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//! 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 crate::ffi::c_int;
use libc::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
#[cfg(target_arch = "loongarch64")]
const UNWIND_DATA_REG: (i32, i32) = (4, 5); // a0, a1
// 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 = "tvos"), 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(_) | EHAction::Filter(_) => {
// 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::Filter(_) if state & uw::_US_FORCE_UNWIND as c_int != 0 => return continue_unwind(exception_object, context),
EHAction::Cleanup(lpad) | EHAction::Catch(lpad) | EHAction::Filter(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(_) | EHAction::Filter(_) => uw::_URC_HANDLER_FOUND,
EHAction::Terminate => uw::_URC_FATAL_PHASE1_ERROR,
}
} else {
match eh_action {
EHAction::None => uw::_URC_CONTINUE_UNWIND,
// Forced unwinding hits a terminate action.
EHAction::Filter(_) if actions as i32 & uw::_UA_FORCE_UNWIND as i32 != 0 => uw::_URC_CONTINUE_UNWIND,
EHAction::Cleanup(lpad) | EHAction::Catch(lpad) | EHAction::Filter(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, any(target_arch = "aarch64", 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)
}