src/jit/ProcessExecutableMemory.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * 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 "jit/ProcessExecutableMemory.h"

#include "mozilla/Array.h"
#include "mozilla/Atomics.h"
#include "mozilla/DebugOnly.h"
#include "mozilla/Maybe.h"
#include "mozilla/TaggedAnonymousMemory.h"
#include "mozilla/XorShift128PlusRNG.h"

#include <errno.h>

#include "jsfriendapi.h"
#include "jsmath.h"

#include "gc/Memory.h"
#ifdef JS_CODEGEN_ARM64
#  include "jit/arm64/vixl/Cpu-vixl.h"
#endif
#include "jit/AtomicOperations.h"
#include "jit/FlushICache.h"  // js::jit::FlushICache
#include "threading/LockGuard.h"
#include "threading/Mutex.h"
#include "util/Memory.h"
#include "util/Poison.h"
#include "util/Windows.h"
#include "vm/MutexIDs.h"

#ifdef XP_WIN
#  include "mozilla/StackWalk_windows.h"
#  include "mozilla/WindowsVersion.h"
#elif defined(__wasi__)
// Nothing.
#else
#  include <sys/mman.h>
#  include <unistd.h>
#endif

#ifdef MOZ_VALGRIND
#  include <valgrind/valgrind.h>
#endif

using namespace js;
using namespace js::jit;

#ifdef XP_WIN
#  if defined(HAVE_64BIT_BUILD)
#    define NEED_JIT_UNWIND_HANDLING
#  endif

static void* ComputeRandomAllocationAddress() {
  /*
   * Inspiration is V8's OS::Allocate in platform-win32.cc.
   *
   * VirtualAlloc takes 64K chunks out of the virtual address space, so we
   * keep 16b alignment.
   *
   * x86: V8 comments say that keeping addresses in the [64MiB, 1GiB) range
   * tries to avoid system default DLL mapping space. In the end, we get 13
   * bits of randomness in our selection.
   * x64: [2GiB, 4TiB), with 25 bits of randomness.
   */
#  ifdef HAVE_64BIT_BUILD
  static const uintptr_t base = 0x0000000080000000;
  static const uintptr_t mask = 0x000003ffffff0000;
#  elif defined(_M_IX86) || defined(__i386__)
  static const uintptr_t base = 0x04000000;
  static const uintptr_t mask = 0x3fff0000;
#  else
#    error "Unsupported architecture"
#  endif

  uint64_t rand = js::GenerateRandomSeed();
  return (void*)(base | (rand & mask));
}

#  ifdef NEED_JIT_UNWIND_HANDLING
static js::JitExceptionHandler sJitExceptionHandler;
#  endif

JS_PUBLIC_API void js::SetJitExceptionHandler(JitExceptionHandler handler) {
#  ifdef NEED_JIT_UNWIND_HANDLING
  MOZ_ASSERT(!sJitExceptionHandler);
  sJitExceptionHandler = handler;
#  else
  // Just do nothing if unwind handling is disabled.
#  endif
}

#  ifdef NEED_JIT_UNWIND_HANDLING
#    if defined(_M_ARM64)
// See the ".xdata records" section of
// https://docs.microsoft.com/en-us/cpp/build/arm64-exception-handling
// These records can have various fields present or absent depending on the
// bits set in the header. Our struct will use one 32-bit slot for unwind codes,
// and no slots for epilog scopes.
struct UnwindInfo {
  uint32_t functionLength : 18;
  uint32_t version : 2;
  uint32_t hasExceptionHandler : 1;
  uint32_t packedEpilog : 1;
  uint32_t epilogCount : 5;
  uint32_t codeWords : 5;
  uint8_t unwindCodes[4];
  uint32_t exceptionHandler;
};
static const unsigned ThunkLength = 20;
#    else
// From documentation for UNWIND_INFO on
// http://msdn.microsoft.com/en-us/library/ddssxxy8.aspx
struct UnwindInfo {
  uint8_t version : 3;
  uint8_t flags : 5;
  uint8_t sizeOfPrologue;
  uint8_t countOfUnwindCodes;
  uint8_t frameRegister : 4;
  uint8_t frameOffset : 4;
  ULONG exceptionHandler;
};
static const unsigned ThunkLength = 12;
#    endif

struct ExceptionHandlerRecord {
  RUNTIME_FUNCTION runtimeFunction;
  UnwindInfo unwindInfo;
  uint8_t thunk[ThunkLength];
};

