runtime/cgo: store M for C-created thread in pthread key

[gostls13.git] / src / runtime / signal_unix.go

// Copyright 2012 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.

//go:build unix

package runtime

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

// sigTabT is the type of an entry in the global sigtable array.
// sigtable is inherently system dependent, and appears in OS-specific files,
// but sigTabT is the same for all Unixy systems.
// The sigtable array is indexed by a system signal number to get the flags
// and printable name of each signal.
type sigTabT struct {
        flags int32
        name  string
}

//go:linkname os_sigpipe os.sigpipe
func os_sigpipe() {
        systemstack(sigpipe)
}

func signame(sig uint32) string {
        if sig >= uint32(len(sigtable)) {
                return ""
        }
        return sigtable[sig].name
}

const (
        _SIG_DFL uintptr = 0
        _SIG_IGN uintptr = 1
)

// sigPreempt is the signal used for non-cooperative preemption.
//
// There's no good way to choose this signal, but there are some
// heuristics:
//
// 1. It should be a signal that's passed-through by debuggers by
// default. On Linux, this is SIGALRM, SIGURG, SIGCHLD, SIGIO,
// SIGVTALRM, SIGPROF, and SIGWINCH, plus some glibc-internal signals.
//
// 2. It shouldn't be used internally by libc in mixed Go/C binaries
// because libc may assume it's the only thing that can handle these
// signals. For example SIGCANCEL or SIGSETXID.
//
// 3. It should be a signal that can happen spuriously without
// consequences. For example, SIGALRM is a bad choice because the
// signal handler can't tell if it was caused by the real process
// alarm or not (arguably this means the signal is broken, but I
// digress). SIGUSR1 and SIGUSR2 are also bad because those are often
// used in meaningful ways by applications.
//
// 4. We need to deal with platforms without real-time signals (like
// macOS), so those are out.
//
// We use SIGURG because it meets all of these criteria, is extremely
// unlikely to be used by an application for its "real" meaning (both
// because out-of-band data is basically unused and because SIGURG
// doesn't report which socket has the condition, making it pretty
// useless), and even if it is, the application has to be ready for
// spurious SIGURG. SIGIO wouldn't be a bad choice either, but is more
// likely to be used for real.
const sigPreempt = _SIGURG

// Stores the signal handlers registered before Go installed its own.
// These signal handlers will be invoked in cases where Go doesn't want to
// handle a particular signal (e.g., signal occurred on a non-Go thread).
// See sigfwdgo for more information on when the signals are forwarded.
//
// This is read by the signal handler; accesses should use
// atomic.Loaduintptr and atomic.Storeuintptr.
var fwdSig [_NSIG]uintptr

// handlingSig is indexed by signal number and is non-zero if we are
// currently handling the signal. Or, to put it another way, whether
// the signal handler is currently set to the Go signal handler or not.
// This is uint32 rather than bool so that we can use atomic instructions.
var handlingSig [_NSIG]uint32

// channels for synchronizing signal mask updates with the signal mask
// thread
var (
        disableSigChan  chan uint32
        enableSigChan   chan uint32
        maskUpdatedChan chan struct{}
)

func init() {
        // _NSIG is the number of signals on this operating system.
        // sigtable should describe what to do for all the possible signals.
        if len(sigtable) != _NSIG {
                print("runtime: len(sigtable)=", len(sigtable), " _NSIG=", _NSIG, "\n")
                throw("bad sigtable len")
        }
}

var signalsOK bool

// Initialize signals.
// Called by libpreinit so runtime may not be initialized.
//
//go:nosplit
//go:nowritebarrierrec
func initsig(preinit bool) {
        if !preinit {
                // It's now OK for signal handlers to run.
                signalsOK = true
        }

        // For c-archive/c-shared this is called by libpreinit with
        // preinit == true.
        if (isarchive || islibrary) && !preinit {
                return
        }

        for i := uint32(0); i < _NSIG; i++ {
                t := &sigtable[i]
                if t.flags == 0 || t.flags&_SigDefault != 0 {
                        continue
                }

                // We don't need to use atomic operations here because
                // there shouldn't be any other goroutines running yet.
                fwdSig[i] = getsig(i)

                if !sigInstallGoHandler(i) {
                        // Even if we are not installing a signal handler,
                        // set SA_ONSTACK if necessary.
                        if fwdSig[i] != _SIG_DFL && fwdSig[i] != _SIG_IGN {
                                setsigstack(i)
                        } else if fwdSig[i] == _SIG_IGN {
                                sigInitIgnored(i)
                        }
                        continue
                }

                handlingSig[i] = 1
                setsig(i, abi.FuncPCABIInternal(sighandler))
        }
}

//go:nosplit
//go:nowritebarrierrec
func sigInstallGoHandler(sig uint32) bool {
        // For some signals, we respect an inherited SIG_IGN handler
        // rather than insist on installing our own default handler.
        // Even these signals can be fetched using the os/signal package.
        switch sig {
        case _SIGHUP, _SIGINT:
                if atomic.Loaduintptr(&fwdSig[sig]) == _SIG_IGN {
                        return false
                }
        }

        if (GOOS == "linux" || GOOS == "android") && !iscgo && sig == sigPerThreadSyscall {
                // sigPerThreadSyscall is the same signal used by glibc for
                // per-thread syscalls on Linux. We use it for the same purpose
                // in non-cgo binaries.
                return true
        }

        t := &sigtable[sig]
        if t.flags&_SigSetStack != 0 {
                return false
        }

        // When built using c-archive or c-shared, only install signal
        // handlers for synchronous signals and SIGPIPE and sigPreempt.
        if (isarchive || islibrary) && t.flags&_SigPanic == 0 && sig != _SIGPIPE && sig != sigPreempt {
                return false
        }

        return true
}

// sigenable enables the Go signal handler to catch the signal sig.
// It is only called while holding the os/signal.handlers lock,
// via os/signal.enableSignal and signal_enable.
func sigenable(sig uint32) {
        if sig >= uint32(len(sigtable)) {
                return
        }

