src/runtime/signal_windows.go - go - Git at Google

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package runtime

 import (
 	"internal/abi"
 	"runtime/internal/sys"
 	"unsafe"
 )

 const (
 	_SEM_FAILCRITICALERRORS = 0x0001
 	_SEM_NOGPFAULTERRORBOX  = 0x0002
 	_SEM_NOOPENFILEERRORBOX = 0x8000

 	_WER_FAULT_REPORTING_NO_UI = 0x0020
 )

 func preventErrorDialogs() {
 	errormode := stdcall0(_GetErrorMode)
 	stdcall1(_SetErrorMode, errormode|_SEM_FAILCRITICALERRORS|_SEM_NOGPFAULTERRORBOX|_SEM_NOOPENFILEERRORBOX)

 	// Disable WER fault reporting UI.
 	// Do this even if WER is disabled as a whole,
 	// as WER might be enabled later with setTraceback("wer")
 	// and we still want the fault reporting UI to be disabled if this happens.
 	var werflags uintptr
 	stdcall2(_WerGetFlags, currentProcess, uintptr(unsafe.Pointer(&werflags)))
 	stdcall1(_WerSetFlags, werflags|_WER_FAULT_REPORTING_NO_UI)
 }

 // enableWER re-enables Windows error reporting without fault reporting UI.
 func enableWER() {
 	// re-enable Windows Error Reporting
 	errormode := stdcall0(_GetErrorMode)
 	if errormode&_SEM_NOGPFAULTERRORBOX != 0 {
 		stdcall1(_SetErrorMode, errormode^_SEM_NOGPFAULTERRORBOX)
 	}
 }

 // in sys_windows_386.s, sys_windows_amd64.s, sys_windows_arm.s, and sys_windows_arm64.s
 func exceptiontramp()
 func firstcontinuetramp()
 func lastcontinuetramp()
 func sehtramp()
 func sigresume()

 func initExceptionHandler() {
 	stdcall2(_AddVectoredExceptionHandler, 1, abi.FuncPCABI0(exceptiontramp))
 	if GOARCH == "386" {
 		// use SetUnhandledExceptionFilter for windows-386.
 		// note: SetUnhandledExceptionFilter handler won't be called, if debugging.
 		stdcall1(_SetUnhandledExceptionFilter, abi.FuncPCABI0(lastcontinuetramp))
 	} else {
 		stdcall2(_AddVectoredContinueHandler, 1, abi.FuncPCABI0(firstcontinuetramp))
 		stdcall2(_AddVectoredContinueHandler, 0, abi.FuncPCABI0(lastcontinuetramp))
 	}
 }

 // isAbort returns true, if context r describes exception raised
 // by calling runtime.abort function.
 //
 //go:nosplit
 func isAbort(r *context) bool {
 	pc := r.ip()
 	if GOARCH == "386" || GOARCH == "amd64" || GOARCH == "arm" {
 		// In the case of an abort, the exception IP is one byte after
 		// the INT3 (this differs from UNIX OSes). Note that on ARM,
 		// this means that the exception IP is no longer aligned.
 		pc--
 	}
 	return isAbortPC(pc)
 }

 // isgoexception reports whether this exception should be translated
 // into a Go panic or throw.
 //
 // It is nosplit to avoid growing the stack in case we're aborting
 // because of a stack overflow.
 //
 //go:nosplit
 func isgoexception(info *exceptionrecord, r *context) bool {
 	// Only handle exception if executing instructions in Go binary
 	// (not Windows library code).
 	// TODO(mwhudson): needs to loop to support shared libs
 	if r.ip() < firstmoduledata.text || firstmoduledata.etext < r.ip() {
 		return false
 	}

 	// Go will only handle some exceptions.
 	switch info.exceptioncode {
 	default:
 		return false
 	case _EXCEPTION_ACCESS_VIOLATION:
 	case _EXCEPTION_IN_PAGE_ERROR:
 	case _EXCEPTION_INT_DIVIDE_BY_ZERO:
 	case _EXCEPTION_INT_OVERFLOW:
 	case _EXCEPTION_FLT_DENORMAL_OPERAND:
 	case _EXCEPTION_FLT_DIVIDE_BY_ZERO:
 	case _EXCEPTION_FLT_INEXACT_RESULT:
 	case _EXCEPTION_FLT_OVERFLOW:
 	case _EXCEPTION_FLT_UNDERFLOW:
 	case _EXCEPTION_BREAKPOINT:
 	case _EXCEPTION_ILLEGAL_INSTRUCTION: // breakpoint arrives this way on arm64
 	}
 	return true
 }

 const (
 	callbackVEH = iota
 	callbackFirstVCH
 	callbackLastVCH
 )

