gcc.rs

// 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 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Implementation of panics backed by libgcc/libunwind (in some form)
//!
//! For background on exception handling and stack unwinding please see
//! "Exception Handling in LLVM" (llvm.org/docs/ExceptionHandling.html) and
//! documents linked from it.
//! These are also good reads:
//!     http://mentorembedded.github.io/cxx-abi/abi-eh.html
//!     http://monoinfinito.wordpress.com/series/exception-handling-in-c/
//!     http://www.airs.com/blog/index.php?s=exception+frames
//!
//! ## A brief summary
//!
//! Exception handling happens in two phases: a search phase and a cleanup
//! phase.
//!
//! In both phases the unwinder walks stack frames from top to bottom using
//! information from the stack frame unwind sections of the current process's
//! modules ("module" here refers to an OS module, i.e. an executable or a
//! dynamic library).
//!
//! For each stack frame, it invokes the associated "personality routine", whose
//! address is also stored in the unwind info section.
//!
//! In the search phase, the job of a personality routine is to examine
//! exception object being thrown, and to decide whether it should be caught at
//! that stack frame.  Once the handler frame has been identified, cleanup phase
//! begins.
//!
//! In the cleanup phase, the unwinder invokes each personality routine again.
//! This time it decides which (if any) cleanup code needs to be run for
//! the current stack frame.  If so, the control is transferred to a special
//! branch in the function body, the "landing pad", which invokes destructors,
//! frees memory, etc.  At the end of the landing pad, control is transferred
//! back to the unwinder and unwinding resumes.
//!
//! Once stack has been unwound down to the handler frame level, unwinding stops
//! and the last personality routine transfers control to the catch block.
//!
//! ## `eh_personality` and `eh_unwind_resume`
//!
//! These language items are used by the compiler when generating unwind info.
//! The first one is the personality routine described above.  The second one
//! allows compilation target to customize the process of resuming unwind at the
//! end of the landing pads. `eh_unwind_resume` is used only if
//! `custom_unwind_resume` flag in the target options is set.

#![allow(private_no_mangle_fns)]

use core::any::Any;
use core::ptr;
use alloc::boxed::Box;

use unwind as uw;
use libc::{c_int, uintptr_t};
use dwarf::eh::{self, EHContext, EHAction};

#[repr(C)]
struct Exception {
    _uwe: uw::_Unwind_Exception,
    cause: Option<Box<Any + Send>>,
}

pub unsafe fn panic(data: Box<Any + Send>) -> u32 {
    let exception = Box::new(Exception {
        _uwe: uw::_Unwind_Exception {
            exception_class: rust_exception_class(),
            exception_cleanup: exception_cleanup,
            private: [0; uw::unwinder_private_data_size],
        },
        cause: Some(data),
    });
    let exception_param = Box::into_raw(exception) as *mut uw::_Unwind_Exception;
    return uw::_Unwind_RaiseException(exception_param) as u32;

    extern "C" fn exception_cleanup(_unwind_code: uw::_Unwind_Reason_Code,
                                    exception: *mut uw::_Unwind_Exception) {
        unsafe {
            let _: Box<Exception> = Box::from_raw(exception as *mut Exception);
        }
    }
}

pub fn payload() -> *mut u8 {
    ptr::null_mut()
}

pub unsafe fn cleanup(ptr: *mut u8) -> Box<Any + Send> {
    let my_ep = ptr as *mut Exception;
    let cause = (*my_ep).cause.take();
    uw::_Unwind_DeleteException(ptr as *mut _);
    cause.unwrap()
}

// Rust's exception class identifier.  This is used by personality routines to
// determine whether the exception was thrown by their own runtime.
fn rust_exception_class() -> uw::_Unwind_Exception_Class {
    // M O Z \0  R U S T -- vendor, language
    0x4d4f5a_00_52555354
}


// Register ids were lifted from LLVM's TargetLowering::getExceptionPointerRegister()
// and TargetLowering::getExceptionSelectorRegister() for each architecture,
// then mapped to DWARF register numbers via register definition tables
// (typically <arch>RegisterInfo.td, search for "DwarfRegNum").
// See also http://llvm.org/docs/WritingAnLLVMBackend.html#defining-a-register.

