path: root/js/src/ds/MemoryProtectionExceptionHandler.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "ds/MemoryProtectionExceptionHandler.h"

#include "mozilla/Assertions.h"
#include "mozilla/Atomics.h"

#if defined(XP_WIN)
# include "jswin.h"
#elif defined(XP_UNIX) && !defined(XP_DARWIN)
# include <signal.h>
# include <sys/types.h>
# include <unistd.h>
#elif defined(XP_DARWIN)
# include <mach/mach.h>
# include <unistd.h>
#endif

#ifdef ANDROID
# include <android/log.h>
#endif

#include "ds/SplayTree.h"

#include "threading/LockGuard.h"
#include "threading/Thread.h"
#include "vm/MutexIDs.h"

namespace js {

/*
 * A class to store the addresses of the regions recognized as protected
 * by this exception handler. We use a splay tree to store these addresses.
 */
class ProtectedRegionTree
{
    struct Region
    {
        uintptr_t first;
        uintptr_t last;

        Region(uintptr_t addr, size_t size) : first(addr),
                                              last(addr + (size - 1)) {}

        static int compare(const Region& A, const Region& B) {
            if (A.last < B.first)
                return -1;
            if (A.first > B.last)
                return 1;
            return 0;
        }
    };

    Mutex lock;
    LifoAlloc alloc;
    SplayTree<Region, Region> tree;

  public:
    ProtectedRegionTree() : lock(mutexid::ProtectedRegionTree),
                            alloc(4096),
                            tree(&alloc) {}

    ~ProtectedRegionTree() { MOZ_ASSERT(tree.empty()); }

    void insert(uintptr_t addr, size_t size) {
        MOZ_ASSERT(addr && size);
        LockGuard<Mutex> guard(lock);
        AutoEnterOOMUnsafeRegion oomUnsafe;
        if (!tree.insert(Region(addr, size)))
            oomUnsafe.crash("Failed to store allocation ID.");
    }

    void remove(uintptr_t addr) {
        MOZ_ASSERT(addr);
        LockGuard<Mutex> guard(lock);
        tree.remove(Region(addr, 1));
    }

    bool isProtected(uintptr_t addr) {
        if (!addr)
            return false;
        LockGuard<Mutex> guard(lock);
        return tree.maybeLookup(Region(addr, 1));
    }
};

static bool sExceptionHandlerInstalled = false;

static ProtectedRegionTree sProtectedRegions;

bool
MemoryProtectionExceptionHandler::isDisabled()
{
    // Disabled everywhere for this release.
    return true;
}

void
MemoryProtectionExceptionHandler::addRegion(void* addr, size_t size)
{
    if (sExceptionHandlerInstalled)
        sProtectedRegions.insert(uintptr_t(addr), size);
}

void
MemoryProtectionExceptionHandler::removeRegion(void* addr)
{
    if (sExceptionHandlerInstalled)
        sProtectedRegions.remove(uintptr_t(addr));
}

/* -------------------------------------------------------------------------- */

/*
 * This helper function attempts to replicate the functionality of
 * mozilla::MOZ_ReportCrash() in an async-signal-safe way.
 */
static MOZ_COLD MOZ_ALWAYS_INLINE void
ReportCrashIfDebug(const char* aStr)
    MOZ_PRETEND_NORETURN_FOR_STATIC_ANALYSIS
{
#ifdef DEBUG
# if defined(XP_WIN)
    DWORD bytesWritten;
    BOOL ret = WriteFile(GetStdHandle(STD_ERROR_HANDLE), aStr,
                         strlen(aStr) + 1, &bytesWritten, nullptr);
    ::__debugbreak();
# elif defined(ANDROID)
    int ret = __android_log_write(ANDROID_LOG_FATAL, "MOZ_CRASH", aStr);
# else
    ssize_t ret = write(STDERR_FILENO, aStr, strlen(aStr) + 1);
# endif
    (void)ret; // Ignore failures; we're already crashing anyway.
#endif
}

/* -------------------------------------------------------------------------- */

#if defined(XP_WIN)

static void* sVectoredExceptionHandler = nullptr;

