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Switch libunwind detection to using weak symbols #9479

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Oct 22, 2024
Merged

Switch libunwind detection to using weak symbols #9479

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5c9c0df
Switch libunwind detection to using weak symbols
alexcrichton Oct 16, 2024
a846526
Fix warnings
alexcrichton Oct 16, 2024
faa0ad4
Run clang-format
alexcrichton Oct 16, 2024
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12 changes: 12 additions & 0 deletions crates/wasmtime/src/runtime/vm/helpers.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
#undef _FORTIFY_SOURCE

#include <setjmp.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

Expand Down Expand Up @@ -116,3 +117,14 @@ __attribute__((weak))
struct JITDescriptor *VERSIONED_SYMBOL(wasmtime_jit_debug_descriptor)() {
return &__jit_debug_descriptor;
}

// For more information about this see `unix/unwind.rs` and the
// `using_libunwind` function. The basic idea is that weak symbols aren't stable
// in Rust so we use a bit of C to work around that.
#ifndef CFG_TARGET_OS_windows
__attribute__((weak)) extern void __unw_add_dynamic_fde();

bool VERSIONED_SYMBOL(wasmtime_using_libunwind)() {
return __unw_add_dynamic_fde != NULL;
}
#endif
44 changes: 6 additions & 38 deletions crates/wasmtime/src/runtime/vm/sys/unix/unwind.rs
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Expand Up @@ -2,8 +2,7 @@

use crate::prelude::*;
use crate::runtime::vm::SendSyncPtr;
use core::ptr::{self, NonNull};
use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
use core::ptr::NonNull;

/// Represents a registration of function unwind information for System V ABI.
pub struct UnwindRegistration {
Expand All @@ -14,6 +13,8 @@ extern "C" {
// libunwind import
fn __register_frame(fde: *const u8);
fn __deregister_frame(fde: *const u8);
#[wasmtime_versioned_export_macros::versioned_link]
fn wasmtime_using_libunwind() -> bool;
}

/// There are two primary unwinders on Unix platforms: libunwind and libgcc.
Expand All @@ -33,43 +34,10 @@ extern "C" {
/// 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(ptr::null_mut(), c"__unw_add_dynamic_fde".as_ptr()).is_null()
};
USING_LIBUNWIND.store(
if looks_like_libunwind {
LIBUNWIND_YES
} else {
LIBUNWIND_NO
},
Relaxed,
);
looks_like_libunwind
}
_ => unreachable!(),
}
// Otherwise delegate to `helpers.c` since weak symbols can't be used from
// Rust at this time.
cfg!(target_os = "macos") || unsafe { wasmtime_using_libunwind() }
}

impl UnwindRegistration {
Expand Down