 // sigFetchGSafe is like getg() but without panicking
 // when TLS is not set.
 // Only implemented on windows/386, which is the only
 // arch that loads TLS when calling getg(). Others
 // use a dedicated register.
 func sigFetchGSafe() *g

 func sigFetchG() *g {
 	if GOARCH == "386" {
 		return sigFetchGSafe()
 	}
 	return getg()
 }

 // sigtrampgo is called from the exception handler function, sigtramp,
 // written in assembly code.
 // Return EXCEPTION_CONTINUE_EXECUTION if the exception is handled,
 // else return EXCEPTION_CONTINUE_SEARCH.
 //
 // It is nosplit for the same reason as exceptionhandler.
 //
 //go:nosplit
 func sigtrampgo(ep *exceptionpointers, kind int) int32 {
 	gp := sigFetchG()
 	if gp == nil {
 		return _EXCEPTION_CONTINUE_SEARCH
 	}

 	var fn func(info *exceptionrecord, r *context, gp *g) int32
 	switch kind {
 	case callbackVEH:
 		fn = exceptionhandler
 	case callbackFirstVCH:
 		fn = firstcontinuehandler
 	case callbackLastVCH:
 		fn = lastcontinuehandler
 	default:
 		throw("unknown sigtramp callback")
 	}

 	// Check if we are running on g0 stack, and if we are,
 	// call fn directly instead of creating the closure.
 	// for the systemstack argument.
 	//
 	// A closure can't be marked as nosplit, so it might
 	// call morestack if we are at the g0 stack limit.
 	// If that happens, the runtime will call abort
 	// and end up in sigtrampgo again.
 	// TODO: revisit this workaround if/when closures
 	// can be compiled as nosplit.
 	//
 	// Note that this scenario should only occur on
 	// TestG0StackOverflow. Any other occurrence should
 	// be treated as a bug.
 	var ret int32
 	if gp != gp.m.g0 {
 		systemstack(func() {
 			ret = fn(ep.record, ep.context, gp)
 		})
 	} else {
 		ret = fn(ep.record, ep.context, gp)
 	}
 	if ret == _EXCEPTION_CONTINUE_SEARCH {
 		return ret
 	}

 	// Check if we need to set up the control flow guard workaround.
 	// On Windows, the stack pointer in the context must lie within
 	// system stack limits when we resume from exception.
 	// Store the resume SP and PC in alternate registers
 	// and return to sigresume on the g0 stack.
 	// sigresume makes no use of the stack at all,
 	// loading SP from RX and jumping to RY, being RX and RY two scratch registers.
 	// Note that blindly smashing RX and RY is only safe because we know sigpanic
 	// will not actually return to the original frame, so the registers
 	// are effectively dead. But this does mean we can't use the
 	// same mechanism for async preemption.
 	if ep.context.ip() == abi.FuncPCABI0(sigresume) {
 		// sigresume has already been set up by a previous exception.
 		return ret
 	}
 	prepareContextForSigResume(ep.context)
 	ep.context.set_sp(gp.m.g0.sched.sp)
 	ep.context.set_ip(abi.FuncPCABI0(sigresume))
 	return ret
 }

 // Called by sigtramp from Windows VEH handler.
 // Return value signals whether the exception has been handled (EXCEPTION_CONTINUE_EXECUTION)
 // or should be made available to other handlers in the chain (EXCEPTION_CONTINUE_SEARCH).
 //
 // This is nosplit to avoid growing the stack until we've checked for
 // _EXCEPTION_BREAKPOINT, which is raised by abort() if we overflow the g0 stack.
 //
 //go:nosplit
 func exceptionhandler(info *exceptionrecord, r *context, gp *g) int32 {
 	if !isgoexception(info, r) {
 		return _EXCEPTION_CONTINUE_SEARCH
 	}

 	if gp.throwsplit || isAbort(r) {
 		// We can't safely sigpanic because it may grow the stack.
 		// Or this is a call to abort.
 		// Don't go through any more of the Windows handler chain.
 		// Crash now.
 		winthrow(info, r, gp)
 	}

 	// After this point, it is safe to grow the stack.