        // SIGPROF is handled specially for profiling.
        if sig == _SIGPROF {
                return
        }

        t := &sigtable[sig]
        if t.flags&_SigNotify != 0 {
                ensureSigM()
                enableSigChan <- sig
                <-maskUpdatedChan
                if atomic.Cas(&handlingSig[sig], 0, 1) {
                        atomic.Storeuintptr(&fwdSig[sig], getsig(sig))
                        setsig(sig, abi.FuncPCABIInternal(sighandler))
                }
        }
}

// sigdisable disables the Go signal handler for the signal sig.
// It is only called while holding the os/signal.handlers lock,
// via os/signal.disableSignal and signal_disable.
func sigdisable(sig uint32) {
        if sig >= uint32(len(sigtable)) {
                return
        }

        // SIGPROF is handled specially for profiling.
        if sig == _SIGPROF {
                return
        }

        t := &sigtable[sig]
        if t.flags&_SigNotify != 0 {
                ensureSigM()
                disableSigChan <- sig
                <-maskUpdatedChan

                // If initsig does not install a signal handler for a
                // signal, then to go back to the state before Notify
                // we should remove the one we installed.
                if !sigInstallGoHandler(sig) {
                        atomic.Store(&handlingSig[sig], 0)
                        setsig(sig, atomic.Loaduintptr(&fwdSig[sig]))
                }
        }
}

// sigignore ignores the signal sig.
// It is only called while holding the os/signal.handlers lock,
// via os/signal.ignoreSignal and signal_ignore.
func sigignore(sig uint32) {
        if sig >= uint32(len(sigtable)) {
                return
        }

        // SIGPROF is handled specially for profiling.
        if sig == _SIGPROF {
                return
        }

        t := &sigtable[sig]
        if t.flags&_SigNotify != 0 {
                atomic.Store(&handlingSig[sig], 0)
                setsig(sig, _SIG_IGN)
        }
}

// clearSignalHandlers clears all signal handlers that are not ignored
// back to the default. This is called by the child after a fork, so that
// we can enable the signal mask for the exec without worrying about
// running a signal handler in the child.
//
//go:nosplit
//go:nowritebarrierrec
func clearSignalHandlers() {
        for i := uint32(0); i < _NSIG; i++ {
                if atomic.Load(&handlingSig[i]) != 0 {
                        setsig(i, _SIG_DFL)
                }
        }
}

// setProcessCPUProfilerTimer is called when the profiling timer changes.
// It is called with prof.signalLock held. hz is the new timer, and is 0 if
// profiling is being disabled. Enable or disable the signal as
// required for -buildmode=c-archive.
func setProcessCPUProfilerTimer(hz int32) {
        if hz != 0 {
                // Enable the Go signal handler if not enabled.
                if atomic.Cas(&handlingSig[_SIGPROF], 0, 1) {
                        h := getsig(_SIGPROF)
                        // If no signal handler was installed before, then we record
                        // _SIG_IGN here. When we turn off profiling (below) we'll start
                        // ignoring SIGPROF signals. We do this, rather than change
                        // to SIG_DFL, because there may be a pending SIGPROF
                        // signal that has not yet been delivered to some other thread.
                        // If we change to SIG_DFL when turning off profiling, the
                        // program will crash when that SIGPROF is delivered. We assume
                        // that programs that use profiling don't want to crash on a
                        // stray SIGPROF. See issue 19320.
                        // We do the change here instead of when turning off profiling,
                        // because there we may race with a signal handler running
                        // concurrently, in particular, sigfwdgo may observe _SIG_DFL and
                        // die. See issue 43828.
                        if h == _SIG_DFL {
                                h = _SIG_IGN
                        }
                        atomic.Storeuintptr(&fwdSig[_SIGPROF], h)
                        setsig(_SIGPROF, abi.FuncPCABIInternal(sighandler))
                }

                var it itimerval
                it.it_interval.tv_sec = 0
                it.it_interval.set_usec(1000000 / hz)
                it.it_value = it.it_interval
                setitimer(_ITIMER_PROF, &it, nil)
        } else {
                setitimer(_ITIMER_PROF, &itimerval{}, nil)

                // If the Go signal handler should be disabled by default,
                // switch back to the signal handler that was installed
                // when we enabled profiling. We don't try to handle the case
                // of a program that changes the SIGPROF handler while Go
                // profiling is enabled.
                if !sigInstallGoHandler(_SIGPROF) {
                        if atomic.Cas(&handlingSig[_SIGPROF], 1, 0) {
                                h := atomic.Loaduintptr(&fwdSig[_SIGPROF])
                                setsig(_SIGPROF, h)
                        }
                }
        }
}

// setThreadCPUProfilerHz makes any thread-specific changes required to
// implement profiling at a rate of hz.
// No changes required on Unix systems when using setitimer.
func setThreadCPUProfilerHz(hz int32) {
        getg().m.profilehz = hz
}

func sigpipe() {
        if signal_ignored(_SIGPIPE) || sigsend(_SIGPIPE) {
                return
        }
        dieFromSignal(_SIGPIPE)
}

// doSigPreempt handles a preemption signal on gp.
func doSigPreempt(gp *g, ctxt *sigctxt) {
        // Check if this G wants to be preempted and is safe to
        // preempt.
        if wantAsyncPreempt(gp) {
                if ok, newpc := isAsyncSafePoint(gp, ctxt.sigpc(), ctxt.sigsp(), ctxt.siglr()); ok {
                        // Adjust the PC and inject a call to asyncPreempt.
                        ctxt.pushCall(abi.FuncPCABI0(asyncPreempt), newpc)
                }
        }