// This function must match the function pointer type PEXCEPTION_HANDLER
// mentioned in:
//   http://msdn.microsoft.com/en-us/library/ssa62fwe.aspx.
// This type is rather elusive in documentation; Wine is the best I've found:
//   http://source.winehq.org/source/include/winnt.h
static DWORD ExceptionHandler(PEXCEPTION_RECORD exceptionRecord,
                              _EXCEPTION_REGISTRATION_RECORD*, PCONTEXT context,
                              _EXCEPTION_REGISTRATION_RECORD**) {
  return sJitExceptionHandler(exceptionRecord, context);
}

PRUNTIME_FUNCTION RuntimeFunctionCallback(DWORD64 ControlPc, PVOID Context);

// For an explanation of the problem being solved here, see
// SetJitExceptionFilter in jsfriendapi.h.
static bool RegisterExecutableMemory(void* p, size_t bytes, size_t pageSize) {
  if (!VirtualAlloc(p, pageSize, MEM_COMMIT, PAGE_READWRITE)) {
    MOZ_CRASH();
  }

  ExceptionHandlerRecord* r = reinterpret_cast<ExceptionHandlerRecord*>(p);
  void* handler = JS_FUNC_TO_DATA_PTR(void*, ExceptionHandler);

  // Because the .xdata format on ARM64 can only encode sizes up to 1M (much
  // too small for our JIT code regions), we register a function table callback
  // to provide RUNTIME_FUNCTIONs at runtime. Windows doesn't seem to care about
  // the size fields on RUNTIME_FUNCTIONs that are created in this way, so the
  // same RUNTIME_FUNCTION can work for any address in the region. We'll set up
  // a generic one now and the callback can just return a pointer to it.

  // All these fields are specified to be offsets from the base of the
  // executable code (which is 'p'), even if they have 'Address' in their
  // names. In particular, exceptionHandler is a ULONG offset which is a
  // 32-bit integer. Since 'p' can be farther than INT32_MAX away from
  // sJitExceptionHandler, we must generate a little thunk inside the
  // record. The record is put on its own page so that we can take away write
  // access to protect against accidental clobbering.

#    if defined(_M_ARM64)
  r->runtimeFunction.BeginAddress = pageSize;
  r->runtimeFunction.UnwindData = offsetof(ExceptionHandlerRecord, unwindInfo);
  static_assert(offsetof(ExceptionHandlerRecord, unwindInfo) % 4 == 0,
                "The ARM64 .pdata format requires that exception information "
                "RVAs be 4-byte aligned.");

  memset(&r->unwindInfo, 0, sizeof(r->unwindInfo));
  r->unwindInfo.hasExceptionHandler = true;
  r->unwindInfo.exceptionHandler = offsetof(ExceptionHandlerRecord, thunk);

  // Use a fake unwind code to make the Windows unwinder do _something_. If the
  // PC and SP both stay unchanged, we'll fail the unwinder's sanity checks and
  // it won't call our exception handler.
  r->unwindInfo.codeWords = 1;  // one 32-bit word gives us up to 4 codes
  r->unwindInfo.unwindCodes[0] =
      0b00000001;  // alloc_s small stack of size 1*16
  r->unwindInfo.unwindCodes[1] = 0b11100100;  // end

  uint32_t* thunk = (uint32_t*)r->thunk;
  uint16_t* addr = (uint16_t*)&handler;

  // xip0/r16 should be safe to clobber: Windows just used it to call our thunk.
  const uint8_t reg = 16;

  // Say `handler` is 0x4444333322221111, then:
  thunk[0] = 0xd2800000 | addr[0] << 5 | reg;  // mov  xip0, 1111
  thunk[1] = 0xf2a00000 | addr[1] << 5 | reg;  // movk xip0, 2222 lsl #0x10
  thunk[2] = 0xf2c00000 | addr[2] << 5 | reg;  // movk xip0, 3333 lsl #0x20
  thunk[3] = 0xf2e00000 | addr[3] << 5 | reg;  // movk xip0, 4444 lsl #0x30
  thunk[4] = 0xd61f0000 | reg << 5;            // br xip0
#    else
  r->runtimeFunction.BeginAddress = pageSize;
  r->runtimeFunction.EndAddress = (DWORD)bytes;
  r->runtimeFunction.UnwindData = offsetof(ExceptionHandlerRecord, unwindInfo);

  r->unwindInfo.version = 1;
  r->unwindInfo.flags = UNW_FLAG_EHANDLER;
  r->unwindInfo.sizeOfPrologue = 0;
  r->unwindInfo.countOfUnwindCodes = 0;
  r->unwindInfo.frameRegister = 0;
  r->unwindInfo.frameOffset = 0;
  r->unwindInfo.exceptionHandler = offsetof(ExceptionHandlerRecord, thunk);

