blob: 809e4619812dec41c90575e81069f039e62e53d4 [file] [log] [blame]
//! 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]: 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 crate::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 <stdin.h>
//
// 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) => (core::ptr::null_mut());
($e:expr) => ($e as *mut u8);
}
}
#[cfg(any(target_arch = "x86_64", target_arch = "arm"))]
#[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: core::ptr::null_mut(),
name: imp::NAME1,
};
static mut TYPE_DESCRIPTOR2: _TypeDescriptor = _TypeDescriptor {
pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ as *const _,
spare: core::ptr::null_mut(),
name: imp::NAME2,
};
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 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<dyn Any + Send> {
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() }
}