unwind.rs

//! Unwind info generation (`.eh_frame`)

use cranelift_codegen::FinalizedMachExceptionHandler;
use cranelift_codegen::ir::Endianness;
use cranelift_codegen::isa::unwind::UnwindInfo;
use cranelift_module::DataId;
use cranelift_object::ObjectProduct;
use gimli::write::{Address, CieId, EhFrame, FrameTable, Section};
use gimli::{Encoding, Format, RunTimeEndian};

use super::emit::{DebugRelocName, address_for_data, address_for_func};
use super::gcc_except_table::{
    Action, ActionKind, ActionTable, CallSite, CallSiteTable, GccExceptTable, TypeInfoTable,
};
use super::object::WriteDebugInfo;
use crate::prelude::*;

pub(crate) const EXCEPTION_HANDLER_CLEANUP: u32 = 0;
pub(crate) const EXCEPTION_HANDLER_CATCH: u32 = 1;

pub(crate) struct UnwindContext {
    endian: RunTimeEndian,
    frame_table: FrameTable,
    cie_id: Option<CieId>,
}

impl UnwindContext {
    #[cfg_attr(not(bootstrap), allow(todo_macro_calls))]
    pub(crate) fn new(module: &mut dyn Module, pic_eh_frame: bool) -> Self {
        let endian = match module.isa().endianness() {
            Endianness::Little => RunTimeEndian::Little,
            Endianness::Big => RunTimeEndian::Big,
        };
        let mut frame_table = FrameTable::default();

        let cie_id = if let Some(mut cie) = module.isa().create_systemv_cie() {
            let ptr_encoding = if pic_eh_frame {
                gimli::DwEhPe(gimli::DW_EH_PE_pcrel.0 | gimli::DW_EH_PE_sdata4.0)
            } else {
                gimli::DW_EH_PE_absptr
            };

            cie.fde_address_encoding = ptr_encoding;

            // FIXME only add personality function and lsda when necessary: https://github.com/rust-lang/rust/blob/1f76d219c906f0112bb1872f33aa977164c53fa6/compiler/rustc_codegen_ssa/src/mir/mod.rs#L200-L204
            if cfg!(feature = "unwinding") {
                let code_ptr_encoding = if pic_eh_frame {
                    if module.isa().triple().architecture == target_lexicon::Architecture::X86_64 {
                        gimli::DwEhPe(
                            gimli::DW_EH_PE_indirect.0
                                | gimli::DW_EH_PE_pcrel.0
                                | gimli::DW_EH_PE_sdata4.0,
                        )
                    } else if let target_lexicon::Architecture::Aarch64(_) =
                        module.isa().triple().architecture
                    {
                        gimli::DwEhPe(
                            gimli::DW_EH_PE_indirect.0
                                | gimli::DW_EH_PE_pcrel.0
                                | gimli::DW_EH_PE_sdata8.0,
                        )
                    } else {
                        todo!()
                    }
                } else {
                    gimli::DwEhPe(gimli::DW_EH_PE_indirect.0 | gimli::DW_EH_PE_absptr.0)
                };

                cie.lsda_encoding = Some(ptr_encoding);

                // FIXME use eh_personality lang item instead
                let personality = module
                    .declare_function(
                        "rust_eh_personality",
                        Linkage::Import,
                        &Signature {
                            params: vec![
                                AbiParam::new(types::I32),
                                AbiParam::new(types::I32),
                                AbiParam::new(types::I64),
                                AbiParam::new(module.target_config().pointer_type()),
                                AbiParam::new(module.target_config().pointer_type()),
                            ],
                            returns: vec![AbiParam::new(types::I32)],
                            call_conv: module.target_config().default_call_conv,
                        },
                    )
                    .unwrap();