  // mov imm64, rax
  r->thunk[0] = 0x48;
  r->thunk[1] = 0xb8;
  memcpy(&r->thunk[2], &handler, 8);

  // jmp rax
  r->thunk[10] = 0xff;
  r->thunk[11] = 0xe0;
#    endif

  DWORD oldProtect;
  if (!VirtualProtect(p, pageSize, PAGE_EXECUTE_READ, &oldProtect)) {
    MOZ_CRASH();
  }

  // XXX NB: The profiler believes this function is only called from the main
  // thread. If that ever becomes untrue, the profiler must be updated
  // immediately.
  AutoSuppressStackWalking suppress;
  return RtlInstallFunctionTableCallback((DWORD64)p | 0x3, (DWORD64)p, bytes,
                                         RuntimeFunctionCallback, NULL, NULL);
}

static void UnregisterExecutableMemory(void* p, size_t bytes, size_t pageSize) {
  // There's no such thing as RtlUninstallFunctionTableCallback, so there's
  // nothing to do here.
}
#  endif

static void* ReserveProcessExecutableMemory(size_t bytes) {
#  ifdef NEED_JIT_UNWIND_HANDLING
  size_t pageSize = gc::SystemPageSize();
  if (sJitExceptionHandler) {
    bytes += pageSize;
  }
#  endif

  void* p = nullptr;
  for (size_t i = 0; i < 10; i++) {
    void* randomAddr = ComputeRandomAllocationAddress();
    p = VirtualAlloc(randomAddr, bytes, MEM_RESERVE, PAGE_NOACCESS);
    if (p) {
      break;
    }
  }

  if (!p) {
    // Try again without randomization.
    p = VirtualAlloc(nullptr, bytes, MEM_RESERVE, PAGE_NOACCESS);
    if (!p) {
      return nullptr;
    }
  }

#  ifdef NEED_JIT_UNWIND_HANDLING
  if (sJitExceptionHandler) {
    if (!RegisterExecutableMemory(p, bytes, pageSize)) {
      VirtualFree(p, 0, MEM_RELEASE);
      return nullptr;
    }

    p = (uint8_t*)p + pageSize;
    bytes -= pageSize;
  }

  RegisterJitCodeRegion((uint8_t*)p, bytes);
#  endif

  return p;
}

static void DeallocateProcessExecutableMemory(void* addr, size_t bytes) {
#  ifdef NEED_JIT_UNWIND_HANDLING
  UnregisterJitCodeRegion((uint8_t*)addr, bytes);

  if (sJitExceptionHandler) {
    size_t pageSize = gc::SystemPageSize();
    addr = (uint8_t*)addr - pageSize;
    UnregisterExecutableMemory(addr, bytes, pageSize);
  }
#  endif

  VirtualFree(addr, 0, MEM_RELEASE);
}

static DWORD ProtectionSettingToFlags(ProtectionSetting protection) {
  switch (protection) {
    case ProtectionSetting::Protected:
      return PAGE_NOACCESS;
    case ProtectionSetting::Writable:
      return PAGE_READWRITE;
    case ProtectionSetting::Executable:
      return PAGE_EXECUTE_READ;
  }
  MOZ_CRASH();
}

[[nodiscard]] static bool CommitPages(void* addr, size_t bytes,
                                      ProtectionSetting protection) {
  void* p = VirtualAlloc(addr, bytes, MEM_COMMIT,
                         ProtectionSettingToFlags(protection));
  if (!p) {
    return false;
  }
  MOZ_RELEASE_ASSERT(p == addr);
  return true;
}

static void DecommitPages(void* addr, size_t bytes) {
  if (!VirtualFree(addr, bytes, MEM_DECOMMIT)) {
    MOZ_CRASH("DecommitPages failed");
  }
}
#elif defined(__wasi__)
static void* ReserveProcessExecutableMemory(size_t bytes) {
  MOZ_CRASH("NYI for WASI.");
  return nullptr;
}
[[nodiscard]] static bool CommitPages(void* addr, size_t bytes,
                                      ProtectionSetting protection) {
  MOZ_CRASH("NYI for WASI.");
  return false;
}
static void DecommitPages(void* addr, size_t bytes) {
  MOZ_CRASH("NYI for WASI.");
}
#else  // !XP_WIN && !__wasi__
#  ifndef MAP_NORESERVE
#    define MAP_NORESERVE 0
#  endif