 	// Make it look like a call to the signal func.
 	// Have to pass arguments out of band since
 	// augmenting the stack frame would break
 	// the unwinding code.
 	gp.sig = info.exceptioncode
 	gp.sigcode0 = info.exceptioninformation[0]
 	gp.sigcode1 = info.exceptioninformation[1]
 	gp.sigpc = r.ip()

 	// Only push runtime·sigpanic if r.ip() != 0.
 	// If r.ip() == 0, probably panicked because of a
 	// call to a nil func. Not pushing that onto sp will
 	// make the trace look like a call to runtime·sigpanic instead.
 	// (Otherwise the trace will end at runtime·sigpanic and we
 	// won't get to see who faulted.)
 	// Also don't push a sigpanic frame if the faulting PC
 	// is the entry of asyncPreempt. In this case, we suspended
 	// the thread right between the fault and the exception handler
 	// starting to run, and we have pushed an asyncPreempt call.
 	// The exception is not from asyncPreempt, so not to push a
 	// sigpanic call to make it look like that. Instead, just
 	// overwrite the PC. (See issue #35773)
 	if r.ip() != 0 && r.ip() != abi.FuncPCABI0(asyncPreempt) {
 		sp := unsafe.Pointer(r.sp())
 		delta := uintptr(sys.StackAlign)
 		sp = add(sp, -delta)
 		r.set_sp(uintptr(sp))
 		if usesLR {
 			*((*uintptr)(sp)) = r.lr()
 			r.set_lr(r.ip())
 		} else {
 			*((*uintptr)(sp)) = r.ip()
 		}
 	}
 	r.set_ip(abi.FuncPCABI0(sigpanic0))
 	return _EXCEPTION_CONTINUE_EXECUTION
 }

 // sehhandler is reached as part of the SEH chain.
 //
 // It is nosplit for the same reason as exceptionhandler.
 //
 //go:nosplit
 func sehhandler(_ *exceptionrecord, _ uint64, _ *context, dctxt *_DISPATCHER_CONTEXT) int32 {
 	g0 := getg()
 	if g0 == nil || g0.m.curg == nil {
 		// No g available, nothing to do here.
 		return _EXCEPTION_CONTINUE_SEARCH_SEH
 	}
 	// The Windows SEH machinery will unwind the stack until it finds
 	// a frame with a handler for the exception or until the frame is
 	// outside the stack boundaries, in which case it will call the
 	// UnhandledExceptionFilter. Unfortunately, it doesn't know about
 	// the goroutine stack, so it will stop unwinding when it reaches the
 	// first frame not running in g0. As a result, neither non-Go exceptions
 	// handlers higher up the stack nor UnhandledExceptionFilter will be called.
 	//
 	// To work around this, manually unwind the stack until the top of the goroutine
 	// stack is reached, and then pass the control back to Windows.
 	gp := g0.m.curg
 	ctxt := dctxt.ctx()
 	var base, sp uintptr
 	for {
 		entry := stdcall3(_RtlLookupFunctionEntry, ctxt.ip(), uintptr(unsafe.Pointer(&base)), 0)
 		if entry == 0 {
 			break
 		}
 		stdcall8(_RtlVirtualUnwind, 0, base, ctxt.ip(), entry, uintptr(unsafe.Pointer(ctxt)), 0, uintptr(unsafe.Pointer(&sp)), 0)
 		if sp < gp.stack.lo || gp.stack.hi <= sp {
 			break
 		}
 	}
 	return _EXCEPTION_CONTINUE_SEARCH_SEH
 }

 // It seems Windows searches ContinueHandler's list even
 // if ExceptionHandler returns EXCEPTION_CONTINUE_EXECUTION.
 // firstcontinuehandler will stop that search,
 // if exceptionhandler did the same earlier.
 //
 // It is nosplit for the same reason as exceptionhandler.
 //
 //go:nosplit
 func firstcontinuehandler(info *exceptionrecord, r *context, gp *g) int32 {
 	if !isgoexception(info, r) {
 		return _EXCEPTION_CONTINUE_SEARCH
 	}
 	return _EXCEPTION_CONTINUE_EXECUTION
 }

 // lastcontinuehandler is reached, because runtime cannot handle
 // current exception. lastcontinuehandler will print crash info and exit.
 //
 // It is nosplit for the same reason as exceptionhandler.
 //
 //go:nosplit
 func lastcontinuehandler(info *exceptionrecord, r *context, gp *g) int32 {
 	if islibrary || isarchive {
 		// Go DLL/archive has been loaded in a non-go program.
 		// If the exception does not originate from go, the go runtime
 		// should not take responsibility of crashing the process.
 		return _EXCEPTION_CONTINUE_SEARCH
 	}