        // Acknowledge the preemption.
        gp.m.preemptGen.Add(1)
        gp.m.signalPending.Store(0)

        if GOOS == "darwin" || GOOS == "ios" {
                pendingPreemptSignals.Add(-1)
        }
}

const preemptMSupported = true

// preemptM sends a preemption request to mp. This request may be
// handled asynchronously and may be coalesced with other requests to
// the M. When the request is received, if the running G or P are
// marked for preemption and the goroutine is at an asynchronous
// safe-point, it will preempt the goroutine. It always atomically
// increments mp.preemptGen after handling a preemption request.
func preemptM(mp *m) {
        // On Darwin, don't try to preempt threads during exec.
        // Issue #41702.
        if GOOS == "darwin" || GOOS == "ios" {
                execLock.rlock()
        }

        if mp.signalPending.CompareAndSwap(0, 1) {
                if GOOS == "darwin" || GOOS == "ios" {
                        pendingPreemptSignals.Add(1)
                }

                // If multiple threads are preempting the same M, it may send many
                // signals to the same M such that it hardly make progress, causing
                // live-lock problem. Apparently this could happen on darwin. See
                // issue #37741.
                // Only send a signal if there isn't already one pending.
                signalM(mp, sigPreempt)
        }

        if GOOS == "darwin" || GOOS == "ios" {
                execLock.runlock()
        }
}

// sigFetchG fetches the value of G safely when running in a signal handler.
// On some architectures, the g value may be clobbered when running in a VDSO.
// See issue #32912.
//
//go:nosplit
func sigFetchG(c *sigctxt) *g {
        switch GOARCH {
        case "arm", "arm64", "ppc64", "ppc64le", "riscv64", "s390x":
                if !iscgo && inVDSOPage(c.sigpc()) {
                        // When using cgo, we save the g on TLS and load it from there
                        // in sigtramp. Just use that.
                        // Otherwise, before making a VDSO call we save the g to the
                        // bottom of the signal stack. Fetch from there.
                        // TODO: in efence mode, stack is sysAlloc'd, so this wouldn't
                        // work.
                        sp := getcallersp()
                        s := spanOf(sp)
                        if s != nil && s.state.get() == mSpanManual && s.base() < sp && sp < s.limit {
                                gp := *(**g)(unsafe.Pointer(s.base()))
                                return gp
                        }
                        return nil
                }
        }
        return getg()
}

// sigtrampgo is called from the signal handler function, sigtramp,
// written in assembly code.
// This is called by the signal handler, and the world may be stopped.
//
// It must be nosplit because getg() is still the G that was running
// (if any) when the signal was delivered, but it's (usually) called
// on the gsignal stack. Until this switches the G to gsignal, the
// stack bounds check won't work.
//
//go:nosplit
//go:nowritebarrierrec
func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
        if sigfwdgo(sig, info, ctx) {
                return
        }
        c := &sigctxt{info, ctx}
        gp := sigFetchG(c)
        setg(gp)
        if gp == nil || (gp.m != nil && gp.m.isExtraInC) {
                if sig == _SIGPROF {
                        // Some platforms (Linux) have per-thread timers, which we use in
                        // combination with the process-wide timer. Avoid double-counting.
                        if validSIGPROF(nil, c) {
                                sigprofNonGoPC(c.sigpc())
                        }
                        return
                }
                if sig == sigPreempt && preemptMSupported && debug.asyncpreemptoff == 0 {
                        // This is probably a signal from preemptM sent
                        // while executing Go code but received while
                        // executing non-Go code.
                        // We got past sigfwdgo, so we know that there is
                        // no non-Go signal handler for sigPreempt.
                        // The default behavior for sigPreempt is to ignore
                        // the signal, so badsignal will be a no-op anyway.
                        if GOOS == "darwin" || GOOS == "ios" {
                                pendingPreemptSignals.Add(-1)
                        }
                        return
                }
                c.fixsigcode(sig)
                // Set g to nil here and badsignal will use g0 by needm.
                // TODO: reuse the current m here by using the gsignal and adjustSignalStack,
                // since the current g maybe a normal goroutine and actually running on the signal stack,
                // it may hit stack split that is not expected here.
                if gp != nil {
                        setg(nil)
                }
                badsignal(uintptr(sig), c)
                // Restore g
                if gp != nil {
                        setg(gp)
                }
                return
        }

        setg(gp.m.gsignal)

        // If some non-Go code called sigaltstack, adjust.
        var gsignalStack gsignalStack
        setStack := adjustSignalStack(sig, gp.m, &gsignalStack)
        if setStack {
                gp.m.gsignal.stktopsp = getcallersp()
        }

        if gp.stackguard0 == stackFork {
                signalDuringFork(sig)
        }

        c.fixsigcode(sig)
        sighandler(sig, info, ctx, gp)
        setg(gp)
        if setStack {
                restoreGsignalStack(&gsignalStack)
        }
}

// If the signal handler receives a SIGPROF signal on a non-Go thread,
// it tries to collect a traceback into sigprofCallers.
// sigprofCallersUse is set to non-zero while sigprofCallers holds a traceback.
var sigprofCallers cgoCallers
var sigprofCallersUse uint32

// sigprofNonGo is called if we receive a SIGPROF signal on a non-Go thread,
// and the signal handler collected a stack trace in sigprofCallers.
// When this is called, sigprofCallersUse will be non-zero.
// g is nil, and what we can do is very limited.
//
// It is called from the signal handling functions written in assembly code that
// are active for cgo programs, cgoSigtramp and sigprofNonGoWrapper, which have
// not verified that the SIGPROF delivery corresponds to the best available
// profiling source for this thread.
//
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
        if prof.hz.Load() != 0 {
                c := &sigctxt{info, ctx}
                // Some platforms (Linux) have per-thread timers, which we use in
                // combination with the process-wide timer. Avoid double-counting.
                if validSIGPROF(nil, c) {
                        n := 0
                        for n < len(sigprofCallers) && sigprofCallers[n] != 0 {
                                n++
                        }
                        cpuprof.addNonGo(sigprofCallers[:n])
                }
        }

        atomic.Store(&sigprofCallersUse, 0)
}

// sigprofNonGoPC is called when a profiling signal arrived on a
// non-Go thread and we have a single PC value, not a stack trace.
// g is nil, and what we can do is very limited.
//
//go:nosplit
//go:nowritebarrierrec
func sigprofNonGoPC(pc uintptr) {
        if prof.hz.Load() != 0 {
                stk := []uintptr{
                        pc,
                        abi.FuncPCABIInternal(_ExternalCode) + sys.PCQuantum,
                }
                cpuprof.addNonGo(stk)
        }
}