static void* ComputeRandomAllocationAddress() {
#  ifdef __OpenBSD__
  // OpenBSD already has random mmap and the idea that all x64 cpus
  // have 48-bit address space is not correct. Returning nullptr
  // allows OpenBSD do to the right thing.
  return nullptr;
#  else
  uint64_t rand = js::GenerateRandomSeed();

#    ifdef HAVE_64BIT_BUILD
  // x64 CPUs have a 48-bit address space and on some platforms the OS will
  // give us access to 47 bits, so to be safe we right shift by 18 to leave
  // 46 bits.
  rand >>= 18;
#    else
  // On 32-bit, right shift by 34 to leave 30 bits, range [0, 1GiB). Then add
  // 512MiB to get range [512MiB, 1.5GiB), or [0x20000000, 0x60000000). This
  // is based on V8 comments in platform-posix.cc saying this range is
  // relatively unpopulated across a variety of kernels.
  rand >>= 34;
  rand += 512 * 1024 * 1024;
#    endif

  // Ensure page alignment.
  uintptr_t mask = ~uintptr_t(gc::SystemPageSize() - 1);
  return (void*)uintptr_t(rand & mask);
#  endif
}

static void* ReserveProcessExecutableMemory(size_t bytes) {
  // Note that randomAddr is just a hint: if the address is not available
  // mmap will pick a different address.
  void* randomAddr = ComputeRandomAllocationAddress();
  void* p = MozTaggedAnonymousMmap(randomAddr, bytes, PROT_NONE,
                                   MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1,
                                   0, "js-executable-memory");
  if (p == MAP_FAILED) {
    return nullptr;
  }
  return p;
}

static void DeallocateProcessExecutableMemory(void* addr, size_t bytes) {
  mozilla::DebugOnly<int> result = munmap(addr, bytes);
  MOZ_ASSERT(!result || errno == ENOMEM);
}

static unsigned ProtectionSettingToFlags(ProtectionSetting protection) {
#  ifdef MOZ_VALGRIND
  // If we're configured for Valgrind and running on it, use a slacker
  // scheme that doesn't change execute permissions, since doing so causes
  // Valgrind a lot of extra overhead re-JITting code that loses and later
  // regains execute permission.  See bug 1338179.
  if (RUNNING_ON_VALGRIND) {
    switch (protection) {
      case ProtectionSetting::Protected:
        return PROT_NONE;
      case ProtectionSetting::Writable:
        return PROT_READ | PROT_WRITE | PROT_EXEC;
      case ProtectionSetting::Executable:
        return PROT_READ | PROT_EXEC;
    }
    MOZ_CRASH();
  }
  // If we get here, we're configured for Valgrind but not running on
  // it, so use the standard scheme.
#  endif
  switch (protection) {
    case ProtectionSetting::Protected:
      return PROT_NONE;
    case ProtectionSetting::Writable:
      return PROT_READ | PROT_WRITE;
    case ProtectionSetting::Executable:
      return PROT_READ | PROT_EXEC;
  }
  MOZ_CRASH();
}

[[nodiscard]] static bool CommitPages(void* addr, size_t bytes,
                                      ProtectionSetting protection) {
  void* p = MozTaggedAnonymousMmap(
      addr, bytes, ProtectionSettingToFlags(protection),
      MAP_FIXED | MAP_PRIVATE | MAP_ANON, -1, 0, "js-executable-memory");
  if (p == MAP_FAILED) {
    return false;
  }
  MOZ_RELEASE_ASSERT(p == addr);
  return true;
}

static void DecommitPages(void* addr, size_t bytes) {
  // Use mmap with MAP_FIXED and PROT_NONE. Inspired by jemalloc's
  // pages_decommit.
  void* p = MozTaggedAnonymousMmap(addr, bytes, PROT_NONE,
                                   MAP_FIXED | MAP_PRIVATE | MAP_ANON, -1, 0,
                                   "js-executable-memory");
  MOZ_RELEASE_ASSERT(addr == p);
}
#endif