/*
 * We can only handle one exception. To guard against races and reentrancy,
 * we set this value the first time we enter the exception handler and never
 * touch it again.
 */
static mozilla::Atomic<bool> sHandlingException(false);

static long __stdcall
VectoredExceptionHandler(EXCEPTION_POINTERS* ExceptionInfo)
{
    EXCEPTION_RECORD* ExceptionRecord = ExceptionInfo->ExceptionRecord;

    // We only handle one kind of exception; ignore all others.
    if (ExceptionRecord->ExceptionCode == EXCEPTION_ACCESS_VIOLATION) {
        // Make absolutely sure we can only get here once.
        if (sHandlingException.compareExchange(false, true)) {
            // Restore the previous handler. We're going to forward to it
            // anyway, and if we crash while doing so we don't want to hang.
            MOZ_ALWAYS_TRUE(RemoveVectoredExceptionHandler(sVectoredExceptionHandler));

            // Get the address that the offending code tried to access.
            uintptr_t address = uintptr_t(ExceptionRecord->ExceptionInformation[1]);

            // If the faulting address is in one of our protected regions, we
            // want to annotate the crash to make it stand out from the crowd.
            if (sProtectedRegions.isProtected(address)) {
                ReportCrashIfDebug("Hit MOZ_CRASH(Tried to access a protected region!)\n");
                MOZ_CRASH_ANNOTATE("MOZ_CRASH(Tried to access a protected region!)");
            }
        }
    }

    // Forward to the previous handler which may be a debugger,
    // the crash reporter or something else entirely.
    return EXCEPTION_CONTINUE_SEARCH;
}

bool
MemoryProtectionExceptionHandler::install()
{
    MOZ_ASSERT(!sExceptionHandlerInstalled);

    // If the exception handler is disabled, report success anyway.
    if (MemoryProtectionExceptionHandler::isDisabled())
        return true;

    // Install our new exception handler.
    sVectoredExceptionHandler = AddVectoredExceptionHandler(/* FirstHandler = */ true,
                                                            VectoredExceptionHandler);

    sExceptionHandlerInstalled = sVectoredExceptionHandler != nullptr;
    return sExceptionHandlerInstalled;
}

void
MemoryProtectionExceptionHandler::uninstall()
{
    if (sExceptionHandlerInstalled) {
        MOZ_ASSERT(!sHandlingException);

        // Restore the previous exception handler.
        MOZ_ALWAYS_TRUE(RemoveVectoredExceptionHandler(sVectoredExceptionHandler));

        sExceptionHandlerInstalled = false;
    }
}

#elif defined(XP_UNIX) && !defined(XP_DARWIN)

static struct sigaction sPrevSEGVHandler = {};

/*
 * We can only handle one exception. To guard against races and reentrancy,
 * we set this value the first time we enter the exception handler and never
 * touch it again.
 */
static mozilla::Atomic<bool> sHandlingException(false);

static void
UnixExceptionHandler(int signum, siginfo_t* info, void* context)
{
    // Make absolutely sure we can only get here once.
    if (sHandlingException.compareExchange(false, true)) {
        // Restore the previous handler. We're going to forward to it
        // anyway, and if we crash while doing so we don't want to hang.
        MOZ_ALWAYS_FALSE(sigaction(SIGSEGV, &sPrevSEGVHandler, nullptr));

        MOZ_ASSERT(signum == SIGSEGV && info->si_signo == SIGSEGV);

        if (info->si_code == SEGV_ACCERR) {
            // Get the address that the offending code tried to access.
            uintptr_t address = uintptr_t(info->si_addr);

            // If the faulting address is in one of our protected regions, we
            // want to annotate the crash to make it stand out from the crowd.
            if (sProtectedRegions.isProtected(address)) {
                ReportCrashIfDebug("Hit MOZ_CRASH(Tried to access a protected region!)\n");
                MOZ_CRASH_ANNOTATE("MOZ_CRASH(Tried to access a protected region!)");
            }
        }
    }

    // Forward to the previous handler which may be a debugger,
    // the crash reporter or something else entirely.
    if (sPrevSEGVHandler.sa_flags & SA_SIGINFO)
        sPrevSEGVHandler.sa_sigaction(signum, info, context);
    else if (sPrevSEGVHandler.sa_handler == SIG_DFL || sPrevSEGVHandler.sa_handler == SIG_IGN)
        sigaction(SIGSEGV, &sPrevSEGVHandler, nullptr);
    else
        sPrevSEGVHandler.sa_handler(signum);

