Auto merge of #92845 - Amanieu:std_personality, r=Mark-Simulacrum

Move EH personality functions to std

These were previously in the panic_unwind crate with dummy stubs in the
panic_abort crate. However it turns out that this is insufficient: we
still need a proper personality function even with -C panic=abort to
handle the following cases:

1) `extern "C-unwind"` still needs to catch foreign exceptions with -C
panic=abort to turn them into aborts. This requires landing pads and a
personality function.

2) ARM EHABI uses the personality function when creating backtraces.
The dummy personality function in panic_abort was causing backtrace
generation to get stuck in a loop since the personality function is
responsible for advancing the unwind state to the next frame.

Fixes #41004

7 files changed, 630 insertions, 0 deletions

diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 50e3acc9400..c918c9ba818 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -594,6 +594,7 @@ pub mod alloc;
 
 // Private support modules
 mod panicking;
+mod personality;
 
 #[path = "../../backtrace/src/lib.rs"]
 #[allow(dead_code, unused_attributes)]
diff --git a/library/std/src/personality.rs b/library/std/src/personality.rs
new file mode 100644
index 00000000000..63f0ad4f16e
--- /dev/null
+++ b/library/std/src/personality.rs
@@ -0,0 +1,46 @@
+//! This module contains the implementation of the `eh_personality` lang item.
+//!
+//! The actual implementation is heavily dependent on the target since Rust
+//! tries to use the native stack unwinding mechanism whenever possible.
+//!
+//! This personality function is still required with `-C panic=abort` because
+//! it is used to catch foreign exceptions from `extern "C-unwind"` and turn
+//! them into aborts.
+//!
+//! Additionally, ARM EHABI uses the personality function when generating
+//! backtraces.
+
+mod dwarf;
+
+#[cfg(not(test))]
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "emscripten")] {
+        mod emcc;
+    } else if #[cfg(target_env = "msvc")] {
+        // This is required by the compiler to exist (e.g., it's a lang item),
+        // but it's never actually called by the compiler because
+        // _CxxFrameHandler3 is the personality function that is always used.
+        // Hence this is just an aborting stub.
+        #[lang = "eh_personality"]
+        fn rust_eh_personality() {
+            core::intrinsics::abort()
+        }
+    } else if #[cfg(any(
+        all(target_family = "windows", target_env = "gnu"),
+        target_os = "psp",
+        target_os = "solid_asp3",
+        all(target_family = "unix", not(target_os = "espidf")),
+        all(target_vendor = "fortanix", target_env = "sgx"),
+    ))] {
+        mod gcc;
+    } else {
+        // Targets that don't support unwinding.
+        // - family=wasm
+        // - os=none ("bare metal" targets)
+        // - os=uefi
+        // - os=espidf
+        // - os=hermit
+        // - nvptx64-nvidia-cuda
+        // - arch=avr
+    }
+}
diff --git a/library/std/src/personality/dwarf/eh.rs b/library/std/src/personality/dwarf/eh.rs
new file mode 100644
index 00000000000..8799137b78f
--- /dev/null
+++ b/library/std/src/personality/dwarf/eh.rs
@@ -0,0 +1,192 @@
+//! Parsing of GCC-style Language-Specific Data Area (LSDA)
+//! For details see:
+//!  * <https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-PDA/LSB-PDA/ehframechpt.html>
+//!  * <https://itanium-cxx-abi.github.io/cxx-abi/exceptions.pdf>
+//!  * <https://www.airs.com/blog/archives/460>
+//!  * <https://www.airs.com/blog/archives/464>
+//!
+//! A reference implementation may be found in the GCC source tree
+//! (`<root>/libgcc/unwind-c.c` as of this writing).
+
+#![allow(non_upper_case_globals)]
+#![allow(unused)]
+
+use super::DwarfReader;
+use core::mem;
+
+pub const DW_EH_PE_omit: u8 = 0xFF;
+pub const DW_EH_PE_absptr: u8 = 0x00;
+