 	// VEH is called before SEH, but arm64 MSVC DLLs use SEH to trap
 	// illegal instructions during runtime initialization to determine
 	// CPU features, so if we make it to the last handler and we're
 	// arm64 and it's an illegal instruction and this is coming from
 	// non-Go code, then assume it's this runtime probing happen, and
 	// pass that onward to SEH.
 	if GOARCH == "arm64" && info.exceptioncode == _EXCEPTION_ILLEGAL_INSTRUCTION &&
 		(r.ip() < firstmoduledata.text || firstmoduledata.etext < r.ip()) {
 		return _EXCEPTION_CONTINUE_SEARCH
 	}

 	winthrow(info, r, gp)
 	return 0 // not reached
 }

 // Always called on g0. gp is the G where the exception occurred.
 //
 //go:nosplit
 func winthrow(info *exceptionrecord, r *context, gp *g) {
 	g0 := getg()

 	if panicking.Load() != 0 { // traceback already printed
 		exit(2)
 	}
 	panicking.Store(1)

 	// In case we're handling a g0 stack overflow, blow away the
 	// g0 stack bounds so we have room to print the traceback. If
 	// this somehow overflows the stack, the OS will trap it.
 	g0.stack.lo = 0
 	g0.stackguard0 = g0.stack.lo + stackGuard
 	g0.stackguard1 = g0.stackguard0

 	print("Exception ", hex(info.exceptioncode), " ", hex(info.exceptioninformation[0]), " ", hex(info.exceptioninformation[1]), " ", hex(r.ip()), "\n")

 	print("PC=", hex(r.ip()), "\n")
 	if g0.m.incgo && gp == g0.m.g0 && g0.m.curg != nil {
 		if iscgo {
 			print("signal arrived during external code execution\n")
 		}
 		gp = g0.m.curg
 	}
 	print("\n")

 	g0.m.throwing = throwTypeRuntime
 	g0.m.caughtsig.set(gp)

 	level, _, docrash := gotraceback()
 	if level > 0 {
 		tracebacktrap(r.ip(), r.sp(), r.lr(), gp)
 		tracebackothers(gp)
 		dumpregs(r)
 	}

 	if docrash {
 		dieFromException(info, r)
 	}

 	exit(2)
 }

 func sigpanic() {
 	gp := getg()
 	if !canpanic() {
 		throw("unexpected signal during runtime execution")
 	}

 	switch gp.sig {
 	case _EXCEPTION_ACCESS_VIOLATION, _EXCEPTION_IN_PAGE_ERROR:
 		if gp.sigcode1 < 0x1000 {
 			panicmem()
 		}
 		if gp.paniconfault {
 			panicmemAddr(gp.sigcode1)
 		}
 		if inUserArenaChunk(gp.sigcode1) {
 			// We could check that the arena chunk is explicitly set to fault,
 			// but the fact that we faulted on accessing it is enough to prove
 			// that it is.
 			print("accessed data from freed user arena ", hex(gp.sigcode1), "\n")
 		} else {
 			print("unexpected fault address ", hex(gp.sigcode1), "\n")
 		}
 		throw("fault")
 	case _EXCEPTION_INT_DIVIDE_BY_ZERO:
 		panicdivide()
 	case _EXCEPTION_INT_OVERFLOW:
 		panicoverflow()
 	case _EXCEPTION_FLT_DENORMAL_OPERAND,
 		_EXCEPTION_FLT_DIVIDE_BY_ZERO,
 		_EXCEPTION_FLT_INEXACT_RESULT,
 		_EXCEPTION_FLT_OVERFLOW,
 		_EXCEPTION_FLT_UNDERFLOW:
 		panicfloat()
 	}
 	throw("fault")
 }

 // Following are not implemented.

 func initsig(preinit bool) {
 }

 func sigenable(sig uint32) {
 }

 func sigdisable(sig uint32) {
 }

 func sigignore(sig uint32) {
 }

 func signame(sig uint32) string {
 	return ""
 }

 //go:nosplit
 func crash() {
 	dieFromException(nil, nil)
 }

 // dieFromException raises an exception that bypasses all exception handlers.
 // This provides the expected exit status for the shell.
 //
 //go:nosplit
 func dieFromException(info *exceptionrecord, r *context) {
 	if info == nil {
 		gp := getg()
 		if gp.sig != 0 {
 			// Try to reconstruct an exception record from
 			// the exception information stored in gp.
 			info = &exceptionrecord{
 				exceptionaddress: gp.sigpc,
 				exceptioncode:    gp.sig,
 				numberparameters: 2,
 			}
 			info.exceptioninformation[0] = gp.sigcode0
 			info.exceptioninformation[1] = gp.sigcode1
 		} else {
 			// By default, a failing Go application exits with exit code 2.
 			// Use this value when gp does not contain exception info.
 			info = &exceptionrecord{
 				exceptioncode: 2,
 			}
 		}
 	}
 	const FAIL_FAST_GENERATE_EXCEPTION_ADDRESS = 0x1
 	stdcall3(_RaiseFailFastException, uintptr(unsafe.Pointer(info)), uintptr(unsafe.Pointer(r)), FAIL_FAST_GENERATE_EXCEPTION_ADDRESS)
 }