                // Use indirection here to support PIC the case where rust_eh_personality is defined in
                // another DSO.
                let personality_ref = module
                    .declare_data("DW.ref.rust_eh_personality", Linkage::Local, false, false)
                    .unwrap();

                let mut personality_ref_data = DataDescription::new();
                // Note: Must not use define_zeroinit. The unwinder can't handle this being in the .bss
                // section.
                let pointer_bytes = usize::from(module.target_config().pointer_bytes());
                personality_ref_data.define(vec![0; pointer_bytes].into_boxed_slice());
                let personality_func_ref =
                    module.declare_func_in_data(personality, &mut personality_ref_data);
                personality_ref_data.write_function_addr(0, personality_func_ref);

                module.define_data(personality_ref, &personality_ref_data).unwrap();

                cie.personality = Some((code_ptr_encoding, address_for_data(personality_ref)));
            }

            Some(frame_table.add_cie(cie))
        } else {
            None
        };

        UnwindContext { endian, frame_table, cie_id }
    }

    pub(crate) fn add_function(
        &mut self,
        module: &mut dyn Module,
        func_id: FuncId,
        context: &Context,
    ) {
        if let target_lexicon::OperatingSystem::MacOSX { .. } =
            module.isa().triple().operating_system
        {
            // The object crate doesn't currently support DW_GNU_EH_PE_absptr, which macOS
            // requires for unwinding tables. In addition on arm64 it currently doesn't
            // support 32bit relocations as we currently use for the unwinding table.
            // See gimli-rs/object#415 and rust-lang/rustc_codegen_cranelift#1371
            return;
        }

        let Some(unwind_info) =
            context.compiled_code().unwrap().create_unwind_info(module.isa()).unwrap()
        else {
            return;
        };

        match unwind_info {
            UnwindInfo::SystemV(unwind_info) => {
                let mut fde = unwind_info.to_fde(address_for_func(func_id));

                // FIXME only add personality function and lsda when necessary: https://github.com/rust-lang/rust/blob/1f76d219c906f0112bb1872f33aa977164c53fa6/compiler/rustc_codegen_ssa/src/mir/mod.rs#L200-L204
                if cfg!(feature = "unwinding") {
                    // FIXME use unique symbol name derived from function name
                    let lsda = module.declare_anonymous_data(false, false).unwrap();

                    let encoding = Encoding {
                        format: Format::Dwarf32,
                        version: 1,
                        address_size: module.isa().frontend_config().pointer_bytes(),
                    };

                    let mut gcc_except_table_data = GccExceptTable {
                        call_sites: CallSiteTable(vec![]),
                        actions: ActionTable::new(),
                        type_info: TypeInfoTable::new(gimli::DW_EH_PE_udata4),
                    };

                    let catch_type = gcc_except_table_data.type_info.add(Address::Constant(0));
                    let catch_action = gcc_except_table_data
                        .actions
                        .add(Action { kind: ActionKind::Catch(catch_type), next_action: None });

                    for call_site in context.compiled_code().unwrap().buffer.call_sites() {
                        if call_site.exception_handlers.is_empty() {
                            gcc_except_table_data.call_sites.0.push(CallSite {
                                start: u64::from(call_site.ret_addr - 1),
                                length: 1,
                                landing_pad: 0,
                                action_entry: None,
                            });
                        }
                        for &handler in call_site.exception_handlers {
                            match handler {
                                FinalizedMachExceptionHandler::Tag(tag, landingpad) => {
                                    match tag.as_u32() {
                                        EXCEPTION_HANDLER_CLEANUP => {
                                            gcc_except_table_data.call_sites.0.push(CallSite {
                                                start: u64::from(call_site.ret_addr - 1),
                                                length: 1,
                                                landing_pad: u64::from(landingpad),
                                                action_entry: None,
                                            })
                                        }
                                        EXCEPTION_HANDLER_CATCH => {
                                            gcc_except_table_data.call_sites.0.push(CallSite {
                                                start: u64::from(call_site.ret_addr - 1),
                                                length: 1,
                                                landing_pad: u64::from(landingpad),
                                                action_entry: Some(catch_action),
                                            })
                                        }
                                        _ => unreachable!(),
                                    }
                                }
                                _ => unreachable!(),
                            }
                        }
                    }

                    let mut gcc_except_table = super::emit::WriterRelocate::new(self.endian);

                    gcc_except_table_data.write(&mut gcc_except_table, encoding).unwrap();