+pub const DW_EH_PE_uleb128: u8 = 0x01;
+pub const DW_EH_PE_udata2: u8 = 0x02;
+pub const DW_EH_PE_udata4: u8 = 0x03;
+pub const DW_EH_PE_udata8: u8 = 0x04;
+pub const DW_EH_PE_sleb128: u8 = 0x09;
+pub const DW_EH_PE_sdata2: u8 = 0x0A;
+pub const DW_EH_PE_sdata4: u8 = 0x0B;
+pub const DW_EH_PE_sdata8: u8 = 0x0C;
+
+pub const DW_EH_PE_pcrel: u8 = 0x10;
+pub const DW_EH_PE_textrel: u8 = 0x20;
+pub const DW_EH_PE_datarel: u8 = 0x30;
+pub const DW_EH_PE_funcrel: u8 = 0x40;
+pub const DW_EH_PE_aligned: u8 = 0x50;
+
+pub const DW_EH_PE_indirect: u8 = 0x80;
+
+#[derive(Copy, Clone)]
+pub struct EHContext<'a> {
+    pub ip: usize,                             // Current instruction pointer
+    pub func_start: usize,                     // Address of the current function
+    pub get_text_start: &'a dyn Fn() -> usize, // Get address of the code section
+    pub get_data_start: &'a dyn Fn() -> usize, // Get address of the data section
+}
+
+pub enum EHAction {
+    None,
+    Cleanup(usize),
+    Catch(usize),
+    Terminate,
+}
+
+pub const USING_SJLJ_EXCEPTIONS: bool = cfg!(all(target_os = "ios", target_arch = "arm"));
+
+pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>) -> Result<EHAction, ()> {
+    if lsda.is_null() {
+        return Ok(EHAction::None);
+    }
+
+    let func_start = context.func_start;
+    let mut reader = DwarfReader::new(lsda);
+
+    let start_encoding = reader.read::<u8>();
+    // base address for landing pad offsets
+    let lpad_base = if start_encoding != DW_EH_PE_omit {
+        read_encoded_pointer(&mut reader, context, start_encoding)?
+    } else {
+        func_start
+    };
+
+    let ttype_encoding = reader.read::<u8>();
+    if ttype_encoding != DW_EH_PE_omit {
+        // Rust doesn't analyze exception types, so we don't care about the type table
+        reader.read_uleb128();
+    }
+
+    let call_site_encoding = reader.read::<u8>();
+    let call_site_table_length = reader.read_uleb128();
+    let action_table = reader.ptr.add(call_site_table_length as usize);
+    let ip = context.ip;
+
+    if !USING_SJLJ_EXCEPTIONS {
+        while reader.ptr < action_table {
+            let cs_start = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_len = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_lpad = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_action = reader.read_uleb128();
+            // Callsite table is sorted by cs_start, so if we've passed the ip, we
+            // may stop searching.
+            if ip < func_start + cs_start {
+                break;
+            }
+            if ip < func_start + cs_start + cs_len {
+                if cs_lpad == 0 {
+                    return Ok(EHAction::None);
+                } else {
+                    let lpad = lpad_base + cs_lpad;
+                    return Ok(interpret_cs_action(cs_action, lpad));
+                }
+            }
+        }
+        // Ip is not present in the table.  This should not happen... but it does: issue #35011.
+        // So rather than returning EHAction::Terminate, we do this.
+        Ok(EHAction::None)
+    } else {
+        // SjLj version:
+        // The "IP" is an index into the call-site table, with two exceptions:
+        // -1 means 'no-action', and 0 means 'terminate'.
+        match ip as isize {
+            -1 => return Ok(EHAction::None),
+            0 => return Ok(EHAction::Terminate),
+            _ => (),
+        }
+        let mut idx = ip;
+        loop {
+            let cs_lpad = reader.read_uleb128();
+            let cs_action = reader.read_uleb128();
+            idx -= 1;
+            if idx == 0 {
+                // Can never have null landing pad for sjlj -- that would have
+                // been indicated by a -1 call site index.
+                let lpad = (cs_lpad + 1) as usize;
+                return Ok(interpret_cs_action(cs_action, lpad));
+            }
+        }