 // gsignalStack is unused on Windows.
 type gsignalStack struct{}
	// Copyright 2011 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package runtime

	import (
	"internal/abi"
	"runtime/internal/sys"
	"unsafe"
	)

	const (
	_SEM_FAILCRITICALERRORS = 0x0001
	_SEM_NOGPFAULTERRORBOX = 0x0002
	_SEM_NOOPENFILEERRORBOX = 0x8000

	_WER_FAULT_REPORTING_NO_UI = 0x0020
	)

	func preventErrorDialogs() {
	errormode := stdcall0(_GetErrorMode)
	stdcall1(_SetErrorMode, errormode\|_SEM_FAILCRITICALERRORS\|_SEM_NOGPFAULTERRORBOX\|_SEM_NOOPENFILEERRORBOX)

	// Disable WER fault reporting UI.
	// Do this even if WER is disabled as a whole,
	// as WER might be enabled later with setTraceback("wer")
	// and we still want the fault reporting UI to be disabled if this happens.
	var werflags uintptr
	stdcall2(_WerGetFlags, currentProcess, uintptr(unsafe.Pointer(&werflags)))
	stdcall1(_WerSetFlags, werflags\|_WER_FAULT_REPORTING_NO_UI)
	}

	// enableWER re-enables Windows error reporting without fault reporting UI.
	func enableWER() {
	// re-enable Windows Error Reporting
	errormode := stdcall0(_GetErrorMode)
	if errormode&_SEM_NOGPFAULTERRORBOX != 0 {
	stdcall1(_SetErrorMode, errormode^_SEM_NOGPFAULTERRORBOX)
	}
	}

	// in sys_windows_386.s, sys_windows_amd64.s, sys_windows_arm.s, and sys_windows_arm64.s
	func exceptiontramp()
	func firstcontinuetramp()
	func lastcontinuetramp()
	func sehtramp()
	func sigresume()

	func initExceptionHandler() {
	stdcall2(_AddVectoredExceptionHandler, 1, abi.FuncPCABI0(exceptiontramp))
	if GOARCH == "386" {
	// use SetUnhandledExceptionFilter for windows-386.
	// note: SetUnhandledExceptionFilter handler won't be called, if debugging.
	stdcall1(_SetUnhandledExceptionFilter, abi.FuncPCABI0(lastcontinuetramp))
	} else {
	stdcall2(_AddVectoredContinueHandler, 1, abi.FuncPCABI0(firstcontinuetramp))
	stdcall2(_AddVectoredContinueHandler, 0, abi.FuncPCABI0(lastcontinuetramp))
	}
	}

	// isAbort returns true, if context r describes exception raised
	// by calling runtime.abort function.
	//
	//go:nosplit
	func isAbort(r *context) bool {
	pc := r.ip()
	if GOARCH == "386" \|\| GOARCH == "amd64" \|\| GOARCH == "arm" {
	// In the case of an abort, the exception IP is one byte after
	// the INT3 (this differs from UNIX OSes). Note that on ARM,
	// this means that the exception IP is no longer aligned.
	pc--
	}
	return isAbortPC(pc)
	}

	// isgoexception reports whether this exception should be translated
	// into a Go panic or throw.
	//
	// It is nosplit to avoid growing the stack in case we're aborting
	// because of a stack overflow.
	//
	//go:nosplit
	func isgoexception(info exceptionrecord, r context) bool {
	// Only handle exception if executing instructions in Go binary
	// (not Windows library code).
	// TODO(mwhudson): needs to loop to support shared libs
	if r.ip() < firstmoduledata.text \|\| firstmoduledata.etext < r.ip() {
	return false
	}

	// Go will only handle some exceptions.
	switch info.exceptioncode {
	default:
	return false
	case _EXCEPTION_ACCESS_VIOLATION:
	case _EXCEPTION_IN_PAGE_ERROR:
	case _EXCEPTION_INT_DIVIDE_BY_ZERO:
	case _EXCEPTION_INT_OVERFLOW:
	case _EXCEPTION_FLT_DENORMAL_OPERAND:
	case _EXCEPTION_FLT_DIVIDE_BY_ZERO:
	case _EXCEPTION_FLT_INEXACT_RESULT:
	case _EXCEPTION_FLT_OVERFLOW:
	case _EXCEPTION_FLT_UNDERFLOW:
	case _EXCEPTION_BREAKPOINT:
	case _EXCEPTION_ILLEGAL_INSTRUCTION: // breakpoint arrives this way on arm64
	}
	return true
	}

	const (
	callbackVEH = iota
	callbackFirstVCH
	callbackLastVCH
	)