// adjustSignalStack adjusts the current stack guard based on the
// stack pointer that is actually in use while handling a signal.
// We do this in case some non-Go code called sigaltstack.
// This reports whether the stack was adjusted, and if so stores the old
// signal stack in *gsigstack.
//
//go:nosplit
func adjustSignalStack(sig uint32, mp *m, gsigStack *gsignalStack) bool {
        sp := uintptr(unsafe.Pointer(&sig))
        if sp >= mp.gsignal.stack.lo && sp < mp.gsignal.stack.hi {
                return false
        }

        var st stackt
        sigaltstack(nil, &st)
        stsp := uintptr(unsafe.Pointer(st.ss_sp))
        if st.ss_flags&_SS_DISABLE == 0 && sp >= stsp && sp < stsp+st.ss_size {
                setGsignalStack(&st, gsigStack)
                return true
        }

        if sp >= mp.g0.stack.lo && sp < mp.g0.stack.hi {
                // The signal was delivered on the g0 stack.
                // This can happen when linked with C code
                // using the thread sanitizer, which collects
                // signals then delivers them itself by calling
                // the signal handler directly when C code,
                // including C code called via cgo, calls a
                // TSAN-intercepted function such as malloc.
                //
                // We check this condition last as g0.stack.lo
                // may be not very accurate (see mstart).
                st := stackt{ss_size: mp.g0.stack.hi - mp.g0.stack.lo}
                setSignalstackSP(&st, mp.g0.stack.lo)
                setGsignalStack(&st, gsigStack)
                return true
        }

        // sp is not within gsignal stack, g0 stack, or sigaltstack. Bad.
        setg(nil)
        needm()
        if st.ss_flags&_SS_DISABLE != 0 {
                noSignalStack(sig)
        } else {
                sigNotOnStack(sig)
        }
        dropm()
        return false
}

// crashing is the number of m's we have waited for when implementing
// GOTRACEBACK=crash when a signal is received.
var crashing int32

// testSigtrap and testSigusr1 are used by the runtime tests. If
// non-nil, it is called on SIGTRAP/SIGUSR1. If it returns true, the
// normal behavior on this signal is suppressed.
var testSigtrap func(info *siginfo, ctxt *sigctxt, gp *g) bool
var testSigusr1 func(gp *g) bool

// sighandler is invoked when a signal occurs. The global g will be
// set to a gsignal goroutine and we will be running on the alternate
// signal stack. The parameter gp will be the value of the global g
// when the signal occurred. The sig, info, and ctxt parameters are
// from the system signal handler: they are the parameters passed when
// the SA is passed to the sigaction system call.
//
// The garbage collector may have stopped the world, so write barriers
// are not allowed.
//
//go:nowritebarrierrec
func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
        // The g executing the signal handler. This is almost always
        // mp.gsignal. See delayedSignal for an exception.
        gsignal := getg()
        mp := gsignal.m
        c := &sigctxt{info, ctxt}

        // Cgo TSAN (not the Go race detector) intercepts signals and calls the
        // signal handler at a later time. When the signal handler is called, the
        // memory may have changed, but the signal context remains old. The
        // unmatched signal context and memory makes it unsafe to unwind or inspect
        // the stack. So we ignore delayed non-fatal signals that will cause a stack
        // inspection (profiling signal and preemption signal).
        // cgo_yield is only non-nil for TSAN, and is specifically used to trigger
        // signal delivery. We use that as an indicator of delayed signals.
        // For delayed signals, the handler is called on the g0 stack (see
        // adjustSignalStack).
        delayedSignal := *cgo_yield != nil && mp != nil && gsignal.stack == mp.g0.stack

        if sig == _SIGPROF {
                // Some platforms (Linux) have per-thread timers, which we use in
                // combination with the process-wide timer. Avoid double-counting.
                if !delayedSignal && validSIGPROF(mp, c) {
                        sigprof(c.sigpc(), c.sigsp(), c.siglr(), gp, mp)
                }
                return
        }

        if sig == _SIGTRAP && testSigtrap != nil && testSigtrap(info, (*sigctxt)(noescape(unsafe.Pointer(c))), gp) {
                return
        }

        if sig == _SIGUSR1 && testSigusr1 != nil && testSigusr1(gp) {
                return
        }

        if (GOOS == "linux" || GOOS == "android") && sig == sigPerThreadSyscall {
                // sigPerThreadSyscall is the same signal used by glibc for
                // per-thread syscalls on Linux. We use it for the same purpose
                // in non-cgo binaries. Since this signal is not _SigNotify,
                // there is nothing more to do once we run the syscall.
                runPerThreadSyscall()
                return
        }

        if sig == sigPreempt && debug.asyncpreemptoff == 0 && !delayedSignal {
                // Might be a preemption signal.
                doSigPreempt(gp, c)
                // Even if this was definitely a preemption signal, it
                // may have been coalesced with another signal, so we
                // still let it through to the application.
        }

        flags := int32(_SigThrow)
        if sig < uint32(len(sigtable)) {
                flags = sigtable[sig].flags
        }
        if !c.sigFromUser() && flags&_SigPanic != 0 && gp.throwsplit {
                // We can't safely sigpanic because it may grow the
                // stack. Abort in the signal handler instead.
                flags = _SigThrow
        }
        if isAbortPC(c.sigpc()) {
                // On many architectures, the abort function just
                // causes a memory fault. Don't turn that into a panic.
                flags = _SigThrow
        }
        if !c.sigFromUser() && flags&_SigPanic != 0 {
                // The signal is going to cause a panic.
                // Arrange the stack so that it looks like the point
                // where the signal occurred made a call to the
                // function sigpanic. Then set the PC to sigpanic.