+    }
+}
+
+fn interpret_cs_action(cs_action: u64, lpad: usize) -> EHAction {
+    if cs_action == 0 {
+        // If cs_action is 0 then this is a cleanup (Drop::drop). We run these
+        // for both Rust panics and foreign exceptions.
+        EHAction::Cleanup(lpad)
+    } else {
+        // Stop unwinding Rust panics at catch_unwind.
+        EHAction::Catch(lpad)
+    }
+}
+
+#[inline]
+fn round_up(unrounded: usize, align: usize) -> Result<usize, ()> {
+    if align.is_power_of_two() { Ok((unrounded + align - 1) & !(align - 1)) } else { Err(()) }
+}
+
+unsafe fn read_encoded_pointer(
+    reader: &mut DwarfReader,
+    context: &EHContext<'_>,
+    encoding: u8,
+) -> Result<usize, ()> {
+    if encoding == DW_EH_PE_omit {
+        return Err(());
+    }
+
+    // DW_EH_PE_aligned implies it's an absolute pointer value
+    if encoding == DW_EH_PE_aligned {
+        reader.ptr = round_up(reader.ptr as usize, mem::size_of::<usize>())? as *const u8;
+        return Ok(reader.read::<usize>());
+    }
+
+    let mut result = match encoding & 0x0F {
+        DW_EH_PE_absptr => reader.read::<usize>(),
+        DW_EH_PE_uleb128 => reader.read_uleb128() as usize,
+        DW_EH_PE_udata2 => reader.read::<u16>() as usize,
+        DW_EH_PE_udata4 => reader.read::<u32>() as usize,
+        DW_EH_PE_udata8 => reader.read::<u64>() as usize,
+        DW_EH_PE_sleb128 => reader.read_sleb128() as usize,
+        DW_EH_PE_sdata2 => reader.read::<i16>() as usize,
+        DW_EH_PE_sdata4 => reader.read::<i32>() as usize,
+        DW_EH_PE_sdata8 => reader.read::<i64>() as usize,
+        _ => return Err(()),
+    };
+
+    result += match encoding & 0x70 {
+        DW_EH_PE_absptr => 0,
+        // relative to address of the encoded value, despite the name
+        DW_EH_PE_pcrel => reader.ptr as usize,
+        DW_EH_PE_funcrel => {
+            if context.func_start == 0 {
+                return Err(());
+            }
+            context.func_start
+        }
+        DW_EH_PE_textrel => (*context.get_text_start)(),
+        DW_EH_PE_datarel => (*context.get_data_start)(),
+        _ => return Err(()),
+    };
+
+    if encoding & DW_EH_PE_indirect != 0 {
+        result = *(result as *const usize);
+    }
+
+    Ok(result)
+}
diff --git a/library/std/src/personality/dwarf/mod.rs b/library/std/src/personality/dwarf/mod.rs
new file mode 100644
index 00000000000..652fbe95a14
--- /dev/null
+++ b/library/std/src/personality/dwarf/mod.rs
@@ -0,0 +1,73 @@
+//! Utilities for parsing DWARF-encoded data streams.
+//! See <http://www.dwarfstd.org>,
+//! DWARF-4 standard, Section 7 - "Data Representation"
+
+// This module is used only by x86_64-pc-windows-gnu for now, but we
+// are compiling it everywhere to avoid regressions.
+#![allow(unused)]
+
+#[cfg(test)]
+mod tests;
+
+pub mod eh;
+
+use core::mem;
+
+pub struct DwarfReader {
+    pub ptr: *const u8,
+}
+
+#[repr(C, packed)]
+struct Unaligned<T>(T);
+
+impl DwarfReader {
+    pub fn new(ptr: *const u8) -> DwarfReader {
+        DwarfReader { ptr }
+    }
+
+    // DWARF streams are packed, so e.g., a u32 would not necessarily be aligned
+    // on a 4-byte boundary. This may cause problems on platforms with strict
+    // alignment requirements. By wrapping data in a "packed" struct, we are
+    // telling the backend to generate "misalignment-safe" code.
+    pub unsafe fn read<T: Copy>(&mut self) -> T {
+        let Unaligned(result) = *(self.ptr as *const Unaligned<T>);
+        self.ptr = self.ptr.add(mem::size_of::<T>());
+        result
+    }
+
+    // ULEB128 and SLEB128 encodings are defined in Section 7.6 - "Variable
+    // Length Data".
+    pub unsafe fn read_uleb128(&mut self) -> u64 {
+        let mut shift: usize = 0;
+        let mut result: u64 = 0;
+        let mut byte: u8;
+        loop {
+            byte = self.read::<u8>();
+            result |= ((byte & 0x7F) as u64) << shift;
