next, however, is more complicated. This is a pointer to a `_ThrowInfo`
64 // structure, and it generally is just intended to just describe the exception
67 // Currently the definition of this type [1] is a little hairy, and the main
68 // oddity (and difference from the online article) is that on 32-bit the
69 // pointers are pointers but on 64-bit the pointers are expressed as 32-bit
70 // offsets from the `__ImageBase` symbol. The `ptr_t` and `ptr!` macro in the
71 // modules below are used to express this.
73 // The maze of type definitions also closely follows what LLVM emits for this
74 // sort of operation. For example, if you compile this C++ code on MSVC and emit
77 // #include <stdint.h>
79 // struct rust_panic {
80 // rust_panic(const rust_panic&);
87 // rust_panic a = {0, 1};
91 // That's essentially what we're trying to emulate. Most of the constant values
92 // below were just copied from LLVM,
94 // In any case, these structures are all constructed in a similar manner, and
95 // it's just somewhat verbose for us.
97 // [1]: http://www.geoffchappell.com/studies/msvc/language/predefined/
99 #[cfg(target_arch = "x86")]
102 pub type ptr_t = *mut u8;
106 core::ptr::null_mut()
114 #[cfg(any(target_arch = "x86_64", target_arch = "arm"))]
117 pub type ptr_t = u32;
120 pub static __ImageBase: u8;
126 (($e as usize) - (&imp::__ImageBase as *const _ as usize)) as u32
132 pub struct _ThrowInfo {
133 pub attributes: c_uint,
134 pub pmfnUnwind: imp::ptr_t,
135 pub pForwardCompat: imp::ptr_t,
136 pub pCatchableTypeArray: imp::ptr_t,
140 pub struct _CatchableTypeArray {
141 pub nCatchableTypes: c_int,
142 pub arrayOfCatchableTypes: [imp::ptr_t; 1],
146 pub struct _CatchableType {
147 pub properties: c_uint,
148 pub pType: imp::ptr_t,
149 pub thisDisplacement: _PMD,
150 pub sizeOrOffset: c_int,
151 pub copyFunction: imp::ptr_t,
162 pub struct _TypeDescriptor {
163 pub pVFTable: *const u8,
168 // Note that we intentionally ignore name mangling rules here: we don't want C++
169 // to be able to catch Rust panics by simply declaring a `struct rust_panic`.
170 const TYPE_NAME: [u8; 11] = *b"rust_panic\0";
172 static mut THROW_INFO: _ThrowInfo = _ThrowInfo {
175 pForwardCompat: ptr!(0),
176 pCatchableTypeArray: ptr!(0),
179 static mut CATCHABLE_TYPE_ARRAY: _CatchableTypeArray =
180 _CatchableTypeArray { nCatchableTypes: 1, arrayOfCatchableTypes: [ptr!(0)] };
182 static mut CATCHABLE_TYPE: _CatchableType = _CatchableType {
185 thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0 },
186 sizeOrOffset: mem::size_of::<[u64; 2]>() as c_int,
187 copyFunction: ptr!(0),
191 // The leading `\x01` byte here is actually a magical signal to LLVM to
192 // *not* apply any other mangling like prefixing with a `_` character.
194 // This symbol is the vtable used by C++'s `std::type_info`. Objects of type
195 // `std::type_info`, type descriptors, have a pointer to this table. Type
196 // descriptors are referenced by the C++ EH structures defined above and
197 // that we construct below.
198 #[link_name = "\x01??_7type_info@@6B@"]
199 static TYPE_INFO_VTABLE: *const u8;
202 // We use #[lang = "eh_catch_typeinfo"] here as this is the type descriptor which
203 // we'll use in LLVM's `catchpad` instruction which ends up also being passed as
204 // an argument to the C++ personality function.
206 // Again, I'm not entirely sure what this is describing, it just seems to work.
207 #[cfg_attr(not(test), lang = "eh_catch_typeinfo")]
208 static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor {
209 pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ as *const _,
210 spare: core::ptr::null_mut(),
214 // Destructor used if the C++ code decides to capture the exception and drop it
215 // without propagating it. The catch part of the try intrinsic will set the
216 // first word of the exception object to 0 so that it is skipped by the
219 // Note that x86 Windows uses the "thiscall" calling convention for C++ member
220 // functions instead of the default "C" calling convention.
