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Diffstat (limited to 'library/panic_unwind/src/seh.rs')
| -rw-r--r-- | library/panic_unwind/src/seh.rs | 335 |
1 files changed, 335 insertions, 0 deletions
diff --git a/library/panic_unwind/src/seh.rs b/library/panic_unwind/src/seh.rs new file mode 100644 index 000000000..9f1eb411f --- /dev/null +++ b/library/panic_unwind/src/seh.rs @@ -0,0 +1,335 @@ +//! 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<dyn Any + Send>`. 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]: https://docs.microsoft.com/en-us/cpp/build/exception-handling-x64 +//! [llvm]: https://llvm.org/docs/ExceptionHandling.html#background-on-windows-exceptions + +#![allow(nonstandard_style)] + +use alloc::boxed::Box; +use core::any::Any; +use core::mem::{self, ManuallyDrop}; +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 +// `_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 <stdint.h> +// +// struct rust_panic { +// rust_panic(const rust_panic&); +// ~rust_panic(); +// +// uint64_t x[2]; +// }; +// +// void foo() { +// rust_panic a = {0, 1}; +// throw a; +// } +// +// That's essentially what we're trying to emulate. Most of the constant values +// below were just copied from LLVM, +// +// In any case, these structures are all constructed in a similar manner, and +// it's just somewhat verbose for us. +// +// [1]: https://www.geoffchappell.com/studies/msvc/language/predefined/ + +#[cfg(target_arch = "x86")] +#[macro_use] +mod imp { + pub type ptr_t = *mut u8; + + macro_rules! ptr { + (0) => { + core::ptr::null_mut() + }; + ($e:expr) => { + $e as *mut u8 + }; + } +} + +#[cfg(not(target_arch = "x86"))] +#[macro_use] +mod imp { + pub type ptr_t = u32; + + extern "C" { + pub static __ImageBase: u8; + } + + macro_rules! ptr { + (0) => (0); + ($e:expr) => { + (($e as usize) - (&imp::__ImageBase as *const _ as usize)) as u32 + } + } +} + +#[repr(C)] +pub struct _ThrowInfo { + pub attributes: c_uint, + pub pmfnUnwind: 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; 1], +} + +#[repr(C)] +pub struct _CatchableType { + pub properties: c_uint, + pub pType: imp::ptr_t, + pub thisDisplacement: _PMD, + pub sizeOrOffset: c_int, + pub copyFunction: 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; 11], +} + +// Note that we intentionally ignore name mangling rules here: we don't want C++ +// to be able to catch Rust panics by simply declaring a `struct rust_panic`. +// +// When modifying, make sure that the type name string exactly matches +// the one used in `compiler/rustc_codegen_llvm/src/intrinsic.rs`. +const TYPE_NAME: [u8; 11] = *b"rust_panic\0"; + +static mut THROW_INFO: _ThrowInfo = _ThrowInfo { + attributes: 0, + pmfnUnwind: ptr!(0), + pForwardCompat: ptr!(0), + pCatchableTypeArray: ptr!(0), +}; + +static mut CATCHABLE_TYPE_ARRAY: _CatchableTypeArray = + _CatchableTypeArray { nCatchableTypes: 1, arrayOfCatchableTypes: [ptr!(0)] }; + +static mut CATCHABLE_TYPE: _CatchableType = _CatchableType { + properties: 0, + pType: ptr!(0), + thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0 }, + sizeOrOffset: mem::size_of::<Exception>() as c_int, + copyFunction: 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; +} + +// This type descriptor is only used when throwing an exception. The catch part +// is handled by the try intrinsic, which generates its own TypeDescriptor. +// +// This is fine since the MSVC runtime uses string comparison on the type name +// to match TypeDescriptors rather than pointer equality. +static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor { + pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ as *const _, + spare: core::ptr::null_mut(), + name: TYPE_NAME, +}; + +// Destructor used if the C++ code decides to capture the exception and drop it +// without propagating it. The catch part of the try intrinsic will set the +// first word of the exception object to 0 so that it is skipped by the +// destructor. +// +// Note that x86 Windows uses the "thiscall" calling convention for C++ member +// functions instead of the default "C" calling convention. +// +// The exception_copy function is a bit special here: it is invoked by the MSVC +// runtime under a try/catch block and the panic that we generate here will be +// used as the result of the exception copy. This is used by the C++ runtime to +// support capturing exceptions with std::exception_ptr, which we can't support +// because Box<dyn Any> isn't clonable. +macro_rules! define_cleanup { + ($abi:tt $abi2:tt) => { + unsafe extern $abi fn exception_cleanup(e: *mut Exception) { + if let Exception { data: Some(b) } = e.read() { + drop(b); + super::__rust_drop_panic(); + } + } + unsafe extern $abi2 fn exception_copy(_dest: *mut Exception, + _src: *mut Exception) + -> *mut Exception { + panic!("Rust panics cannot be copied"); + } + } +} +cfg_if::cfg_if! { + if #[cfg(target_arch = "x86")] { + define_cleanup!("thiscall" "thiscall-unwind"); + } else { + define_cleanup!("C" "C-unwind"); + } +} + +pub unsafe fn panic(data: Box<dyn Any + Send>) -> 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 ManuallyDrop is needed here since we don't want Exception to be + // dropped when unwinding. Instead it will be dropped by exception_cleanup + // which is invoked by the C++ runtime. + let mut exception = ManuallyDrop::new(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, + // 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.pmfnUnwind as *mut _ as *mut u32, ptr!(exception_cleanup) as u32); + 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_TYPE as *const _) as u32, + ); + atomic_store( + &mut CATCHABLE_TYPE.pType as *mut _ as *mut u32, + ptr!(&TYPE_DESCRIPTOR as *const _) as u32, + ); + atomic_store( + &mut CATCHABLE_TYPE.copyFunction as *mut _ as *mut u32, + ptr!(exception_copy) as u32, + ); + + extern "system-unwind" { + fn _CxxThrowException(pExceptionObject: *mut c_void, pThrowInfo: *mut u8) -> !; + } + + _CxxThrowException(throw_ptr, &mut THROW_INFO as *mut _ as *mut _); +} + +pub unsafe fn cleanup(payload: *mut u8) -> Box<dyn Any + Send> { + // A null payload here means that we got here from the catch (...) of + // __rust_try. This happens when a non-Rust foreign exception is caught. + if payload.is_null() { + super::__rust_foreign_exception(); + } else { + 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 +// 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() { + core::intrinsics::abort() +} |