template <size_t NumBits>
class PageBitSet {
  using WordType = uint32_t;
  static const size_t BitsPerWord = sizeof(WordType) * 8;

  static_assert((NumBits % BitsPerWord) == 0,
                "NumBits must be a multiple of BitsPerWord");
  static const size_t NumWords = NumBits / BitsPerWord;

  mozilla::Array<WordType, NumWords> words_;

  uint32_t indexToWord(uint32_t index) const {
    MOZ_ASSERT(index < NumBits);
    return index / BitsPerWord;
  }
  WordType indexToBit(uint32_t index) const {
    MOZ_ASSERT(index < NumBits);
    return WordType(1) << (index % BitsPerWord);
  }

 public:
  void init() { mozilla::PodArrayZero(words_); }
  bool contains(size_t index) const {
    uint32_t word = indexToWord(index);
    return words_[word] & indexToBit(index);
  }
  void insert(size_t index) {
    MOZ_ASSERT(!contains(index));
    uint32_t word = indexToWord(index);
    words_[word] |= indexToBit(index);
  }
  void remove(size_t index) {
    MOZ_ASSERT(contains(index));
    uint32_t word = indexToWord(index);
    words_[word] &= ~indexToBit(index);
  }

#ifdef DEBUG
  bool empty() const {
    for (size_t i = 0; i < NumWords; i++) {
      if (words_[i] != 0) {
        return false;
      }
    }
    return true;
  }
#endif
};

// Per-process executable memory allocator. It reserves a block of memory of
// MaxCodeBytesPerProcess bytes, then allocates/deallocates pages from that.
//
// This has a number of benefits compared to raw mmap/VirtualAlloc:
//
// * More resillient against certain attacks.
//
// * Behaves more consistently across platforms: it avoids the 64K granularity
//   issues on Windows, for instance.
//
// * On x64, near jumps can be used for jumps to other JIT pages.
//
// * On Win64, we have to register the exception handler only once (at process
//   startup). This saves some memory and avoids RtlAddFunctionTable profiler
//   deadlocks.
class ProcessExecutableMemory {
  static_assert(
      (MaxCodeBytesPerProcess % ExecutableCodePageSize) == 0,
      "MaxCodeBytesPerProcess must be a multiple of ExecutableCodePageSize");
  static const size_t MaxCodePages =
      MaxCodeBytesPerProcess / ExecutableCodePageSize;

  // Start of the MaxCodeBytesPerProcess memory block or nullptr if
  // uninitialized. Note that this is NOT guaranteed to be aligned to
  // ExecutableCodePageSize.
  uint8_t* base_;

  // The fields below should only be accessed while we hold the lock.
  Mutex lock_;

  // pagesAllocated_ is an Atomic so that bytesAllocated does not have to
  // take the lock.
  mozilla::Atomic<size_t, mozilla::ReleaseAcquire> pagesAllocated_;

  // Page where we should try to allocate next.
  size_t cursor_;

  mozilla::Maybe<mozilla::non_crypto::XorShift128PlusRNG> rng_;
  PageBitSet<MaxCodePages> pages_;

 public:
  ProcessExecutableMemory()
      : base_(nullptr),
        lock_(mutexid::ProcessExecutableRegion),
        pagesAllocated_(0),
        cursor_(0),
        rng_(),
        pages_() {}

  [[nodiscard]] bool init() {
    pages_.init();

    MOZ_RELEASE_ASSERT(!initialized());
    MOZ_RELEASE_ASSERT(gc::SystemPageSize() <= ExecutableCodePageSize);

    void* p = ReserveProcessExecutableMemory(MaxCodeBytesPerProcess);
    if (!p) {
      return false;
    }

    base_ = static_cast<uint8_t*>(p);

    mozilla::Array<uint64_t, 2> seed;
    GenerateXorShift128PlusSeed(seed);
    rng_.emplace(seed[0], seed[1]);
    return true;
  }

  uint8_t* base() const { return base_; }

  bool initialized() const { return base_ != nullptr; }

  size_t bytesAllocated() const {
    MOZ_ASSERT(pagesAllocated_ <= MaxCodePages);
    return pagesAllocated_ * ExecutableCodePageSize;
  }

  void release() {
#if defined(__wasi__)
    MOZ_ASSERT(!initialized());
#else
    MOZ_ASSERT(initialized());
    MOZ_ASSERT(pages_.empty());
    MOZ_ASSERT(pagesAllocated_ == 0);
    DeallocateProcessExecutableMemory(base_, MaxCodeBytesPerProcess);
    base_ = nullptr;
    rng_.reset();
    MOZ_ASSERT(!initialized());
#endif
  }