	// sigFetchGSafe is like getg() but without panicking
	// when TLS is not set.
	// Only implemented on windows/386, which is the only
	// arch that loads TLS when calling getg(). Others
	// use a dedicated register.
	func sigFetchGSafe() *g

	func sigFetchG() *g {
	if GOARCH == "386" {
	return sigFetchGSafe()
	}
	return getg()
	}

	// sigtrampgo is called from the exception handler function, sigtramp,
	// written in assembly code.
	// Return EXCEPTION_CONTINUE_EXECUTION if the exception is handled,
	// else return EXCEPTION_CONTINUE_SEARCH.
	//
	// It is nosplit for the same reason as exceptionhandler.
	//
	//go:nosplit
	func sigtrampgo(ep *exceptionpointers, kind int) int32 {
	gp := sigFetchG()
	if gp == nil {
	return _EXCEPTION_CONTINUE_SEARCH
	}

	var fn func(info exceptionrecord, r context, gp *g) int32
	switch kind {
	case callbackVEH:
	fn = exceptionhandler
	case callbackFirstVCH:
	fn = firstcontinuehandler
	case callbackLastVCH:
	fn = lastcontinuehandler
	default:
	throw("unknown sigtramp callback")
	}

	// Check if we are running on g0 stack, and if we are,
	// call fn directly instead of creating the closure.
	// for the systemstack argument.
	//
	// A closure can't be marked as nosplit, so it might
	// call morestack if we are at the g0 stack limit.
	// If that happens, the runtime will call abort
	// and end up in sigtrampgo again.
	// TODO: revisit this workaround if/when closures
	// can be compiled as nosplit.
	//
	// Note that this scenario should only occur on
	// TestG0StackOverflow. Any other occurrence should
	// be treated as a bug.
	var ret int32
	if gp != gp.m.g0 {
	systemstack(func() {
	ret = fn(ep.record, ep.context, gp)
	})
	} else {
	ret = fn(ep.record, ep.context, gp)
	}
	if ret == _EXCEPTION_CONTINUE_SEARCH {
	return ret
	}

	// Check if we need to set up the control flow guard workaround.
	// On Windows, the stack pointer in the context must lie within
	// system stack limits when we resume from exception.
	// Store the resume SP and PC in alternate registers
	// and return to sigresume on the g0 stack.
	// sigresume makes no use of the stack at all,
	// loading SP from RX and jumping to RY, being RX and RY two scratch registers.
	// Note that blindly smashing RX and RY is only safe because we know sigpanic
	// will not actually return to the original frame, so the registers
	// are effectively dead. But this does mean we can't use the
	// same mechanism for async preemption.
	if ep.context.ip() == abi.FuncPCABI0(sigresume) {
	// sigresume has already been set up by a previous exception.
	return ret
	}
	prepareContextForSigResume(ep.context)
	ep.context.set_sp(gp.m.g0.sched.sp)
	ep.context.set_ip(abi.FuncPCABI0(sigresume))
	return ret
	}

	// Called by sigtramp from Windows VEH handler.
	// Return value signals whether the exception has been handled (EXCEPTION_CONTINUE_EXECUTION)
	// or should be made available to other handlers in the chain (EXCEPTION_CONTINUE_SEARCH).
	//
	// This is nosplit to avoid growing the stack until we've checked for
	// _EXCEPTION_BREAKPOINT, which is raised by abort() if we overflow the g0 stack.
	//
	//go:nosplit
	func exceptionhandler(info exceptionrecord, r context, gp *g) int32 {
	if !isgoexception(info, r) {
	return _EXCEPTION_CONTINUE_SEARCH
	}

	if gp.throwsplit \|\| isAbort(r) {
	// We can't safely sigpanic because it may grow the stack.
	// Or this is a call to abort.
	// Don't go through any more of the Windows handler chain.
	// Crash now.
	winthrow(info, r, gp)
	}

	// After this point, it is safe to grow the stack.

