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 (50bba836b642)

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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326
/* 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 <string>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <dlfcn.h>
#include <unistd.h>
#include <errno.h>
#include <algorithm>
#include <fcntl.h>
#include "ElfLoader.h"
#include "BaseElf.h"
#include "CustomElf.h"
#include "Mappable.h"
#include "Logging.h"
#include "Utils.h"
#include <inttypes.h>

// From Utils.h
mozilla::Atomic<size_t, mozilla::ReleaseAcquire> gPageSize;

#if defined(ANDROID)
#  include <sys/syscall.h>
#  include <sys/system_properties.h>
#  include <math.h>

#  include <android/api-level.h>
#  if __ANDROID_API__ < 8
/* Android API < 8 doesn't provide sigaltstack */

extern "C" {

inline int sigaltstack(const stack_t* ss, stack_t* oss) {
  return syscall(__NR_sigaltstack, ss, oss);
}

} /* extern "C" */
#  endif /* __ANDROID_API__ */
#endif   /* ANDROID */

#ifdef __ARM_EABI__
extern "C" MOZ_EXPORT const void* __gnu_Unwind_Find_exidx(void* pc, int* pcount)
    __attribute__((weak));
#endif

/* Ideally we'd #include <link.h>, but that's a world of pain
 * Moreover, not all versions of android support it, so we need a weak
 * reference. */
extern "C" MOZ_EXPORT int dl_iterate_phdr(dl_phdr_cb callback, void* data)
    __attribute__((weak));

/* Pointer to the PT_DYNAMIC section of the executable or library
 * containing this code. */
extern "C" Elf::Dyn _DYNAMIC[];

/**
 * dlfcn.h replacements functions
 */

void* __wrap_dlopen(const char* path, int flags) {
  RefPtr<LibHandle> handle = ElfLoader::Singleton.Load(path, flags);
  if (handle) handle->AddDirectRef();
  return handle;
}

const char* __wrap_dlerror(void) {
  const char* error = ElfLoader::Singleton.lastError.exchange(nullptr);
  if (error) {
    // Return a custom error if available.
    return error;
  }
  // Or fallback to the system error.
  return dlerror();
}

void* __wrap_dlsym(void* handle, const char* symbol) {
  if (!handle) {
    ElfLoader::Singleton.lastError = "dlsym(NULL, sym) unsupported";
    return nullptr;
  }
  if (handle != RTLD_DEFAULT && handle != RTLD_NEXT) {
    LibHandle* h = reinterpret_cast<LibHandle*>(handle);
    return h->GetSymbolPtr(symbol);
  }

  ElfLoader::Singleton.lastError = nullptr;  // Use system dlerror.
  return dlsym(handle, symbol);
}

int __wrap_dlclose(void* handle) {
  if (!handle) {
    ElfLoader::Singleton.lastError = "No handle given to dlclose()";
    return -1;
  }
  reinterpret_cast<LibHandle*>(handle)->ReleaseDirectRef();
  return 0;
}

int __wrap_dladdr(void* addr, Dl_info* info) {
  RefPtr<LibHandle> handle = ElfLoader::Singleton.GetHandleByPtr(addr);
  if (!handle) {
    return dladdr(addr, info);
  }
  info->dli_fname = handle->GetPath();
  info->dli_fbase = handle->GetBase();
  return 1;
}

class DlIteratePhdrHelper {
 public:
  DlIteratePhdrHelper() {
    int pipefd[2];
    valid_pipe = (pipe(pipefd) == 0);
    read_fd.reset(pipefd[0]);
    write_fd.reset(pipefd[1]);
  }

  int fill_and_call(dl_phdr_cb callback, const void* l_addr, const char* l_name,
                    void* data);

 private:
  bool valid_pipe;
  AutoCloseFD read_fd;
  AutoCloseFD write_fd;
};

// This function is called for each shared library iterated over by
// dl_iterate_phdr, and is used to fill a dl_phdr_info which is then
// sent through to the dl_iterate_phdr callback.
int DlIteratePhdrHelper::fill_and_call(dl_phdr_cb callback, const void* l_addr,
                                       const char* l_name, void* data) {
  dl_phdr_info info;
  info.dlpi_addr = reinterpret_cast<Elf::Addr>(l_addr);
  info.dlpi_name = l_name;
  info.dlpi_phdr = nullptr;
  info.dlpi_phnum = 0;