222 // The exception_copy function is a bit special here: it is invoked by the MSVC
223 // runtime under a try/catch block and the panic that we generate here will be
224 // used as the result of the exception copy. This is used by the C++ runtime to
225 // support capturing exceptions with std::exception_ptr, which we can't support
226 // because Box<dyn Any> isn't clonable.
227 macro_rules! define_cleanup {
229 unsafe extern $abi fn exception_cleanup(e: *mut [u64; 2]) {
232 super::__rust_drop_panic();
236 unsafe extern $abi fn exception_copy(_dest: *mut [u64; 2],
239 panic!("Rust panics cannot be copied");
244 if #[cfg(target_arch = "x86")] {
245 define_cleanup!("thiscall");
247 define_cleanup!("C");
251 pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
252 use core::intrinsics::atomic_store;
254 // _CxxThrowException executes entirely on this stack frame, so there's no
255 // need to otherwise transfer `data` to the heap. We just pass a stack
256 // pointer to this function.
258 // The first argument is the payload being thrown (our two pointers), and
259 // the second argument is the type information object describing the
260 // exception (constructed above).
261 let ptrs = mem::transmute::<_, raw::TraitObject>(data);
262 let mut ptrs = [ptrs.data as u64, ptrs.vtable as u64];
263 let throw_ptr = ptrs.as_mut_ptr() as *mut _;
265 // This... may seems surprising, and justifiably so. On 32-bit MSVC the
266 // pointers between these structure are just that, pointers. On 64-bit MSVC,
267 // however, the pointers between structures are rather expressed as 32-bit
268 // offsets from `__ImageBase`.
270 // Consequently, on 32-bit MSVC we can declare all these pointers in the
271 // `static`s above. On 64-bit MSVC, we would have to express subtraction of
272 // pointers in statics, which Rust does not currently allow, so we can't
275 // The next best thing, then is to fill in these structures at runtime
276 // (panicking is already the "slow path" anyway). So here we reinterpret all
277 // of these pointer fields as 32-bit integers and then store the
278 // relevant value into it (atomically, as concurrent panics may be
279 // happening). Technically the runtime will probably do a nonatomic read of
280 // these fields, but in theory they never read the *wrong* value so it
281 // shouldn't be too bad...
283 // In any case, we basically need to do something like this until we can
284 // express more operations in statics (and we may never be able to).
285 atomic_store(&mut THROW_INFO.pmfnUnwind as *mut _ as *mut u32, ptr!(exception_cleanup) as u32);
287 &mut THROW_INFO.pCatchableTypeArray as *mut _ as *mut u32,
288 ptr!(&CATCHABLE_TYPE_ARRAY as *const _) as u32,
291 &mut CATCHABLE_TYPE_ARRAY.arrayOfCatchableTypes[0] as *mut _ as *mut u32,
292 ptr!(&CATCHABLE_TYPE as *const _) as u32,
295 &mut CATCHABLE_TYPE.pType as *mut _ as *mut u32,
296 ptr!(&TYPE_DESCRIPTOR as *const _) as u32,
299 &mut CATCHABLE_TYPE.copyFunction as *mut _ as *mut u32,
300 ptr!(exception_copy) as u32,
305 pub fn _CxxThrowException(pExceptionObject: *mut c_void, pThrowInfo: *mut u8) -> !;
308 _CxxThrowException(throw_ptr, &mut THROW_INFO as *mut _ as *mut _);
311 pub fn payload() -> [u64; 2] {
315 pub unsafe fn cleanup(payload: [u64; 2]) -> Box<dyn Any + Send> {
316 mem::transmute(raw::TraitObject { data: payload[0] as *mut _, vtable: payload[1] as *mut _ })
319 // This is required by the compiler to exist (e.g., it's a lang item), but
320 // it's never actually called by the compiler because __C_specific_handler
321 // or _except_handler3 is the personality function that is always used.
322 // Hence this is just an aborting stub.
323 #[lang = "eh_personality"]
325 fn rust_eh_personality() {
326 unsafe { core::intrinsics::abort() }