	// Make it look like a call to the signal func.
	// Have to pass arguments out of band since
	// augmenting the stack frame would break
	// the unwinding code.
	gp.sig = info.exceptioncode
	gp.sigcode0 = info.exceptioninformation[0]
	gp.sigcode1 = info.exceptioninformation[1]
	gp.sigpc = r.ip()

	// Only push runtime·sigpanic if r.ip() != 0.
	// If r.ip() == 0, probably panicked because of a
	// call to a nil func. Not pushing that onto sp will
	// make the trace look like a call to runtime·sigpanic instead.
	// (Otherwise the trace will end at runtime·sigpanic and we
	// won't get to see who faulted.)
	// Also don't push a sigpanic frame if the faulting PC
	// is the entry of asyncPreempt. In this case, we suspended
	// the thread right between the fault and the exception handler
	// starting to run, and we have pushed an asyncPreempt call.
	// The exception is not from asyncPreempt, so not to push a
	// sigpanic call to make it look like that. Instead, just
	// overwrite the PC. (See issue #35773)
	if r.ip() != 0 && r.ip() != abi.FuncPCABI0(asyncPreempt) {
	sp := unsafe.Pointer(r.sp())
	delta := uintptr(sys.StackAlign)
	sp = add(sp, -delta)
	r.set_sp(uintptr(sp))
	if usesLR {
	((uintptr)(sp)) = r.lr()
	r.set_lr(r.ip())
	} else {
	((uintptr)(sp)) = r.ip()
	}
	}
	r.set_ip(abi.FuncPCABI0(sigpanic0))
	return _EXCEPTION_CONTINUE_EXECUTION
	}

	// sehhandler is reached as part of the SEH chain.
	//
	// It is nosplit for the same reason as exceptionhandler.
	//
	//go:nosplit
	func sehhandler(_ exceptionrecord, _ uint64, _ context, dctxt *_DISPATCHER_CONTEXT) int32 {
	g0 := getg()
	if g0 == nil \|\| g0.m.curg == nil {
	// No g available, nothing to do here.
	return _EXCEPTION_CONTINUE_SEARCH_SEH
	}
	// The Windows SEH machinery will unwind the stack until it finds
	// a frame with a handler for the exception or until the frame is
	// outside the stack boundaries, in which case it will call the
	// UnhandledExceptionFilter. Unfortunately, it doesn't know about
	// the goroutine stack, so it will stop unwinding when it reaches the
	// first frame not running in g0. As a result, neither non-Go exceptions
	// handlers higher up the stack nor UnhandledExceptionFilter will be called.
	//
	// To work around this, manually unwind the stack until the top of the goroutine
	// stack is reached, and then pass the control back to Windows.
	gp := g0.m.curg
	ctxt := dctxt.ctx()
	var base, sp uintptr
	for {
	entry := stdcall3(_RtlLookupFunctionEntry, ctxt.ip(), uintptr(unsafe.Pointer(&base)), 0)
	if entry == 0 {
	break
	}
	stdcall8(_RtlVirtualUnwind, 0, base, ctxt.ip(), entry, uintptr(unsafe.Pointer(ctxt)), 0, uintptr(unsafe.Pointer(&sp)), 0)
	if sp < gp.stack.lo \|\| gp.stack.hi <= sp {
	break
	}
	}
	return _EXCEPTION_CONTINUE_SEARCH_SEH
	}

	// It seems Windows searches ContinueHandler's list even
	// if ExceptionHandler returns EXCEPTION_CONTINUE_EXECUTION.
	// firstcontinuehandler will stop that search,
	// if exceptionhandler did the same earlier.
	//
	// It is nosplit for the same reason as exceptionhandler.
	//
	//go:nosplit
	func firstcontinuehandler(info exceptionrecord, r context, gp *g) int32 {
	if !isgoexception(info, r) {
	return _EXCEPTION_CONTINUE_SEARCH
	}
	return _EXCEPTION_CONTINUE_EXECUTION
	}

	// lastcontinuehandler is reached, because runtime cannot handle
	// current exception. lastcontinuehandler will print crash info and exit.
	//
	// It is nosplit for the same reason as exceptionhandler.
	//
	//go:nosplit
	func lastcontinuehandler(info exceptionrecord, r context, gp *g) int32 {
	if islibrary \|\| isarchive {
	// Go DLL/archive has been loaded in a non-go program.
	// If the exception does not originate from go, the go runtime
	// should not take responsibility of crashing the process.
	return _EXCEPTION_CONTINUE_SEARCH
	}