                // Have to pass arguments out of band since
                // augmenting the stack frame would break
                // the unwinding code.
                gp.sig = sig
                gp.sigcode0 = uintptr(c.sigcode())
                gp.sigcode1 = uintptr(c.fault())
                gp.sigpc = c.sigpc()

                c.preparePanic(sig, gp)
                return
        }

        if c.sigFromUser() || flags&_SigNotify != 0 {
                if sigsend(sig) {
                        return
                }
        }

        if c.sigFromUser() && signal_ignored(sig) {
                return
        }

        if flags&_SigKill != 0 {
                dieFromSignal(sig)
        }

        // _SigThrow means that we should exit now.
        // If we get here with _SigPanic, it means that the signal
        // was sent to us by a program (c.sigFromUser() is true);
        // in that case, if we didn't handle it in sigsend, we exit now.
        if flags&(_SigThrow|_SigPanic) == 0 {
                return
        }

        mp.throwing = throwTypeRuntime
        mp.caughtsig.set(gp)

        if crashing == 0 {
                startpanic_m()
        }

        if sig < uint32(len(sigtable)) {
                print(sigtable[sig].name, "\n")
        } else {
                print("Signal ", sig, "\n")
        }

        print("PC=", hex(c.sigpc()), " m=", mp.id, " sigcode=", c.sigcode(), "\n")
        if mp.incgo && gp == mp.g0 && mp.curg != nil {
                print("signal arrived during cgo execution\n")
                // Switch to curg so that we get a traceback of the Go code
                // leading up to the cgocall, which switched from curg to g0.
                gp = mp.curg
        }
        if sig == _SIGILL || sig == _SIGFPE {
                // It would be nice to know how long the instruction is.
                // Unfortunately, that's complicated to do in general (mostly for x86
                // and s930x, but other archs have non-standard instruction lengths also).
                // Opt to print 16 bytes, which covers most instructions.
                const maxN = 16
                n := uintptr(maxN)
                // We have to be careful, though. If we're near the end of
                // a page and the following page isn't mapped, we could
                // segfault. So make sure we don't straddle a page (even though
                // that could lead to printing an incomplete instruction).
                // We're assuming here we can read at least the page containing the PC.
                // I suppose it is possible that the page is mapped executable but not readable?
                pc := c.sigpc()
                if n > physPageSize-pc%physPageSize {
                        n = physPageSize - pc%physPageSize
                }
                print("instruction bytes:")
                b := (*[maxN]byte)(unsafe.Pointer(pc))
                for i := uintptr(0); i < n; i++ {
                        print(" ", hex(b[i]))
                }
                println()
        }
        print("\n")

        level, _, docrash := gotraceback()
        if level > 0 {
                goroutineheader(gp)
                tracebacktrap(c.sigpc(), c.sigsp(), c.siglr(), gp)
                if crashing > 0 && gp != mp.curg && mp.curg != nil && readgstatus(mp.curg)&^_Gscan == _Grunning {
                        // tracebackothers on original m skipped this one; trace it now.
                        goroutineheader(mp.curg)
                        traceback(^uintptr(0), ^uintptr(0), 0, mp.curg)
                } else if crashing == 0 {
                        tracebackothers(gp)
                        print("\n")
                }
                dumpregs(c)
        }

        if docrash {
                crashing++
                if crashing < mcount()-int32(extraMCount) {
                        // There are other m's that need to dump their stacks.
                        // Relay SIGQUIT to the next m by sending it to the current process.
                        // All m's that have already received SIGQUIT have signal masks blocking
                        // receipt of any signals, so the SIGQUIT will go to an m that hasn't seen it yet.
                        // When the last m receives the SIGQUIT, it will fall through to the call to
                        // crash below. Just in case the relaying gets botched, each m involved in
                        // the relay sleeps for 5 seconds and then does the crash/exit itself.
                        // In expected operation, the last m has received the SIGQUIT and run
                        // crash/exit and the process is gone, all long before any of the
                        // 5-second sleeps have finished.
                        print("\n-----\n\n")
                        raiseproc(_SIGQUIT)
                        usleep(5 * 1000 * 1000)
                }
                crash()
        }

        printDebugLog()

        exit(2)
}

// sigpanic turns a synchronous signal into a run-time panic.
// If the signal handler sees a synchronous panic, it arranges the
// stack to look like the function where the signal occurred called
// sigpanic, sets the signal's PC value to sigpanic, and returns from
// the signal handler. The effect is that the program will act as
// though the function that got the signal simply called sigpanic
// instead.
//
// This must NOT be nosplit because the linker doesn't know where
// sigpanic calls can be injected.
//
// The signal handler must not inject a call to sigpanic if
// getg().throwsplit, since sigpanic may need to grow the stack.
//
// This is exported via linkname to assembly in runtime/cgo.
//
//go:linkname sigpanic
func sigpanic() {
        gp := getg()
        if !canpanic() {
                throw("unexpected signal during runtime execution")
        }

        switch gp.sig {
        case _SIGBUS:
                if gp.sigcode0 == _BUS_ADRERR && gp.sigcode1 < 0x1000 {
                        panicmem()
                }
                // Support runtime/debug.SetPanicOnFault.
                if gp.paniconfault {
                        panicmemAddr(gp.sigcode1)
                }
                print("unexpected fault address ", hex(gp.sigcode1), "\n")
                throw("fault")
        case _SIGSEGV:
                if (gp.sigcode0 == 0 || gp.sigcode0 == _SEGV_MAPERR || gp.sigcode0 == _SEGV_ACCERR) && gp.sigcode1 < 0x1000 {
                        panicmem()
                }
                // Support runtime/debug.SetPanicOnFault.
                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 _SIGFPE:
                switch gp.sigcode0 {
                case _FPE_INTDIV:
                        panicdivide()
                case _FPE_INTOVF:
                        panicoverflow()
                }
                panicfloat()
        }

        if gp.sig >= uint32(len(sigtable)) {
                // can't happen: we looked up gp.sig in sigtable to decide to call sigpanic
                throw("unexpected signal value")
        }
        panic(errorString(sigtable[gp.sig].name))
}