  void assertValidAddress(void* p, size_t bytes) const {
    MOZ_RELEASE_ASSERT(p >= base_ &&
                       uintptr_t(p) + bytes <=
                           uintptr_t(base_) + MaxCodeBytesPerProcess);
  }

  bool containsAddress(const void* p) const {
    return p >= base_ &&
           uintptr_t(p) < uintptr_t(base_) + MaxCodeBytesPerProcess;
  }

  void* allocate(size_t bytes, ProtectionSetting protection,
                 MemCheckKind checkKind);
  void deallocate(void* addr, size_t bytes, bool decommit);
};

void* ProcessExecutableMemory::allocate(size_t bytes,
                                        ProtectionSetting protection,
                                        MemCheckKind checkKind) {
  MOZ_ASSERT(initialized());
  MOZ_ASSERT(bytes > 0);
  MOZ_ASSERT((bytes % ExecutableCodePageSize) == 0);

  size_t numPages = bytes / ExecutableCodePageSize;

  // Take the lock and try to allocate.
  void* p = nullptr;
  {
    LockGuard<Mutex> guard(lock_);
    MOZ_ASSERT(pagesAllocated_ <= MaxCodePages);

    // Check if we have enough pages available.
    if (pagesAllocated_ + numPages >= MaxCodePages) {
      return nullptr;
    }

    MOZ_ASSERT(bytes <= MaxCodeBytesPerProcess);

    // Maybe skip a page to make allocations less predictable.
    size_t page = cursor_ + (rng_.ref().next() % 2);

    for (size_t i = 0; i < MaxCodePages; i++) {
      // Make sure page + numPages - 1 is a valid index.
      if (page + numPages > MaxCodePages) {
        page = 0;
      }

      bool available = true;
      for (size_t j = 0; j < numPages; j++) {
        if (pages_.contains(page + j)) {
          available = false;
          break;
        }
      }
      if (!available) {
        page++;
        continue;
      }

      // Mark the pages as unavailable.
      for (size_t j = 0; j < numPages; j++) {
        pages_.insert(page + j);
      }

      pagesAllocated_ += numPages;
      MOZ_ASSERT(pagesAllocated_ <= MaxCodePages);

      // If we allocated a small number of pages, move cursor_ to the
      // next page. We don't do this for larger allocations to avoid
      // skipping a large number of small holes.
      if (numPages <= 2) {
        cursor_ = page + numPages;
      }

      p = base_ + page * ExecutableCodePageSize;
      break;
    }
    if (!p) {
      return nullptr;
    }
  }

  // Commit the pages after releasing the lock.
  if (!CommitPages(p, bytes, protection)) {
    deallocate(p, bytes, /* decommit = */ false);
    return nullptr;
  }

  SetMemCheckKind(p, bytes, checkKind);

  return p;
}

void ProcessExecutableMemory::deallocate(void* addr, size_t bytes,
                                         bool decommit) {
  MOZ_ASSERT(initialized());
  MOZ_ASSERT(addr);
  MOZ_ASSERT((uintptr_t(addr) % gc::SystemPageSize()) == 0);
  MOZ_ASSERT(bytes > 0);
  MOZ_ASSERT((bytes % ExecutableCodePageSize) == 0);

  assertValidAddress(addr, bytes);

  size_t firstPage =
      (static_cast<uint8_t*>(addr) - base_) / ExecutableCodePageSize;
  size_t numPages = bytes / ExecutableCodePageSize;

  // Decommit before taking the lock.
  MOZ_MAKE_MEM_NOACCESS(addr, bytes);
  if (decommit) {
    DecommitPages(addr, bytes);
  }