	// VEH is called before SEH, but arm64 MSVC DLLs use SEH to trap
	// illegal instructions during runtime initialization to determine
	// CPU features, so if we make it to the last handler and we're
	// arm64 and it's an illegal instruction and this is coming from
	// non-Go code, then assume it's this runtime probing happen, and
	// pass that onward to SEH.
	if GOARCH == "arm64" && info.exceptioncode == _EXCEPTION_ILLEGAL_INSTRUCTION &&
	(r.ip() < firstmoduledata.text \|\| firstmoduledata.etext < r.ip()) {
	return _EXCEPTION_CONTINUE_SEARCH
	}

	winthrow(info, r, gp)
	return 0 // not reached
	}

	// Always called on g0. gp is the G where the exception occurred.
	//
	//go:nosplit
	func winthrow(info exceptionrecord, r context, gp *g) {
	g0 := getg()

	if panicking.Load() != 0 { // traceback already printed
	exit(2)
	}
	panicking.Store(1)

	// In case we're handling a g0 stack overflow, blow away the
	// g0 stack bounds so we have room to print the traceback. If
	// this somehow overflows the stack, the OS will trap it.
	g0.stack.lo = 0
	g0.stackguard0 = g0.stack.lo + stackGuard
	g0.stackguard1 = g0.stackguard0

	print("Exception ", hex(info.exceptioncode), " ", hex(info.exceptioninformation[0]), " ", hex(info.exceptioninformation[1]), " ", hex(r.ip()), "\n")

	print("PC=", hex(r.ip()), "\n")
	if g0.m.incgo && gp == g0.m.g0 && g0.m.curg != nil {
	if iscgo {
	print("signal arrived during external code execution\n")
	}
	gp = g0.m.curg
	}
	print("\n")

	g0.m.throwing = throwTypeRuntime
	g0.m.caughtsig.set(gp)

	level, _, docrash := gotraceback()
	if level > 0 {
	tracebacktrap(r.ip(), r.sp(), r.lr(), gp)
	tracebackothers(gp)
	dumpregs(r)
	}

	if docrash {
	dieFromException(info, r)
	}

	exit(2)
	}

	func sigpanic() {
	gp := getg()
	if !canpanic() {
	throw("unexpected signal during runtime execution")
	}

	switch gp.sig {
	case _EXCEPTION_ACCESS_VIOLATION, _EXCEPTION_IN_PAGE_ERROR:
	if gp.sigcode1 < 0x1000 {
	panicmem()
	}
	if gp.paniconfault {
	panicmemAddr(gp.sigcode1)
	}
	if inUserArenaChunk(gp.sigcode1) {
	// We could check that the arena chunk is explicitly set to fault,
	// but the fact that we faulted on accessing it is enough to prove
	// that it is.
	print("accessed data from freed user arena ", hex(gp.sigcode1), "\n")
	} else {
	print("unexpected fault address ", hex(gp.sigcode1), "\n")
	}
	throw("fault")
	case _EXCEPTION_INT_DIVIDE_BY_ZERO:
	panicdivide()
	case _EXCEPTION_INT_OVERFLOW:
	panicoverflow()
	case _EXCEPTION_FLT_DENORMAL_OPERAND,
	_EXCEPTION_FLT_DIVIDE_BY_ZERO,
	_EXCEPTION_FLT_INEXACT_RESULT,
	_EXCEPTION_FLT_OVERFLOW,
	_EXCEPTION_FLT_UNDERFLOW:
	panicfloat()
	}
	throw("fault")
	}

	// Following are not implemented.

	func initsig(preinit bool) {
	}

	func sigenable(sig uint32) {
	}

	func sigdisable(sig uint32) {
	}

	func sigignore(sig uint32) {
	}

	func signame(sig uint32) string {
	return ""
	}

	//go:nosplit
	func crash() {
	dieFromException(nil, nil)
	}

	// dieFromException raises an exception that bypasses all exception handlers.
	// This provides the expected exit status for the shell.
	//
	//go:nosplit
	func dieFromException(info exceptionrecord, r context) {
	if info == nil {
	gp := getg()
	if gp.sig != 0 {
	// Try to reconstruct an exception record from
	// the exception information stored in gp.
	info = &exceptionrecord{
	exceptionaddress: gp.sigpc,
	exceptioncode: gp.sig,
	numberparameters: 2,
	}
	info.exceptioninformation[0] = gp.sigcode0
	info.exceptioninformation[1] = gp.sigcode1
	} else {
	// By default, a failing Go application exits with exit code 2.
	// Use this value when gp does not contain exception info.
	info = &exceptionrecord{
	exceptioncode: 2,
	}
	}
	}
	const FAIL_FAST_GENERATE_EXCEPTION_ADDRESS = 0x1
	stdcall3(_RaiseFailFastException, uintptr(unsafe.Pointer(info)), uintptr(unsafe.Pointer(r)), FAIL_FAST_GENERATE_EXCEPTION_ADDRESS)
	}

	// gsignalStack is unused on Windows.
	type gsignalStack struct{}