+            shift += 7;
+            if byte & 0x80 == 0 {
+                break;
+            }
+        }
+        result
+    }
+
+    pub unsafe fn read_sleb128(&mut self) -> i64 {
+        let mut shift: u32 = 0;
+        let mut result: u64 = 0;
+        let mut byte: u8;
+        loop {
+            byte = self.read::<u8>();
+            result |= ((byte & 0x7F) as u64) << shift;
+            shift += 7;
+            if byte & 0x80 == 0 {
+                break;
+            }
+        }
+        // sign-extend
+        if shift < u64::BITS && (byte & 0x40) != 0 {
+            result |= (!0 as u64) << shift;
+        }
+        result as i64
+    }
+}
diff --git a/library/std/src/personality/dwarf/tests.rs b/library/std/src/personality/dwarf/tests.rs
new file mode 100644
index 00000000000..1644f37083a
--- /dev/null
+++ b/library/std/src/personality/dwarf/tests.rs
@@ -0,0 +1,19 @@
+use super::*;
+
+#[test]
+fn dwarf_reader() {
+    let encoded: &[u8] = &[1, 2, 3, 4, 5, 6, 7, 0xE5, 0x8E, 0x26, 0x9B, 0xF1, 0x59, 0xFF, 0xFF];
+
+    let mut reader = DwarfReader::new(encoded.as_ptr());
+
+    unsafe {
+        assert!(reader.read::<u8>() == u8::to_be(1u8));
+        assert!(reader.read::<u16>() == u16::to_be(0x0203));
+        assert!(reader.read::<u32>() == u32::to_be(0x04050607));
+
+        assert!(reader.read_uleb128() == 624485);
+        assert!(reader.read_sleb128() == -624485);
+
+        assert!(reader.read::<i8>() == i8::to_be(-1));
+    }
+}
diff --git a/library/std/src/personality/emcc.rs b/library/std/src/personality/emcc.rs
new file mode 100644
index 00000000000..f942bdf18c1
--- /dev/null
+++ b/library/std/src/personality/emcc.rs
@@ -0,0 +1,20 @@
+//! On Emscripten Rust panics are wrapped in C++ exceptions, so we just forward
+//! to `__gxx_personality_v0` which is provided by Emscripten.
+
+use libc::c_int;
+use unwind as uw;
+
+// This is required by the compiler to exist (e.g., it's a lang item), but it's
+// never actually called by the compiler.  Emscripten EH doesn't use a
+// personality function at all, it instead uses __cxa_find_matching_catch.
+// Wasm error handling would use __gxx_personality_wasm0.
+#[lang = "eh_personality"]
+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 {
+    core::intrinsics::abort()
+}
diff --git a/library/std/src/personality/gcc.rs b/library/std/src/personality/gcc.rs
new file mode 100644
index 00000000000..7f0b0439cf0
--- /dev/null
+++ b/library/std/src/personality/gcc.rs
@@ -0,0 +1,279 @@
+//! 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:
+//!  * <https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html>
+//!  * <https://monoinfinito.wordpress.com/series/exception-handling-in-c/>
+//!  * <https://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.
+
+use super::dwarf::eh::{self, EHAction, EHContext};
+use libc::{c_int, uintptr_t};
+use unwind as uw;
+
+// 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 https://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(target_arch = "m68k")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // D0, D1
+
+#[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(any(target_arch = "sparc", target_arch = "sparc64"))]
+const UNWIND_DATA_REG: (i32, i32) = (24, 25); // I0, I1
+
+#[cfg(target_arch = "hexagon")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1
+
+#[cfg(any(target_arch = "riscv64", target_arch = "riscv32"))]
+const UNWIND_DATA_REG: (i32, i32) = (10, 11); // x10, x11
+
+// 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
+
+cfg_if::cfg_if! {
+    if #[cfg(all(target_arch = "arm", not(target_os = "ios"), not(target_os = "watchos"), not(target_os = "netbsd")))] {
+        // ARM EHABI personality routine.
+        // https://infocenter.arm.com/help/topic/com.arm.doc.ihi0038b/IHI0038B_ehabi.pdf
+        //
+        // iOS uses the default routine instead since it uses SjLj unwinding.