// dieFromSignal kills the program with a signal.
// This provides the expected exit status for the shell.
// This is only called with fatal signals expected to kill the process.
//
//go:nosplit
//go:nowritebarrierrec
func dieFromSignal(sig uint32) {
        unblocksig(sig)
        // Mark the signal as unhandled to ensure it is forwarded.
        atomic.Store(&handlingSig[sig], 0)
        raise(sig)

        // That should have killed us. On some systems, though, raise
        // sends the signal to the whole process rather than to just
        // the current thread, which means that the signal may not yet
        // have been delivered. Give other threads a chance to run and
        // pick up the signal.
        osyield()
        osyield()
        osyield()

        // If that didn't work, try _SIG_DFL.
        setsig(sig, _SIG_DFL)
        raise(sig)

        osyield()
        osyield()
        osyield()

        // If we are still somehow running, just exit with the wrong status.
        exit(2)
}

// raisebadsignal is called when a signal is received on a non-Go
// thread, and the Go program does not want to handle it (that is, the
// program has not called os/signal.Notify for the signal).
func raisebadsignal(sig uint32, c *sigctxt) {
        if sig == _SIGPROF {
                // Ignore profiling signals that arrive on non-Go threads.
                return
        }

        var handler uintptr
        if sig >= _NSIG {
                handler = _SIG_DFL
        } else {
                handler = atomic.Loaduintptr(&fwdSig[sig])
        }

        // Reset the signal handler and raise the signal.
        // We are currently running inside a signal handler, so the
        // signal is blocked. We need to unblock it before raising the
        // signal, or the signal we raise will be ignored until we return
        // from the signal handler. We know that the signal was unblocked
        // before entering the handler, or else we would not have received
        // it. That means that we don't have to worry about blocking it
        // again.
        unblocksig(sig)
        setsig(sig, handler)

        // If we're linked into a non-Go program we want to try to
        // avoid modifying the original context in which the signal
        // was raised. If the handler is the default, we know it
        // is non-recoverable, so we don't have to worry about
        // re-installing sighandler. At this point we can just
        // return and the signal will be re-raised and caught by
        // the default handler with the correct context.
        //
        // On FreeBSD, the libthr sigaction code prevents
        // this from working so we fall through to raise.
        if GOOS != "freebsd" && (isarchive || islibrary) && handler == _SIG_DFL && !c.sigFromUser() {
                return
        }

        raise(sig)

        // Give the signal a chance to be delivered.
        // In almost all real cases the program is about to crash,
        // so sleeping here is not a waste of time.
        usleep(1000)

        // If the signal didn't cause the program to exit, restore the
        // Go signal handler and carry on.
        //
        // We may receive another instance of the signal before we
        // restore the Go handler, but that is not so bad: we know
        // that the Go program has been ignoring the signal.
        setsig(sig, abi.FuncPCABIInternal(sighandler))
}

//go:nosplit
func crash() {
        // OS X core dumps are linear dumps of the mapped memory,
        // from the first virtual byte to the last, with zeros in the gaps.
        // Because of the way we arrange the address space on 64-bit systems,
        // this means the OS X core file will be >128 GB and even on a zippy
        // workstation can take OS X well over an hour to write (uninterruptible).
        // Save users from making that mistake.
        if GOOS == "darwin" && GOARCH == "amd64" {
                return
        }

        dieFromSignal(_SIGABRT)
}

// ensureSigM starts one global, sleeping thread to make sure at least one thread
// is available to catch signals enabled for os/signal.
func ensureSigM() {
        if maskUpdatedChan != nil {
                return
        }
        maskUpdatedChan = make(chan struct{})
        disableSigChan = make(chan uint32)
        enableSigChan = make(chan uint32)
        go func() {
                // Signal masks are per-thread, so make sure this goroutine stays on one
                // thread.
                LockOSThread()
                defer UnlockOSThread()
                // The sigBlocked mask contains the signals not active for os/signal,
                // initially all signals except the essential. When signal.Notify()/Stop is called,
                // sigenable/sigdisable in turn notify this thread to update its signal
                // mask accordingly.
                sigBlocked := sigset_all
                for i := range sigtable {
                        if !blockableSig(uint32(i)) {
                                sigdelset(&sigBlocked, i)
                        }
                }
                sigprocmask(_SIG_SETMASK, &sigBlocked, nil)
                for {
                        select {
                        case sig := <-enableSigChan:
                                if sig > 0 {
                                        sigdelset(&sigBlocked, int(sig))
                                }
                        case sig := <-disableSigChan:
                                if sig > 0 && blockableSig(sig) {
                                        sigaddset(&sigBlocked, int(sig))
                                }
                        }
                        sigprocmask(_SIG_SETMASK, &sigBlocked, nil)
                        maskUpdatedChan <- struct{}{}
                }
        }()
}

// This is called when we receive a signal when there is no signal stack.
// This can only happen if non-Go code calls sigaltstack to disable the
// signal stack.
func noSignalStack(sig uint32) {
        println("signal", sig, "received on thread with no signal stack")
        throw("non-Go code disabled sigaltstack")
}

// This is called if we receive a signal when there is a signal stack
// but we are not on it. This can only happen if non-Go code called
// sigaction without setting the SS_ONSTACK flag.
func sigNotOnStack(sig uint32) {
        println("signal", sig, "received but handler not on signal stack")
        throw("non-Go code set up signal handler without SA_ONSTACK flag")
}