                    let mut data = DataDescription::new();
                    data.define(gcc_except_table.writer.into_vec().into_boxed_slice());
                    data.set_segment_section("", ".gcc_except_table");

                    for reloc in &gcc_except_table.relocs {
                        match reloc.name {
                            DebugRelocName::Section(_id) => unreachable!(),
                            DebugRelocName::Symbol(id) => {
                                let id = id.try_into().unwrap();
                                if id & 1 << 31 == 0 {
                                    let func_ref = module
                                        .declare_func_in_data(FuncId::from_u32(id), &mut data);
                                    data.write_function_addr(reloc.offset, func_ref);
                                } else {
                                    let gv = module.declare_data_in_data(
                                        DataId::from_u32(id & !(1 << 31)),
                                        &mut data,
                                    );
                                    data.write_data_addr(reloc.offset, gv, 0);
                                }
                            }
                        };
                    }

                    module.define_data(lsda, &data).unwrap();
                    fde.lsda = Some(address_for_data(lsda));
                }

                self.frame_table.add_fde(self.cie_id.unwrap(), fde);
            }
            UnwindInfo::WindowsX64(_) | UnwindInfo::WindowsArm64(_) => {
                // Windows does not have debug info for its unwind info.
            }
            unwind_info => unimplemented!("{:?}", unwind_info),
        }
    }

    pub(crate) fn emit(self, product: &mut ObjectProduct) {
        let mut eh_frame = EhFrame::from(super::emit::WriterRelocate::new(self.endian));
        self.frame_table.write_eh_frame(&mut eh_frame).unwrap();

        if !eh_frame.0.writer.slice().is_empty() {
            let id = eh_frame.id();
            let section_id = product.add_debug_section(id, eh_frame.0.writer.into_vec());
            let mut section_map = FxHashMap::default();
            section_map.insert(id, section_id);

            for reloc in &eh_frame.0.relocs {
                product.add_debug_reloc(&section_map, &section_id, reloc);
            }
        }
    }

    #[cfg(all(feature = "jit", windows))]
    pub(crate) unsafe fn register_jit(self, _jit_module: &cranelift_jit::JITModule) {}

    #[cfg(all(feature = "jit", not(windows)))]
    pub(crate) unsafe fn register_jit(self, jit_module: &cranelift_jit::JITModule) {
        use std::mem::ManuallyDrop;

        let mut eh_frame = EhFrame::from(super::emit::WriterRelocate::new(self.endian));
        self.frame_table.write_eh_frame(&mut eh_frame).unwrap();

        if eh_frame.0.writer.slice().is_empty() {
            return;
        }

        let mut eh_frame = eh_frame.0.relocate_for_jit(jit_module);

        // GCC expects a terminating "empty" length, so write a 0 length at the end of the table.
        eh_frame.extend(&[0, 0, 0, 0]);

        // FIXME support unregistering unwind tables once cranelift-jit supports deallocating
        // individual functions
        let eh_frame = ManuallyDrop::new(eh_frame);

        // =======================================================================
        // Everything after this line up to the end of the file is loosely based on
        // https://github.com/bytecodealliance/wasmtime/blob/4471a82b0c540ff48960eca6757ccce5b1b5c3e4/crates/jit/src/unwind/systemv.rs
        #[cfg(target_os = "macos")]
        unsafe {
            // On macOS, `__register_frame` takes a pointer to a single FDE
            let start = eh_frame.as_ptr();
            let end = start.add(eh_frame.len());
            let mut current = start;

            // Walk all of the entries in the frame table and register them
            while current < end {
                let len = std::ptr::read::<u32>(current as *const u32) as usize;

                // Skip over the CIE
                if current != start {
                    __register_frame(current);
                }

                // Move to the next table entry (+4 because the length itself is not inclusive)
                current = current.add(len + 4);
            }
        }
        #[cfg(not(target_os = "macos"))]
        {
            // On other platforms, `__register_frame` will walk the FDEs until an entry of length 0
            unsafe { __register_frame(eh_frame.as_ptr()) };
        }
    }
}

unsafe extern "C" {
    // libunwind import
    fn __register_frame(fde: *const u8);
}