// Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! Windows SEH //! //! On Windows (currently only on MSVC), the default exception handling //! mechanism is Structured Exception Handling (SEH). This is quite different //! than Dwarf-based exception handling (e.g. what other unix platforms use) in //! terms of compiler internals, so LLVM is required to have a good deal of //! extra support for SEH. //! //! In a nutshell, what happens here is: //! //! 1. The `panic` function calls the standard Windows function //! `_CxxThrowException` to throw a C++-like exception, triggering the //! unwinding process. //! 2. All landing pads generated by the compiler use the personality function //! `__CxxFrameHandler3`, a function in the CRT, and the unwinding code in //! Windows will use this personality function to execute all cleanup code on //! the stack. //! 3. All compiler-generated calls to `invoke` have a landing pad set as a //! `cleanuppad` LLVM instruction, which indicates the start of the cleanup //! routine. The personality (in step 2, defined in the CRT) is responsible //! for running the cleanup routines. //! 4. Eventually the "catch" code in the `try` intrinsic (generated by the //! compiler) is executed and indicates that control should come back to //! Rust. This is done via a `catchswitch` plus a `catchpad` instruction in //! LLVM IR terms, finally returning normal control to the program with a //! `catchret` instruction. //! //! Some specific differences from the gcc-based exception handling are: //! //! * Rust has no custom personality function, it is instead *always* //! `__CxxFrameHandler3`. Additionally, no extra filtering is performed, so we //! end up catching any C++ exceptions that happen to look like the kind we're //! throwing. Note that throwing an exception into Rust is undefined behavior //! anyway, so this should be fine. //! * We've got some data to transmit across the unwinding boundary, //! specifically a `Box`. Like with Dwarf exceptions //! these two pointers are stored as a payload in the exception itself. On //! MSVC, however, there's no need for an extra heap allocation because the //! call stack is preserved while filter functions are being executed. This //! means that the pointers are passed directly to `_CxxThrowException` which //! are then recovered in the filter function to be written to the stack frame //! of the `try` intrinsic. //! //! [win64]: http://msdn.microsoft.com/en-us/library/1eyas8tf.aspx //! [llvm]: http://llvm.org/docs/ExceptionHandling.html#background-on-windows-exceptions #![allow(nonstandard_style)] #![allow(private_no_mangle_fns)] use alloc::boxed::Box; use core::any::Any; use core::mem; use core::raw; use windows as c; use libc::{c_int, c_uint}; // 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 // `_CxxThrowException`. // // This function takes two arguments. The first is a pointer to the data we're // passing in, which in this case is our trait object. Pretty easy to find! The // next, however, is more complicated. This is a pointer to a `_ThrowInfo` // structure, and it generally is just intended to just describe the exception // being thrown. // // Currently the definition of this type [1] is a little hairy, and the main // oddity (and difference from the online article) is that on 32-bit the // pointers are pointers but on 64-bit the pointers are expressed as 32-bit // offsets from the `__ImageBase` symbol. The `ptr_t` and `ptr!` macro in the // modules below are used to express this. // // The maze of type definitions also closely follows what LLVM emits for this // sort of operation. For example, if you compile this C++ code on MSVC and emit // the LLVM IR: // // #include // // void foo() { // uint64_t a[2] = {0, 1}; // throw a; // } // // That's essentially what we're trying to emulate. Most of the constant values // below were just copied from LLVM, I'm at least not 100% sure what's going on // everywhere. For example the `.PA_K\0` and `.PEA_K\0` strings below (stuck in // the names of a few of these) I'm not actually sure what they do, but it seems // to mirror what LLVM does! // // In any case, these structures are all constructed in a similar manner, and // it's just somewhat verbose for us. // // [1]: http://www.geoffchappell.com/studies/msvc/language/predefined/ #[cfg(target_arch = "x86")] #[macro_use] mod imp { pub type ptr_t = *mut u8; pub const OFFSET: i32 = 4; pub const NAME1: [u8; 7] = [b'.', b'P', b'A', b'_', b'K', 0, 0]; pub const NAME2: [u8; 7] = [b'.', b'P', b'A', b'X', 0, 0, 0]; macro_rules! ptr { (0) => (0 as *mut u8); ($e:expr) => ($e as *mut u8); } } #[cfg(target_arch = "x86_64")] #[macro_use] mod imp { pub type ptr_t = u32; pub const OFFSET: i32 = 8; pub const NAME1: [u8; 7] = [b'.', b'P', b'E', b'A', b'_', b'K', 0]; pub const NAME2: [u8; 7] = [b'.', b'P', b'E', b'A', b'X', 0, 0]; extern "C" { pub static __ImageBase: u8; } macro_rules! ptr { (0) => (0); ($e:expr) => { (($e as usize) - (&imp::__ImageBase as *const _ as usize)) as u32 } } } #[repr(C)] pub struct _ThrowInfo { pub attributes: c_uint, pub pnfnUnwind: imp::ptr_t, pub pForwardCompat: imp::ptr_t, pub pCatchableTypeArray: imp::ptr_t, } #[repr(C)] pub struct _CatchableTypeArray { pub nCatchableTypes: c_int, pub arrayOfCatchableTypes: [imp::ptr_t; 2], } #[repr(C)] pub struct _CatchableType { pub properties: c_uint, pub pType: imp::ptr_t, pub thisDisplacement: _PMD, pub sizeOrOffset: c_int, pub copy_function: imp::ptr_t, } #[repr(C)] pub struct _PMD { pub mdisp: c_int, pub pdisp: c_int, pub vdisp: c_int, } #[repr(C)] pub struct _TypeDescriptor { pub pVFTable: *const u8, pub spare: *mut u8, pub name: [u8; 7], } static mut THROW_INFO: _ThrowInfo = _ThrowInfo { attributes: 0, pnfnUnwind: ptr!(0), pForwardCompat: ptr!(0), pCatchableTypeArray: ptr!(0), }; static mut CATCHABLE_TYPE_ARRAY: _CatchableTypeArray = _CatchableTypeArray { nCatchableTypes: 2, arrayOfCatchableTypes: [ptr!(0), ptr!(0)], }; static mut CATCHABLE_TYPE1: _CatchableType = _CatchableType { properties: 1, pType: ptr!(0), thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0, }, sizeOrOffset: imp::OFFSET, copy_function: ptr!(0), }; static mut CATCHABLE_TYPE2: _CatchableType = _CatchableType { properties: 1, pType: ptr!(0), thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0, }, sizeOrOffset: imp::OFFSET, copy_function: ptr!(0), }; 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. #[link_name = "\x01??_7type_info@@6B@"] static TYPE_INFO_VTABLE: *const u8; } // We use #[lang = "msvc_try_filter"] 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")] static mut TYPE_DESCRIPTOR1: _TypeDescriptor = _TypeDescriptor { pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ as *const _, spare: 0 as *mut _, name: imp::NAME1, }; static mut TYPE_DESCRIPTOR2: _TypeDescriptor = _TypeDescriptor { pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ as *const _, spare: 0 as *mut _, name: imp::NAME2, }; pub unsafe fn panic(data: Box) -> u32 { use core::intrinsics::atomic_store; // _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 mut ptrs_ptr = ptrs.as_mut_ptr(); // This... may seems surprising, and justifiably so. On 32-bit MSVC the // pointers between these structure are just that, pointers. On 64-bit MSVC, // however, the pointers between structures are rather expressed as 32-bit // offsets from `__ImageBase`. // // Consequently, on 32-bit MSVC we can declare all these pointers in the // `static`s above. On 64-bit MSVC, we would have to express subtraction of // pointers in statics, which Rust does not currently allow, so we can't // actually do that. // // The next best thing, then is to fill in these structures at runtime // (panicking is already the "slow path" anyway). So here we reinterpret all // of these pointer fields as 32-bit integers and then store the // relevant value into it (atomically, as concurrent panics may be // happening). Technically the runtime will probably do a nonatomic read of // these fields, but in theory they never read the *wrong* value so it // shouldn't be too bad... // // In any case, we basically need to do something like this until we can // express more operations in statics (and we may never be able to). atomic_store(&mut THROW_INFO.pCatchableTypeArray as *mut _ as *mut u32, ptr!(&CATCHABLE_TYPE_ARRAY as *const _) as u32); atomic_store(&mut CATCHABLE_TYPE_ARRAY.arrayOfCatchableTypes[0] as *mut _ as *mut u32, ptr!(&CATCHABLE_TYPE1 as *const _) as u32); atomic_store(&mut CATCHABLE_TYPE_ARRAY.arrayOfCatchableTypes[1] as *mut _ as *mut u32, ptr!(&CATCHABLE_TYPE2 as *const _) as u32); atomic_store(&mut CATCHABLE_TYPE1.pType as *mut _ as *mut u32, ptr!(&TYPE_DESCRIPTOR1 as *const _) as u32); atomic_store(&mut CATCHABLE_TYPE2.pType as *mut _ as *mut u32, ptr!(&TYPE_DESCRIPTOR2 as *const _) as u32); c::_CxxThrowException(&mut ptrs_ptr as *mut _ as *mut _, &mut THROW_INFO as *mut _ as *mut _); u32::max_value() } pub fn payload() -> [u64; 2] { [0; 2] } pub unsafe fn cleanup(payload: [u64; 2]) -> Box { mem::transmute(raw::TraitObject { data: payload[0] as *mut _, vtable: payload[1] as *mut _, }) } // This is required by the compiler to exist (e.g. it's a lang item), but // it's never actually called by the compiler because __C_specific_handler // or _except_handler3 is the personality function that is always used. // Hence this is just an aborting stub. #[lang = "eh_personality"] #[cfg(not(test))] fn rust_eh_personality() { unsafe { ::core::intrinsics::abort() } }