    // If we reach here, we're returning to let the default signal handler deal
    // with the exception. This is technically undefined behavior, but
    // everything seems to do it, and it removes us from the crash stack.
}

bool
MemoryProtectionExceptionHandler::install()
{
    MOZ_ASSERT(!sExceptionHandlerInstalled);

    // If the exception handler is disabled, report success anyway.
    if (MemoryProtectionExceptionHandler::isDisabled())
        return true;

    // Install our new exception handler and save the previous one.
    struct sigaction faultHandler = {};
    faultHandler.sa_flags = SA_SIGINFO | SA_NODEFER;
    faultHandler.sa_sigaction = UnixExceptionHandler;
    sigemptyset(&faultHandler.sa_mask);
    sExceptionHandlerInstalled = !sigaction(SIGSEGV, &faultHandler, &sPrevSEGVHandler);

    return sExceptionHandlerInstalled;
}

void
MemoryProtectionExceptionHandler::uninstall()
{
    if (sExceptionHandlerInstalled) {
        MOZ_ASSERT(!sHandlingException);

        // Restore the previous exception handler.
        MOZ_ALWAYS_FALSE(sigaction(SIGSEGV, &sPrevSEGVHandler, nullptr));

        sExceptionHandlerInstalled = false;
    }
}

#elif defined(XP_DARWIN)

/*
 * The fact that we need to be able to forward to other exception handlers
 * makes this code much more complicated. The forwarding logic and the
 * structures required to make it work are heavily based on the code at
 * www.ravenbrook.com/project/mps/prototype/2013-06-24/machtest/machtest/main.c
 */

/* -------------------------------------------------------------------------- */
/*                Begin Mach definitions and helper functions                 */
/* -------------------------------------------------------------------------- */

/*
 * These are the message IDs associated with each exception type.
 * We'll be using sIDRequest64, but we need the others for forwarding.
 */
static const mach_msg_id_t sIDRequest32 = 2401;
static const mach_msg_id_t sIDRequestState32 = 2402;
static const mach_msg_id_t sIDRequestStateIdentity32 = 2403;

static const mach_msg_id_t sIDRequest64 = 2405;
static const mach_msg_id_t sIDRequestState64 = 2406;
static const mach_msg_id_t sIDRequestStateIdentity64 = 2407;

/*
 * Each message ID has an associated Mach message structure.
 * We use the preprocessor to make defining them a little less arduous.
 */
# define REQUEST_HEADER_FIELDS\
    mach_msg_header_t header;

# define REQUEST_IDENTITY_FIELDS\
    mach_msg_body_t msgh_body;\
    mach_msg_port_descriptor_t thread;\
    mach_msg_port_descriptor_t task;

# define REQUEST_GENERAL_FIELDS(bits)\
    NDR_record_t NDR;\
    exception_type_t exception;\
    mach_msg_type_number_t code_count;\
    int##bits##_t code[2];

# define REQUEST_STATE_FIELDS\
    int flavor;\
    mach_msg_type_number_t old_state_count;\
    natural_t old_state[THREAD_STATE_MAX];

# define REQUEST_TRAILER_FIELDS\
    mach_msg_trailer_t trailer;

# define EXCEPTION_REQUEST(bits)\
    struct ExceptionRequest##bits\
    {\
        REQUEST_HEADER_FIELDS\
        REQUEST_IDENTITY_FIELDS\
        REQUEST_GENERAL_FIELDS(bits)\
        REQUEST_TRAILER_FIELDS\
    };\

# define EXCEPTION_REQUEST_STATE(bits)\
    struct ExceptionRequestState##bits\
    {\
        REQUEST_HEADER_FIELDS\
        REQUEST_GENERAL_FIELDS(bits)\
        REQUEST_STATE_FIELDS\
        REQUEST_TRAILER_FIELDS\
    };\