#[cfg(target_arch = "x86")]
const UNWIND_DATA_REG: (i32, i32) = (0, 2); // EAX, EDX

#[cfg(target_arch = "x86_64")]
const UNWIND_DATA_REG: (i32, i32) = (0, 1); // RAX, RDX

#[cfg(any(target_arch = "arm", target_arch = "aarch64"))]
const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1 / X0, X1

#[cfg(any(target_arch = "mips", target_arch = "mips64"))]
const UNWIND_DATA_REG: (i32, i32) = (4, 5); // A0, A1

#[cfg(any(target_arch = "powerpc", target_arch = "powerpc64"))]
const UNWIND_DATA_REG: (i32, i32) = (3, 4); // R3, R4 / X3, X4

#[cfg(target_arch = "s390x")]
const UNWIND_DATA_REG: (i32, i32) = (6, 7); // R6, R7

#[cfg(target_arch = "sparc64")]
const UNWIND_DATA_REG: (i32, i32) = (24, 25); // I0, I1

// The following code is based on GCC's C and C++ personality routines.  For reference, see:
// https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/libsupc++/eh_personality.cc
// https://github.com/gcc-mirror/gcc/blob/trunk/libgcc/unwind-c.c

// The personality routine for most of our targets, except ARM, which has a slightly different ABI
// (however, iOS goes here as it uses SjLj unwinding).  Also, the 64-bit Windows implementation
// lives in seh64_gnu.rs
#[cfg(all(any(target_os = "ios", not(target_arch = "arm"))))]
#[lang = "eh_personality"]
#[no_mangle]
#[allow(unused)]
unsafe extern "C" fn rust_eh_personality(version: c_int,
                                         actions: uw::_Unwind_Action,
                                         exception_class: uw::_Unwind_Exception_Class,
                                         exception_object: *mut uw::_Unwind_Exception,
                                         context: *mut uw::_Unwind_Context)
                                         -> uw::_Unwind_Reason_Code {
    if version != 1 {
        return uw::_URC_FATAL_PHASE1_ERROR;
    }
    let eh_action = find_eh_action(context);
    if actions as i32 & uw::_UA_SEARCH_PHASE as i32 != 0 {
        match eh_action {
            EHAction::None |
            EHAction::Cleanup(_) => return uw::_URC_CONTINUE_UNWIND,
            EHAction::Catch(_) => return uw::_URC_HANDLER_FOUND,
            EHAction::Terminate => return uw::_URC_FATAL_PHASE1_ERROR,
        }
    } else {
        match eh_action {
            EHAction::None => return uw::_URC_CONTINUE_UNWIND,
            EHAction::Cleanup(lpad) |
            EHAction::Catch(lpad) => {
                uw::_Unwind_SetGR(context, UNWIND_DATA_REG.0, exception_object as uintptr_t);
                uw::_Unwind_SetGR(context, UNWIND_DATA_REG.1, 0);
                uw::_Unwind_SetIP(context, lpad);
                return uw::_URC_INSTALL_CONTEXT;
            }
            EHAction::Terminate => return uw::_URC_FATAL_PHASE2_ERROR,
        }
    }
}

// ARM EHABI personality routine.
// http://infocenter.arm.com/help/topic/com.arm.doc.ihi0038b/IHI0038B_ehabi.pdf
#[cfg(all(target_arch = "arm", not(target_os = "ios")))]
#[lang = "eh_personality"]
#[no_mangle]
unsafe extern "C" fn rust_eh_personality(state: uw::_Unwind_State,
                                         exception_object: *mut uw::_Unwind_Exception,
                                         context: *mut uw::_Unwind_Context)
                                         -> uw::_Unwind_Reason_Code {
    let state = state as c_int;
    let action = state & uw::_US_ACTION_MASK as c_int;
    let search_phase = if action == uw::_US_VIRTUAL_UNWIND_FRAME as c_int {
        // Backtraces on ARM will call the personality routine with
        // state == _US_VIRTUAL_UNWIND_FRAME | _US_FORCE_UNWIND. In those cases
        // we want to continue unwinding the stack, otherwise all our backtraces
        // would end at __rust_try
        if state & uw::_US_FORCE_UNWIND as c_int != 0 {
            return continue_unwind(exception_object, context);
        }
        true
    } else if action == uw::_US_UNWIND_FRAME_STARTING as c_int {
        false
    } else if action == uw::_US_UNWIND_FRAME_RESUME as c_int {
        return continue_unwind(exception_object, context);
    } else {
        return uw::_URC_FAILURE;
    };

    // The DWARF unwinder assumes that _Unwind_Context holds things like the function
    // and LSDA pointers, however ARM EHABI places them into the exception object.
    // To preserve signatures of functions like _Unwind_GetLanguageSpecificData(), which
    // take only the context pointer, GCC personality routines stash a pointer to exception_object
    // in the context, using location reserved for ARM's "scratch register" (r12).
    uw::_Unwind_SetGR(context,
                      uw::UNWIND_POINTER_REG,
                      exception_object as uw::_Unwind_Ptr);
    // ...A more principled approach would be to provide the full definition of ARM's
    // _Unwind_Context in our libunwind bindings and fetch the required data from there directly,
    // bypassing DWARF compatibility functions.

