unix.rs

use libc::{c_void, siginfo_t};
use nix::sys::signal::{sigaction, SaFlags, SigAction, SigHandler, SigSet, SIGBUS, SIGSEGV};

/// `__register_frame` and `__deregister_frame` on macos take a single fde as an
/// argument, so we need to parse the fde table here.
///
/// This is a pretty direct port of llvm's fde handling code:
///     https://llvm.org/doxygen/RTDyldMemoryManager_8cpp_source.html.
#[allow(clippy::cast_ptr_alignment)]
#[cfg(target_os = "macos")]
pub unsafe fn visit_fde(addr: *mut u8, size: usize, visitor: extern "C" fn(*mut u8)) {
    unsafe fn process_fde(entry: *mut u8, visitor: extern "C" fn(*mut u8)) -> *mut u8 {
        let mut p = entry;
        let length = (p as *const u32).read_unaligned();
        p = p.add(4);
        let offset = (p as *const u32).read_unaligned();

        if offset != 0 {
            visitor(entry);
        }
        p.add(length as usize)
    }

    let mut p = addr;
    let end = p.add(size);

    loop {
        if p >= end {
            break;
        }

        p = process_fde(p, visitor);
    }
}

#[cfg(not(target_os = "macos"))]
pub unsafe fn visit_fde(addr: *mut u8, _size: usize, visitor: extern "C" fn(*mut u8)) {
    visitor(addr);
}

extern "C" {
    #[cfg_attr(nightly, unwind(allowed))]
    fn throw_trap(ty: i32) -> !;
}

pub unsafe fn install_signal_handler() {
    let sa = SigAction::new(
        SigHandler::SigAction(signal_trap_handler),
        SaFlags::SA_ONSTACK | SaFlags::SA_SIGINFO,
        SigSet::empty(),
    );
    sigaction(SIGSEGV, &sa).unwrap();
    sigaction(SIGBUS, &sa).unwrap();
}

#[cfg_attr(nightly, unwind(allowed))]
extern "C" fn signal_trap_handler(
    _signum: ::nix::libc::c_int,
    _siginfo: *mut siginfo_t,
    _ucontext: *mut c_void,
) {
    unsafe {
        // Apparently, we can unwind from arbitary instructions, as long
        // as we don't need to catch the exception inside the function that
        // was interrupted.
        //
        // This works on macos, not sure about linux.
        throw_trap(2);
    }
}