# define EXCEPTION_REQUEST_STATE_IDENTITY(bits)\
    struct ExceptionRequestStateIdentity##bits\
    {\
        REQUEST_HEADER_FIELDS\
        REQUEST_IDENTITY_FIELDS\
        REQUEST_GENERAL_FIELDS(bits)\
        REQUEST_STATE_FIELDS\
        REQUEST_TRAILER_FIELDS\
    };\

/* This is needed because not all fields are naturally aligned on 64-bit. */
# ifdef  __MigPackStructs
#  pragma pack(4)
# endif

EXCEPTION_REQUEST(32)
EXCEPTION_REQUEST(64)
EXCEPTION_REQUEST_STATE(32)
EXCEPTION_REQUEST_STATE(64)
EXCEPTION_REQUEST_STATE_IDENTITY(32)
EXCEPTION_REQUEST_STATE_IDENTITY(64)

/* We use this as a common base when forwarding to the previous handler. */
union ExceptionRequestUnion {
    mach_msg_header_t header;
    ExceptionRequest32 r32;
    ExceptionRequest64 r64;
    ExceptionRequestState32 rs32;
    ExceptionRequestState64 rs64;
    ExceptionRequestStateIdentity32 rsi32;
    ExceptionRequestStateIdentity64 rsi64;
};

/* This isn't really a full Mach message, but it's all we need to send. */
struct ExceptionReply
{
    mach_msg_header_t header;
    NDR_record_t NDR;
    kern_return_t RetCode;
};

# ifdef  __MigPackStructs
#  pragma pack()
# endif

# undef EXCEPTION_REQUEST_STATE_IDENTITY
# undef EXCEPTION_REQUEST_STATE
# undef EXCEPTION_REQUEST
# undef REQUEST_STATE_FIELDS
# undef REQUEST_GENERAL_FIELDS
# undef REQUEST_IDENTITY_FIELDS
# undef REQUEST_HEADER_FIELDS

/*
 * The exception handler we're forwarding to may not have the same behavior
 * or thread state flavor as what we're using. These macros help populate
 * the fields of the message we're about to send to the previous handler.
 */
# define COPY_REQUEST_COMMON(bits, id)\
    dst.header = src.header;\
    dst.header.msgh_id = id;\
    dst.header.msgh_size = static_cast<mach_msg_size_t>(sizeof(dst) - sizeof(dst.trailer));\
    dst.NDR = src.NDR;\
    dst.exception = src.exception;\
    dst.code_count = src.code_count;\
    dst.code[0] = int##bits##_t(src.code[0]);\
    dst.code[1] = int##bits##_t(src.code[1]);

# define COPY_REQUEST_IDENTITY\
    dst.msgh_body = src.msgh_body;\
    dst.thread = src.thread;\
    dst.task = src.task;

# define COPY_REQUEST_STATE(flavor, stateCount, state)\
    mach_msg_size_t stateSize = stateCount * sizeof(natural_t);\
    dst.header.msgh_size = static_cast<mach_msg_size_t>(sizeof(dst) - sizeof(dst.trailer) -\
                                                        sizeof(dst.old_state) + stateSize);\
    dst.flavor = flavor;\
    dst.old_state_count = stateCount;\
    memcpy(dst.old_state, state, stateSize);

# define COPY_EXCEPTION_REQUEST(bits)\
    static void\
    CopyExceptionRequest##bits(ExceptionRequest64& src,\
                               ExceptionRequest##bits& dst)\
    {\
        COPY_REQUEST_COMMON(bits, sIDRequest##bits)\
        COPY_REQUEST_IDENTITY\
    }

# define COPY_EXCEPTION_REQUEST_STATE(bits)\
    static void\
    CopyExceptionRequestState##bits(ExceptionRequest64& src,\
                                    ExceptionRequestState##bits& dst,\
                                    thread_state_flavor_t flavor,\
                                    mach_msg_type_number_t stateCount,\
                                    thread_state_t state)\
    {\
        COPY_REQUEST_COMMON(bits, sIDRequestState##bits)\
        COPY_REQUEST_STATE(flavor, stateCount, state)\
    }