// signalDuringFork is called if we receive a signal while doing a fork.
// We do not want signals at that time, as a signal sent to the process
// group may be delivered to the child process, causing confusion.
// This should never be called, because we block signals across the fork;
// this function is just a safety check. See issue 18600 for background.
func signalDuringFork(sig uint32) {
        println("signal", sig, "received during fork")
        throw("signal received during fork")
}

// This runs on a foreign stack, without an m or a g. No stack split.
//
//go:nosplit
//go:norace
//go:nowritebarrierrec
func badsignal(sig uintptr, c *sigctxt) {
        if !iscgo && !cgoHasExtraM {
                // There is no extra M. needm will not be able to grab
                // an M. Instead of hanging, just crash.
                // Cannot call split-stack function as there is no G.
                writeErrStr("fatal: bad g in signal handler\n")
                exit(2)
                *(*uintptr)(unsafe.Pointer(uintptr(123))) = 2
        }
        needm()
        if !sigsend(uint32(sig)) {
                // A foreign thread received the signal sig, and the
                // Go code does not want to handle it.
                raisebadsignal(uint32(sig), c)
        }
        dropm()
}

//go:noescape
func sigfwd(fn uintptr, sig uint32, info *siginfo, ctx unsafe.Pointer)

// Determines if the signal should be handled by Go and if not, forwards the
// signal to the handler that was installed before Go's. Returns whether the
// signal was forwarded.
// This is called by the signal handler, and the world may be stopped.
//
//go:nosplit
//go:nowritebarrierrec
func sigfwdgo(sig uint32, info *siginfo, ctx unsafe.Pointer) bool {
        if sig >= uint32(len(sigtable)) {
                return false
        }
        fwdFn := atomic.Loaduintptr(&fwdSig[sig])
        flags := sigtable[sig].flags

        // If we aren't handling the signal, forward it.
        if atomic.Load(&handlingSig[sig]) == 0 || !signalsOK {
                // If the signal is ignored, doing nothing is the same as forwarding.
                if fwdFn == _SIG_IGN || (fwdFn == _SIG_DFL && flags&_SigIgn != 0) {
                        return true
                }
                // We are not handling the signal and there is no other handler to forward to.
                // Crash with the default behavior.
                if fwdFn == _SIG_DFL {
                        setsig(sig, _SIG_DFL)
                        dieFromSignal(sig)
                        return false
                }

                sigfwd(fwdFn, sig, info, ctx)
                return true
        }

        // This function and its caller sigtrampgo assumes SIGPIPE is delivered on the
        // originating thread. This property does not hold on macOS (golang.org/issue/33384),
        // so we have no choice but to ignore SIGPIPE.
        if (GOOS == "darwin" || GOOS == "ios") && sig == _SIGPIPE {
                return true
        }

        // If there is no handler to forward to, no need to forward.
        if fwdFn == _SIG_DFL {
                return false
        }

        c := &sigctxt{info, ctx}
        // Only forward synchronous signals and SIGPIPE.
        // Unfortunately, user generated SIGPIPEs will also be forwarded, because si_code
        // is set to _SI_USER even for a SIGPIPE raised from a write to a closed socket
        // or pipe.
        if (c.sigFromUser() || flags&_SigPanic == 0) && sig != _SIGPIPE {
                return false
        }
        // Determine if the signal occurred inside Go code. We test that:
        //   (1) we weren't in VDSO page,
        //   (2) we were in a goroutine (i.e., m.curg != nil), and
        //   (3) we weren't in CGO.
        //   (4) we weren't in dropped extra m.
        gp := sigFetchG(c)
        if gp != nil && gp.m != nil && gp.m.curg != nil && !gp.m.isExtraInC && !gp.m.incgo {
                return false
        }

        // Signal not handled by Go, forward it.
        if fwdFn != _SIG_IGN {
                sigfwd(fwdFn, sig, info, ctx)
        }

        return true
}

// sigsave saves the current thread's signal mask into *p.
// This is used to preserve the non-Go signal mask when a non-Go
// thread calls a Go function.
// This is nosplit and nowritebarrierrec because it is called by needm
// which may be called on a non-Go thread with no g available.
//
//go:nosplit
//go:nowritebarrierrec
func sigsave(p *sigset) {
        sigprocmask(_SIG_SETMASK, nil, p)
}

// msigrestore sets the current thread's signal mask to sigmask.
// This is used to restore the non-Go signal mask when a non-Go thread
// calls a Go function.
// This is nosplit and nowritebarrierrec because it is called by dropm
// after g has been cleared.
//
//go:nosplit
//go:nowritebarrierrec
func msigrestore(sigmask sigset) {
        sigprocmask(_SIG_SETMASK, &sigmask, nil)
}

// sigsetAllExiting is used by sigblock(true) when a thread is
// exiting. sigset_all is defined in OS specific code, and per GOOS
// behavior may override this default for sigsetAllExiting: see
// osinit().
var sigsetAllExiting = sigset_all

// sigblock blocks signals in the current thread's signal mask.
// This is used to block signals while setting up and tearing down g
// when a non-Go thread calls a Go function. When a thread is exiting
// we use the sigsetAllExiting value, otherwise the OS specific
// definition of sigset_all is used.
// This is nosplit and nowritebarrierrec because it is called by needm
// which may be called on a non-Go thread with no g available.
//
//go:nosplit
//go:nowritebarrierrec
func sigblock(exiting bool) {
        if exiting {
                sigprocmask(_SIG_SETMASK, &sigsetAllExiting, nil)
                return
        }
        sigprocmask(_SIG_SETMASK, &sigset_all, nil)
}