  LockGuard<Mutex> guard(lock_);
  MOZ_ASSERT(numPages <= pagesAllocated_);
  pagesAllocated_ -= numPages;

  for (size_t i = 0; i < numPages; i++) {
    pages_.remove(firstPage + i);
  }

  // Move the cursor back so we can reuse pages instead of fragmenting the
  // whole region.
  if (firstPage < cursor_) {
    cursor_ = firstPage;
  }
}

static ProcessExecutableMemory execMemory;

void* js::jit::AllocateExecutableMemory(size_t bytes,
                                        ProtectionSetting protection,
                                        MemCheckKind checkKind) {
  return execMemory.allocate(bytes, protection, checkKind);
}

void js::jit::DeallocateExecutableMemory(void* addr, size_t bytes) {
  execMemory.deallocate(addr, bytes, /* decommit = */ true);
}

bool js::jit::InitProcessExecutableMemory() {
#ifdef JS_CODEGEN_ARM64
  // Initialize instruction cache flushing.
  vixl::CPU::SetUp();
#elif defined(__wasi__)
  return true;
#endif
  return execMemory.init();
}

void js::jit::ReleaseProcessExecutableMemory() { execMemory.release(); }

size_t js::jit::LikelyAvailableExecutableMemory() {
  // Round down available memory to the closest MB.
  return MaxCodeBytesPerProcess -
         AlignBytes(execMemory.bytesAllocated(), 0x100000U);
}

bool js::jit::CanLikelyAllocateMoreExecutableMemory() {
  // Use a 8 MB buffer.
  static const size_t BufferSize = 8 * 1024 * 1024;

  MOZ_ASSERT(execMemory.bytesAllocated() <= MaxCodeBytesPerProcess);

  return execMemory.bytesAllocated() + BufferSize <= MaxCodeBytesPerProcess;
}

bool js::jit::AddressIsInExecutableMemory(const void* p) {
  return execMemory.containsAddress(p);
}

bool js::jit::ReprotectRegion(void* start, size_t size,
                              ProtectionSetting protection,
                              MustFlushICache flushICache) {
  // Flush ICache when making code executable, before we modify |size|.
  if (flushICache == MustFlushICache::LocalThreadOnly ||
      flushICache == MustFlushICache::AllThreads) {
    MOZ_ASSERT(protection == ProtectionSetting::Executable);
    bool codeIsThreadLocal = flushICache == MustFlushICache::LocalThreadOnly;
    jit::FlushICache(start, size, codeIsThreadLocal);
  }

  // Calculate the start of the page containing this region,
  // and account for this extra memory within size.
  size_t pageSize = gc::SystemPageSize();
  intptr_t startPtr = reinterpret_cast<intptr_t>(start);
  intptr_t pageStartPtr = startPtr & ~(pageSize - 1);
  void* pageStart = reinterpret_cast<void*>(pageStartPtr);
  size += (startPtr - pageStartPtr);

  // Round size up
  size += (pageSize - 1);
  size &= ~(pageSize - 1);

  MOZ_ASSERT((uintptr_t(pageStart) % pageSize) == 0);

  execMemory.assertValidAddress(pageStart, size);

  // On weak memory systems, make sure new code is visible on all cores before
  // addresses of the code are made public.  Now is the latest moment in time
  // when we can do that, and we're assuming that every other thread that has
  // written into the memory that is being reprotected here has synchronized
  // with this thread in such a way that the memory writes have become visible
  // and we therefore only need to execute the fence once here.  See bug 1529933
  // for a longer discussion of why this is both necessary and sufficient.
  //
  // We use the C++ fence here -- and not AtomicOperations::fenceSeqCst() --
  // primarily because ReprotectRegion will be called while we construct our own
  // jitted atomics.  But the C++ fence is sufficient and correct, too.
#ifdef __wasi__
  MOZ_CRASH("NYI FOR WASI.");
#else
  std::atomic_thread_fence(std::memory_order_seq_cst);

#  ifdef XP_WIN
  DWORD oldProtect;
  DWORD flags = ProtectionSettingToFlags(protection);
  if (!VirtualProtect(pageStart, size, flags, &oldProtect)) {
    return false;
  }
#  else
  unsigned flags = ProtectionSettingToFlags(protection);
  if (mprotect(pageStart, size, flags)) {
    return false;
  }
#  endif
#endif  // __wasi__

  execMemory.assertValidAddress(pageStart, size);
  return true;
}

#if defined(XP_WIN) && defined(NEED_JIT_UNWIND_HANDLING)
static PRUNTIME_FUNCTION RuntimeFunctionCallback(DWORD64 ControlPc,
                                                 PVOID Context) {
  MOZ_ASSERT(sJitExceptionHandler);

  // RegisterExecutableMemory already set up the runtime function in the
  // exception-data page preceding the allocation.
  uint8_t* p = execMemory.base();
  if (!p) {
    return nullptr;
  }
  return (PRUNTIME_FUNCTION)(p - gc::SystemPageSize());
}
#endif