# define COPY_EXCEPTION_REQUEST_STATE_IDENTITY(bits)\
    static void\
    CopyExceptionRequestStateIdentity##bits(ExceptionRequest64& src,\
                                            ExceptionRequestStateIdentity##bits& dst,\
                                            thread_state_flavor_t flavor,\
                                            mach_msg_type_number_t stateCount,\
                                            thread_state_t state)\
    {\
        COPY_REQUEST_COMMON(bits, sIDRequestStateIdentity##bits)\
        COPY_REQUEST_IDENTITY\
        COPY_REQUEST_STATE(flavor, stateCount, state)\
    }

COPY_EXCEPTION_REQUEST(32)
COPY_EXCEPTION_REQUEST_STATE(32)
COPY_EXCEPTION_REQUEST_STATE_IDENTITY(32)
COPY_EXCEPTION_REQUEST(64)
COPY_EXCEPTION_REQUEST_STATE(64)
COPY_EXCEPTION_REQUEST_STATE_IDENTITY(64)

# undef COPY_EXCEPTION_REQUEST_STATE_IDENTITY
# undef COPY_EXCEPTION_REQUEST_STATE
# undef COPY_EXCEPTION_REQUEST
# undef COPY_REQUEST_STATE
# undef COPY_REQUEST_IDENTITY
# undef COPY_REQUEST_COMMON

/* -------------------------------------------------------------------------- */
/*                 End Mach definitions and helper functions                  */
/* -------------------------------------------------------------------------- */

/* Every Mach exception handler is parameterized by these four properties. */
struct MachExceptionParameters
{
    exception_mask_t mask;
    mach_port_t port;
    exception_behavior_t behavior;
    thread_state_flavor_t flavor;
};

struct ExceptionHandlerState
{
    MachExceptionParameters current;
    MachExceptionParameters previous;

    /* Each Mach exception handler runs in its own thread. */
    Thread handlerThread;
};

/* This choice of ID is arbitrary, but must not match our exception ID. */
static const mach_msg_id_t sIDQuit = 42;

static ExceptionHandlerState sMachExceptionState;

/*
 * The meat of our exception handler. This thread waits for an exception
 * message, annotates the exception if needed, then forwards it to the
 * previously installed handler (which will likely terminate the process).
 */
static void
MachExceptionHandler()
{
    kern_return_t ret;
    MachExceptionParameters& current = sMachExceptionState.current;
    MachExceptionParameters& previous = sMachExceptionState.previous;

    // We use the simplest kind of 64-bit exception message here.
    ExceptionRequest64 request = {};
    request.header.msgh_local_port = current.port;
    request.header.msgh_size = static_cast<mach_msg_size_t>(sizeof(request));
    ret = mach_msg(&request.header, MACH_RCV_MSG, 0, request.header.msgh_size,
                   current.port, MACH_MSG_TIMEOUT_NONE, MACH_PORT_NULL);

    // Restore the previous handler. We're going to forward to it
    // anyway, and if we crash while doing so we don't want to hang.
    task_set_exception_ports(mach_task_self(), previous.mask, previous.port,
                             previous.behavior, previous.flavor);

    // If we failed even receiving the message, just give up.
    if (ret != MACH_MSG_SUCCESS)
        MOZ_CRASH("MachExceptionHandler: mach_msg failed to receive a message!");

    // Terminate the thread if we're shutting down.
    if (request.header.msgh_id == sIDQuit)
        return;

    // The only other valid message ID is the one associated with the
    // EXCEPTION_DEFAULT | MACH_EXCEPTION_CODES behavior we chose.
    if (request.header.msgh_id != sIDRequest64)
        MOZ_CRASH("MachExceptionHandler: Unexpected Message ID!");

    // Make sure we can understand the exception we received.
    if (request.exception != EXC_BAD_ACCESS || request.code_count != 2)
        MOZ_CRASH("MachExceptionHandler: Unexpected exception type!");

    // Get the address that the offending code tried to access.
    uintptr_t address = uintptr_t(request.code[1]);

    // If the faulting address is inside one of our protected regions, we
    // want to annotate the crash to make it stand out from the crowd.
    if (sProtectedRegions.isProtected(address)) {
        ReportCrashIfDebug("Hit MOZ_CRASH(Tried to access a protected region!)\n");
        MOZ_CRASH_ANNOTATE("MOZ_CRASH(Tried to access a protected region!)");
    }