    let eh_action = find_eh_action(context);
    if search_phase {
        match eh_action {
            EHAction::None |
            EHAction::Cleanup(_) => return continue_unwind(exception_object, context),
            EHAction::Catch(_) => return uw::_URC_HANDLER_FOUND,
            EHAction::Terminate => return uw::_URC_FAILURE,
        }
    } else {
        match eh_action {
            EHAction::None => return continue_unwind(exception_object, context),
            EHAction::Cleanup(lpad) |
            EHAction::Catch(lpad) => {
                uw::_Unwind_SetGR(context, UNWIND_DATA_REG.0, exception_object as uintptr_t);
                uw::_Unwind_SetGR(context, UNWIND_DATA_REG.1, 0);
                uw::_Unwind_SetIP(context, lpad);
                return uw::_URC_INSTALL_CONTEXT;
            }
            EHAction::Terminate => return uw::_URC_FAILURE,
        }
    }

    // On ARM EHABI the personality routine is responsible for actually
    // unwinding a single stack frame before returning (ARM EHABI Sec. 6.1).
    unsafe fn continue_unwind(exception_object: *mut uw::_Unwind_Exception,
                              context: *mut uw::_Unwind_Context)
                              -> uw::_Unwind_Reason_Code {
        if __gnu_unwind_frame(exception_object, context) == uw::_URC_NO_REASON {
            uw::_URC_CONTINUE_UNWIND
        } else {
            uw::_URC_FAILURE
        }
    }
    // defined in libgcc
    extern "C" {
        fn __gnu_unwind_frame(exception_object: *mut uw::_Unwind_Exception,
                              context: *mut uw::_Unwind_Context)
                              -> uw::_Unwind_Reason_Code;
    }
}

unsafe fn find_eh_action(context: *mut uw::_Unwind_Context) -> EHAction {
    let lsda = uw::_Unwind_GetLanguageSpecificData(context) as *const u8;
    let mut ip_before_instr: c_int = 0;
    let ip = uw::_Unwind_GetIPInfo(context, &mut ip_before_instr);
    let eh_context = EHContext {
        // The return address points 1 byte past the call instruction,
        // which could be in the next IP range in LSDA range table.
        ip: if ip_before_instr != 0 { ip } else { ip - 1 },
        func_start: uw::_Unwind_GetRegionStart(context),
        get_text_start: &|| uw::_Unwind_GetTextRelBase(context),
        get_data_start: &|| uw::_Unwind_GetDataRelBase(context),
    };
    eh::find_eh_action(lsda, &eh_context)
}

// See docs in the `unwind` module.
#[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
#[lang = "eh_unwind_resume"]
#[unwind]
unsafe extern "C" fn rust_eh_unwind_resume(panic_ctx: *mut u8) -> ! {
    uw::_Unwind_Resume(panic_ctx as *mut uw::_Unwind_Exception);
}

// Frame unwind info registration
//
// Each module's image contains a frame unwind info section (usually
// ".eh_frame").  When a module is loaded/unloaded into the process, the
// unwinder must be informed about the location of this section in memory. The
// methods of achieving that vary by the platform.  On some (e.g. Linux), the
// unwinder can discover unwind info sections on its own (by dynamically
// enumerating currently loaded modules via the dl_iterate_phdr() API and
// finding their ".eh_frame" sections); Others, like Windows, require modules
// to actively register their unwind info sections via unwinder API.
//
// This module defines two symbols which are referenced and called from
// rsbegin.rs to register our information with the GCC runtime. The
// implementation of stack unwinding is (for now) deferred to libgcc_eh, however
// Rust crates use these Rust-specific entry points to avoid potential clashes
// with any GCC runtime.
#[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
pub mod eh_frame_registry {
    extern "C" {
        fn __register_frame_info(eh_frame_begin: *const u8, object: *mut u8);
        fn __deregister_frame_info(eh_frame_begin: *const u8, object: *mut u8);
    }

    #[no_mangle]
    pub unsafe extern "C" fn rust_eh_register_frames(eh_frame_begin: *const u8, object: *mut u8) {
        __register_frame_info(eh_frame_begin, object);
    }

    #[no_mangle]
    pub unsafe extern "C" fn rust_eh_unregister_frames(eh_frame_begin: *const u8,
                                                       object: *mut u8) {
        __deregister_frame_info(eh_frame_begin, object);
    }
}