// unblocksig removes sig from the current thread's signal mask.
// This is nosplit and nowritebarrierrec because it is called from
// dieFromSignal, which can be called by sigfwdgo while running in the
// signal handler, on the signal stack, with no g available.
//
//go:nosplit
//go:nowritebarrierrec
func unblocksig(sig uint32) {
        var set sigset
        sigaddset(&set, int(sig))
        sigprocmask(_SIG_UNBLOCK, &set, nil)
}

// minitSignals is called when initializing a new m to set the
// thread's alternate signal stack and signal mask.
func minitSignals() {
        minitSignalStack()
        minitSignalMask()
}

// minitSignalStack is called when initializing a new m to set the
// alternate signal stack. If the alternate signal stack is not set
// for the thread (the normal case) then set the alternate signal
// stack to the gsignal stack. If the alternate signal stack is set
// for the thread (the case when a non-Go thread sets the alternate
// signal stack and then calls a Go function) then set the gsignal
// stack to the alternate signal stack. We also set the alternate
// signal stack to the gsignal stack if cgo is not used (regardless
// of whether it is already set). Record which choice was made in
// newSigstack, so that it can be undone in unminit.
func minitSignalStack() {
        mp := getg().m
        var st stackt
        sigaltstack(nil, &st)
        if st.ss_flags&_SS_DISABLE != 0 || !iscgo {
                signalstack(&mp.gsignal.stack)
                mp.newSigstack = true
        } else {
                setGsignalStack(&st, &mp.goSigStack)
                mp.newSigstack = false
        }
}

// minitSignalMask is called when initializing a new m to set the
// thread's signal mask. When this is called all signals have been
// blocked for the thread.  This starts with m.sigmask, which was set
// either from initSigmask for a newly created thread or by calling
// sigsave if this is a non-Go thread calling a Go function. It
// removes all essential signals from the mask, thus causing those
// signals to not be blocked. Then it sets the thread's signal mask.
// After this is called the thread can receive signals.
func minitSignalMask() {
        nmask := getg().m.sigmask
        for i := range sigtable {
                if !blockableSig(uint32(i)) {
                        sigdelset(&nmask, i)
                }
        }
        sigprocmask(_SIG_SETMASK, &nmask, nil)
}

// unminitSignals is called from dropm, via unminit, to undo the
// effect of calling minit on a non-Go thread.
//
//go:nosplit
func unminitSignals() {
        if getg().m.newSigstack {
                st := stackt{ss_flags: _SS_DISABLE}
                sigaltstack(&st, nil)
        } else {
                // We got the signal stack from someone else. Restore
                // the Go-allocated stack in case this M gets reused
                // for another thread (e.g., it's an extram). Also, on
                // Android, libc allocates a signal stack for all
                // threads, so it's important to restore the Go stack
                // even on Go-created threads so we can free it.
                restoreGsignalStack(&getg().m.goSigStack)
        }
}

// blockableSig reports whether sig may be blocked by the signal mask.
// We never want to block the signals marked _SigUnblock;
// these are the synchronous signals that turn into a Go panic.
// We never want to block the preemption signal if it is being used.
// In a Go program--not a c-archive/c-shared--we never want to block
// the signals marked _SigKill or _SigThrow, as otherwise it's possible
// for all running threads to block them and delay their delivery until
// we start a new thread. When linked into a C program we let the C code
// decide on the disposition of those signals.
func blockableSig(sig uint32) bool {
        flags := sigtable[sig].flags
        if flags&_SigUnblock != 0 {
                return false
        }
        if sig == sigPreempt && preemptMSupported && debug.asyncpreemptoff == 0 {
                return false
        }
        if isarchive || islibrary {
                return true
        }
        return flags&(_SigKill|_SigThrow) == 0
}

// gsignalStack saves the fields of the gsignal stack changed by
// setGsignalStack.
type gsignalStack struct {
        stack       stack
        stackguard0 uintptr
        stackguard1 uintptr
        stktopsp    uintptr
}

// setGsignalStack sets the gsignal stack of the current m to an
// alternate signal stack returned from the sigaltstack system call.
// It saves the old values in *old for use by restoreGsignalStack.
// This is used when handling a signal if non-Go code has set the
// alternate signal stack.
//
//go:nosplit
//go:nowritebarrierrec
func setGsignalStack(st *stackt, old *gsignalStack) {
        gp := getg()
        if old != nil {
                old.stack = gp.m.gsignal.stack
                old.stackguard0 = gp.m.gsignal.stackguard0
                old.stackguard1 = gp.m.gsignal.stackguard1
                old.stktopsp = gp.m.gsignal.stktopsp
        }
        stsp := uintptr(unsafe.Pointer(st.ss_sp))
        gp.m.gsignal.stack.lo = stsp
        gp.m.gsignal.stack.hi = stsp + st.ss_size
        gp.m.gsignal.stackguard0 = stsp + _StackGuard
        gp.m.gsignal.stackguard1 = stsp + _StackGuard
}

// restoreGsignalStack restores the gsignal stack to the value it had
// before entering the signal handler.
//
//go:nosplit
//go:nowritebarrierrec
func restoreGsignalStack(st *gsignalStack) {
        gp := getg().m.gsignal
        gp.stack = st.stack
        gp.stackguard0 = st.stackguard0
        gp.stackguard1 = st.stackguard1
        gp.stktopsp = st.stktopsp
}

// signalstack sets the current thread's alternate signal stack to s.
//
//go:nosplit
func signalstack(s *stack) {
        st := stackt{ss_size: s.hi - s.lo}
        setSignalstackSP(&st, s.lo)
        sigaltstack(&st, nil)
}

// setsigsegv is used on darwin/arm64 to fake a segmentation fault.
//
// This is exported via linkname to assembly in runtime/cgo.
//
//go:nosplit
//go:linkname setsigsegv
func setsigsegv(pc uintptr) {
        gp := getg()
        gp.sig = _SIGSEGV
        gp.sigpc = pc
        gp.sigcode0 = _SEGV_MAPERR
        gp.sigcode1 = 0 // TODO: emulate si_addr
}