    // Forward to the previous handler which may be a debugger, the unix
    // signal handler, the crash reporter or something else entirely.
    if (previous.port != MACH_PORT_NULL) {
        mach_msg_type_number_t stateCount;
        thread_state_data_t state;
        if ((uint32_t(previous.behavior) & ~MACH_EXCEPTION_CODES) != EXCEPTION_DEFAULT) {
            // If the previous handler requested thread state, get it here.
            stateCount = THREAD_STATE_MAX;
            ret = thread_get_state(request.thread.name, previous.flavor, state, &stateCount);
            if (ret != KERN_SUCCESS)
                MOZ_CRASH("MachExceptionHandler: Could not get the thread state to forward!");
        }

        // Depending on the behavior of the previous handler, the forwarded
        // exception message will have a different set of fields.
        // Of particular note is that exception handlers that lack
        // MACH_EXCEPTION_CODES will get 32-bit fields even on 64-bit
        // systems. It appears that OSX simply truncates these fields.
        ExceptionRequestUnion forward;
        switch (uint32_t(previous.behavior)) {
          case EXCEPTION_DEFAULT:
             CopyExceptionRequest32(request, forward.r32);
             break;
          case EXCEPTION_DEFAULT | MACH_EXCEPTION_CODES:
             CopyExceptionRequest64(request, forward.r64);
             break;
          case EXCEPTION_STATE:
             CopyExceptionRequestState32(request, forward.rs32,
                                         previous.flavor, stateCount, state);
             break;
          case EXCEPTION_STATE | MACH_EXCEPTION_CODES:
             CopyExceptionRequestState64(request, forward.rs64,
                                         previous.flavor, stateCount, state);
             break;
          case EXCEPTION_STATE_IDENTITY:
             CopyExceptionRequestStateIdentity32(request, forward.rsi32,
                                                 previous.flavor, stateCount, state);
             break;
          case EXCEPTION_STATE_IDENTITY | MACH_EXCEPTION_CODES:
             CopyExceptionRequestStateIdentity64(request, forward.rsi64,
                                                 previous.flavor, stateCount, state);
             break;
          default:
             MOZ_CRASH("MachExceptionHandler: Unknown previous handler behavior!");
        }

        // Forward the generated message to the old port. The local and remote
        // port fields *and their rights* are swapped on arrival, so we need to
        // swap them back first.
        forward.header.msgh_bits = (request.header.msgh_bits & ~MACH_MSGH_BITS_PORTS_MASK) |
            MACH_MSGH_BITS(MACH_MSGH_BITS_LOCAL(request.header.msgh_bits),
                           MACH_MSGH_BITS_REMOTE(request.header.msgh_bits));
        forward.header.msgh_local_port = forward.header.msgh_remote_port;
        forward.header.msgh_remote_port = previous.port;
        ret = mach_msg(&forward.header, MACH_SEND_MSG, forward.header.msgh_size, 0,
                       MACH_PORT_NULL, MACH_MSG_TIMEOUT_NONE, MACH_PORT_NULL);
        if (ret != MACH_MSG_SUCCESS)
            MOZ_CRASH("MachExceptionHandler: Failed to forward to the previous handler!");
    } else {
        // There was no previous task-level exception handler, so defer to the
        // host level one instead. We set the return code to KERN_FAILURE to
        // indicate that we did not handle the exception.
        // The reply message ID is always the request ID + 100.
        ExceptionReply reply = {};
        reply.header.msgh_bits =
            MACH_MSGH_BITS(MACH_MSGH_BITS_REMOTE(request.header.msgh_bits), 0);
        reply.header.msgh_size = static_cast<mach_msg_size_t>(sizeof(reply));
        reply.header.msgh_remote_port = request.header.msgh_remote_port;
        reply.header.msgh_local_port = MACH_PORT_NULL;
        reply.header.msgh_id = request.header.msgh_id + 100;
        reply.NDR = request.NDR;
        reply.RetCode = KERN_FAILURE;
        ret = mach_msg(&reply.header, MACH_SEND_MSG, reply.header.msgh_size, 0,
                       MACH_PORT_NULL, MACH_MSG_TIMEOUT_NONE, MACH_PORT_NULL);
        if (ret != MACH_MSG_SUCCESS)
            MOZ_CRASH("MachExceptionHandler: Failed to forward to the host level!");
    }
}

