DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Header

Mercurial (b6d82b1a6b02)

VCS Links

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772
/* -*- 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 "jsutil.h"

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

#ifdef XP_WIN
#  include "mozilla/StackWalk_windows.h"
#  include "mozilla/WindowsVersion.h"
#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;

JS_FRIEND_API void js::SetJitExceptionHandler(JitExceptionHandler handler) {
  MOZ_ASSERT(!sJitExceptionHandler);
  sJitExceptionHandler = handler;
}

#    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();
}

static MOZ_MUST_USE 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");
  }
}
#else  // !XP_WIN
static void* ComputeRandomAllocationAddress() {
  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);
}

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_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();
}

static MOZ_MUST_USE 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,
                  mozilla::recordreplay::Behavior::DontPreserve>
      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_() {}

  MOZ_MUST_USE 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() {
    MOZ_ASSERT(initialized());
    MOZ_ASSERT(pages_.empty());
    MOZ_ASSERT(pagesAllocated_ == 0);
    DeallocateProcessExecutableMemory(base_, MaxCodeBytesPerProcess);
    base_ = nullptr;
    rng_.reset();
    MOZ_ASSERT(!initialized());
  }

  void assertValidAddress(void* p, size_t bytes) const {
    MOZ_RELEASE_ASSERT(p >= base_ &&
                       uintptr_t(p) + bytes <=
                           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();
#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::ReprotectRegion(void* start, size_t size,
                              ProtectionSetting protection) {
  // 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.
  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

  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