wasmer-compiler 5.0.2

// This file contains code from external sources.
// Attributions: https://github.com/wasmerio/wasmer/blob/main/docs/ATTRIBUTIONS.md

//! Module for System V ABI unwind registry.

use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};

use crate::types::unwind::CompiledFunctionUnwindInfoReference;

/// Represents a registry of function unwind information for System V ABI.
pub struct UnwindRegistry {
    registrations: Vec<usize>,
    published: bool,
}

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

/// There are two primary unwinders on Unix platforms: libunwind and libgcc.
///
/// Unfortunately their interface to `__register_frame` is different. The
/// libunwind library takes a pointer to an individual FDE while libgcc takes a
/// null-terminated list of FDEs. This means we need to know what unwinder
/// is being used at runtime.
///
/// This detection is done currently by looking for a libunwind-specific symbol.
/// This specific symbol was somewhat recommended by LLVM's
/// "RTDyldMemoryManager.cpp" file which says:
///
/// > We use the presence of __unw_add_dynamic_fde to detect libunwind.
///
/// I'll note that there's also a different libunwind project at
/// https://www.nongnu.org/libunwind/ but that doesn't appear to have
/// `__register_frame` so I don't think that interacts with this.
fn using_libunwind() -> bool {
    static USING_LIBUNWIND: AtomicUsize = AtomicUsize::new(LIBUNWIND_UNKNOWN);

    const LIBUNWIND_UNKNOWN: usize = 0;
    const LIBUNWIND_YES: usize = 1;
    const LIBUNWIND_NO: usize = 2;

    // On macOS the libgcc interface is never used so libunwind is always used.
    if cfg!(target_os = "macos") {
        return true;
    }

    // On other platforms the unwinder can vary. Sometimes the unwinder is
    // selected at build time and sometimes it differs at build time and runtime
    // (or at least I think that's possible). Fall back to a `libc::dlsym` to
    // figure out what we're using and branch based on that.
    //
    // Note that the result of `libc::dlsym` is cached to only look this up
    // once.
    match USING_LIBUNWIND.load(Relaxed) {
        LIBUNWIND_YES => true,
        LIBUNWIND_NO => false,
        LIBUNWIND_UNKNOWN => {
            let looks_like_libunwind = unsafe {
                !libc::dlsym(
                    std::ptr::null_mut(),
                    "__unw_add_dynamic_fde\0".as_ptr().cast(),
                )
                .is_null()
            };
            USING_LIBUNWIND.store(
                if looks_like_libunwind {
                    LIBUNWIND_YES
                } else {
                    LIBUNWIND_NO
                },
                Relaxed,
            );
            looks_like_libunwind
        }
        _ => unreachable!(),
    }
}

impl UnwindRegistry {
    /// Creates a new unwind registry with the given base address.
    pub fn new() -> Self {
        Self {
            registrations: Vec::new(),
            published: false,
        }
    }

    /// Registers a function given the start offset, length, and unwind information.
    pub fn register(
        &mut self,
        _base_address: usize,
        _func_start: u32,
        _func_len: u32,
        info: &CompiledFunctionUnwindInfoReference,
    ) -> Result<(), String> {
        match info {
            CompiledFunctionUnwindInfoReference::Dwarf => {}
            _ => return Err(format!("unsupported unwind information {info:?}")),
        };
        Ok(())
    }

    /// Publishes all registered functions.
    pub fn publish(&mut self, eh_frame: Option<&[u8]>) -> Result<(), String> {
        if self.published {
            return Err("unwind registry has already been published".to_string());
        }

        if let Some(eh_frame) = eh_frame {
            unsafe {
                self.register_frames(eh_frame);
            }
        }

        self.published = true;

        Ok(())
    }

    #[allow(clippy::cast_ptr_alignment)]
    unsafe fn register_frames(&mut self, eh_frame: &[u8]) {
        if !using_libunwind() {
            // Registering an empty `eh_frame` (i.e. which
            // contains empty FDEs) cause problems on Linux when
            // deregistering it. We must avoid this
            // scenario. Usually, this is handled upstream by the
            // compilers.
            debug_assert_ne!(
                eh_frame,
                &[0, 0, 0, 0],
                "`eh_frame` seems to contain empty FDEs"
            );

            // On gnu (libgcc), `__register_frame` will walk the FDEs until an entry of length 0
            let ptr = eh_frame.as_ptr();
            __register_frame(ptr);
            self.registrations.push(ptr as usize);
        } else {
            // For libunwind, `__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 and zero-length FDEs.
                // LLVM's libunwind emits a warning on zero-length FDEs.
                if current != start && len != 0 {
                    __register_frame(current);
                    self.registrations.push(current as usize);
                }

                // Move to the next table entry (+4 because the length itself is not inclusive)
                current = current.add(len + 4);
            }
        }
    }
}

impl Drop for UnwindRegistry {
    fn drop(&mut self) {
        if self.published {
            unsafe {
                // libgcc stores the frame entries as a linked list in decreasing sort order
                // based on the PC value of the registered entry.
                //
                // As we store the registrations in increasing order, it would be O(N^2) to
                // deregister in that order.
                //
                // To ensure that we just pop off the first element in the list upon every
                // deregistration, walk our list of registrations backwards.
                for fde in self.registrations.iter().rev() {
                    __deregister_frame(*fde as *const _);
                }
            }
        }
    }
}