static void
TerminateMachExceptionHandlerThread()
{
    // Send a simple quit message to the exception handler thread.
    mach_msg_header_t msg;
    msg.msgh_bits = MACH_MSGH_BITS(MACH_MSG_TYPE_COPY_SEND, 0);
    msg.msgh_size = static_cast<mach_msg_size_t>(sizeof(msg));
    msg.msgh_remote_port = sMachExceptionState.current.port;
    msg.msgh_local_port = MACH_PORT_NULL;
    msg.msgh_reserved = 0;
    msg.msgh_id = sIDQuit;
    kern_return_t ret = mach_msg(&msg, MACH_SEND_MSG, sizeof(msg), 0, MACH_PORT_NULL,
                                 MACH_MSG_TIMEOUT_NONE, MACH_PORT_NULL);

    if (ret == MACH_MSG_SUCCESS)
        sMachExceptionState.handlerThread.join();
    else
        MOZ_CRASH("MachExceptionHandler: Handler thread failed to terminate!");
}

bool
MemoryProtectionExceptionHandler::install()
{
    MOZ_ASSERT(!sExceptionHandlerInstalled);

    // If the exception handler is disabled, report success anyway.
    if (MemoryProtectionExceptionHandler::isDisabled())
        return true;

    kern_return_t ret;
    mach_port_t task = mach_task_self();

    // Allocate a new exception port with receive rights.
    sMachExceptionState.current = {};
    MachExceptionParameters& current = sMachExceptionState.current;
    ret = mach_port_allocate(task, MACH_PORT_RIGHT_RECEIVE, &current.port);
    if (ret != KERN_SUCCESS)
        return false;

    // Give the new port send rights as well.
    ret = mach_port_insert_right(task, current.port, current.port, MACH_MSG_TYPE_MAKE_SEND);
    if (ret != KERN_SUCCESS) {
        mach_port_deallocate(task, current.port);
        current = {};
        return false;
    }

    // Start the thread that will receive the messages from our exception port.
    if (!sMachExceptionState.handlerThread.init(MachExceptionHandler)) {
        mach_port_deallocate(task, current.port);
        current = {};
        return false;
    }

    // Set the other properties of our new exception handler.
    current.mask = EXC_MASK_BAD_ACCESS;
    current.behavior = exception_behavior_t(EXCEPTION_DEFAULT | MACH_EXCEPTION_CODES);
    current.flavor = THREAD_STATE_NONE;

    // Tell the task to use our exception handler, and save the previous one.
    sMachExceptionState.previous = {};
    MachExceptionParameters& previous = sMachExceptionState.previous;
    mach_msg_type_number_t previousCount = 1;
    ret = task_swap_exception_ports(task, current.mask, current.port, current.behavior,
                                    current.flavor, &previous.mask, &previousCount,
                                    &previous.port, &previous.behavior, &previous.flavor);
    if (ret != KERN_SUCCESS) {
        TerminateMachExceptionHandlerThread();
        mach_port_deallocate(task, current.port);
        previous = {};
        current = {};
        return false;
    }

    // We should have info on the previous exception handler, even if it's null.
    MOZ_ASSERT(previousCount == 1);

    sExceptionHandlerInstalled = true;
    return sExceptionHandlerInstalled;
}

void
MemoryProtectionExceptionHandler::uninstall()
{
    if (sExceptionHandlerInstalled) {
        mach_port_t task = mach_task_self();

        // Restore the previous exception handler.
        MachExceptionParameters& previous = sMachExceptionState.previous;
        task_set_exception_ports(task, previous.mask, previous.port,
                                 previous.behavior, previous.flavor);

        TerminateMachExceptionHandlerThread();

        // Release the Mach IPC port we used.
        mach_port_deallocate(task, sMachExceptionState.current.port);

        sMachExceptionState.current = {};
        sMachExceptionState.previous = {};

        sExceptionHandlerInstalled = false;
    }
}

#else

#error "This platform is not supported!"

#endif

} /* namespace js */