+        #[lang = "eh_personality"]
+        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 = match find_eh_action(context) {
+                Ok(action) => action,
+                Err(_) => return uw::_URC_FAILURE,
+            };
+            if search_phase {
+                match eh_action {
+                    EHAction::None | EHAction::Cleanup(_) => {
+                        return continue_unwind(exception_object, context);
+                    }
+                    EHAction::Catch(_) => {
+                        // EHABI requires the personality routine to update the
+                        // SP value in the barrier cache of the exception object.
+                        (*exception_object).private[5] =
+                            uw::_Unwind_GetGR(context, uw::UNWIND_SP_REG);
+                        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;
+            }
+        }
+    } else {
+        // Default personality routine, which is used directly on most targets
+        // and indirectly on Windows x86_64 via SEH.
+        unsafe extern "C" fn rust_eh_personality_impl(
+            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 = match find_eh_action(context) {
+                Ok(action) => action,
+                Err(_) => return uw::_URC_FATAL_PHASE1_ERROR,
+            };
+            if actions as i32 & uw::_UA_SEARCH_PHASE as i32 != 0 {
+                match eh_action {
+                    EHAction::None | EHAction::Cleanup(_) => uw::_URC_CONTINUE_UNWIND,
+                    EHAction::Catch(_) => uw::_URC_HANDLER_FOUND,
+                    EHAction::Terminate => uw::_URC_FATAL_PHASE1_ERROR,
+                }
+            } else {
+                match eh_action {
+                    EHAction::None => 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);
+                        uw::_URC_INSTALL_CONTEXT
+                    }
+                    EHAction::Terminate => uw::_URC_FATAL_PHASE2_ERROR,
+                }
+            }
+        }
+
+        cfg_if::cfg_if! {
+            if #[cfg(all(windows, target_arch = "x86_64", target_env = "gnu"))] {
+                // On x86_64 MinGW targets, the unwinding mechanism is SEH however the unwind
+                // handler data (aka LSDA) uses GCC-compatible encoding.
+                #[lang = "eh_personality"]
+                #[allow(nonstandard_style)]
+                unsafe extern "C" fn rust_eh_personality(
+                    exceptionRecord: *mut uw::EXCEPTION_RECORD,
+                    establisherFrame: uw::LPVOID,
+                    contextRecord: *mut uw::CONTEXT,
+                    dispatcherContext: *mut uw::DISPATCHER_CONTEXT,
+                ) -> uw::EXCEPTION_DISPOSITION {
+                    uw::_GCC_specific_handler(
+                        exceptionRecord,
+                        establisherFrame,
+                        contextRecord,
+                        dispatcherContext,
+                        rust_eh_personality_impl,
+                    )
+                }
+            } else {
+                // The personality routine for most of our targets.
+                #[lang = "eh_personality"]
+                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 {
+                    rust_eh_personality_impl(
+                        version,
+                        actions,
+                        exception_class,
+                        exception_object,
+                        context,
+                    )
+                }
+            }
+        }
+    }
+}
+
+unsafe fn find_eh_action(context: *mut uw::_Unwind_Context) -> Result<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 = -1` has special meaning, so use wrapping sub to allow for that
+        ip: if ip_before_instr != 0 { ip } else { ip.wrapping_sub(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)
+}