  // Assuming l_addr points to Elf headers (in most cases, this is true),
  // get the Phdr location from there.
  // Unfortunately, when l_addr doesn't point to Elf headers, it may point
  // to unmapped memory, or worse, unreadable memory. The only way to detect
  // the latter without causing a SIGSEGV is to use the pointer in a system
  // call that will try to read from there, and return an EFAULT error if
  // it can't. One such system call is write(). It used to be possible to
  // use a file descriptor on /dev/null for these kind of things, but recent
  // Linux kernels never return an EFAULT error when using /dev/null.
  // So instead, we use a self pipe. We do however need to read() from the
  // read end of the pipe as well so as to not fill up the pipe buffer and
  // block on subsequent writes.
  // In the unlikely event reads from or write to the pipe fail for some
  // other reason than EFAULT, we don't try any further and just skip setting
  // the Phdr location for all subsequent libraries, rather than trying to
  // start over with a new pipe.
  int can_read = true;
  if (valid_pipe) {
    int ret;
    char raw_ehdr[sizeof(Elf::Ehdr)];
    static_assert(sizeof(raw_ehdr) < PIPE_BUF, "PIPE_BUF is too small");
    do {
      // writes are atomic when smaller than PIPE_BUF, per POSIX.1-2008.
      ret = write(write_fd, l_addr, sizeof(raw_ehdr));
    } while (ret == -1 && errno == EINTR);
    if (ret != sizeof(raw_ehdr)) {
      if (ret == -1 && errno == EFAULT) {
        can_read = false;
      } else {
        valid_pipe = false;
      }
    } else {
      size_t nbytes = 0;
      do {
        // Per POSIX.1-2008, interrupted reads can return a length smaller
        // than the given one instead of failing with errno EINTR.
        ret = read(read_fd, raw_ehdr + nbytes, sizeof(raw_ehdr) - nbytes);
        if (ret > 0) nbytes += ret;
      } while ((nbytes != sizeof(raw_ehdr) && ret > 0) ||
               (ret == -1 && errno == EINTR));
      if (nbytes != sizeof(raw_ehdr)) {
        valid_pipe = false;
      }
    }
  }

  if (valid_pipe && can_read) {
    const Elf::Ehdr* ehdr = Elf::Ehdr::validate(l_addr);
    if (ehdr) {
      info.dlpi_phdr = reinterpret_cast<const Elf::Phdr*>(
          reinterpret_cast<const char*>(ehdr) + ehdr->e_phoff);
      info.dlpi_phnum = ehdr->e_phnum;
    }
  }

  return callback(&info, sizeof(dl_phdr_info), data);
}

int __wrap_dl_iterate_phdr(dl_phdr_cb callback, void* data) {
  DlIteratePhdrHelper helper;
  AutoLock lock(&ElfLoader::Singleton.handlesMutex);

  if (dl_iterate_phdr) {
    for (ElfLoader::LibHandleList::reverse_iterator it =
             ElfLoader::Singleton.handles.rbegin();
         it < ElfLoader::Singleton.handles.rend(); ++it) {
      BaseElf* elf = (*it)->AsBaseElf();
      if (!elf) {
        continue;
      }
      int ret = helper.fill_and_call(callback, (*it)->GetBase(),
                                     (*it)->GetPath(), data);
      if (ret) return ret;
    }
    return dl_iterate_phdr(callback, data);
  }

  /* For versions of Android that don't support dl_iterate_phdr (< 5.0),
   * we go through the debugger helper data, which is known to be racy, but
   * there's not much we can do about this :( . */
  if (!ElfLoader::Singleton.dbg) return -1;

  for (ElfLoader::DebuggerHelper::iterator it =
           ElfLoader::Singleton.dbg.begin();
       it < ElfLoader::Singleton.dbg.end(); ++it) {
    int ret = helper.fill_and_call(callback, it->l_addr, it->l_name, data);
    if (ret) return ret;
  }
  return 0;
}

#ifdef __ARM_EABI__
const void* __wrap___gnu_Unwind_Find_exidx(void* pc, int* pcount) {
  RefPtr<LibHandle> handle = ElfLoader::Singleton.GetHandleByPtr(pc);
  if (handle) return handle->FindExidx(pcount);
  if (__gnu_Unwind_Find_exidx) return __gnu_Unwind_Find_exidx(pc, pcount);
  *pcount = 0;
  return nullptr;
}
#endif

/**
 * faulty.lib public API
 */

MFBT_API size_t __dl_get_mappable_length(void* handle) {
  if (!handle) return 0;
  return reinterpret_cast<LibHandle*>(handle)->GetMappableLength();
}

MFBT_API void* __dl_mmap(void* handle, void* addr, size_t length,
                         off_t offset) {
  if (!handle) return nullptr;
  return reinterpret_cast<LibHandle*>(handle)->MappableMMap(addr, length,
                                                            offset);
}

MFBT_API void __dl_munmap(void* handle, void* addr, size_t length) {
  if (!handle) return;
  return reinterpret_cast<LibHandle*>(handle)->MappableMUnmap(addr, length);
}

MFBT_API bool IsSignalHandlingBroken() {
  return ElfLoader::Singleton.isSignalHandlingBroken();
}

namespace {

/**
 * Returns the part after the last '/' for the given path
 */
const char* LeafName(const char* path) {
  const char* lastSlash = strrchr(path, '/');
  if (lastSlash) return lastSlash + 1;
  return path;
}

#if defined(ANDROID)
/**
 * Return the current Android version, or 0 on failure.
 */
int GetAndroidSDKVersion() {
  static int version = 0;
  if (version) {
    return version;
  }

  char version_string[PROP_VALUE_MAX] = {'\0'};
  int len = __system_property_get("ro.build.version.sdk", version_string);
  if (len) {
    version = static_cast<int>(strtol(version_string, nullptr, 10));
  }
  return version;
}
#endif

/**
 * Run the given lambda while holding the internal lock of the system linker.
 * To take the lock, we call the system dl_iterate_phdr and invoke the lambda
 * from the callback, which is called while the lock is held. Return true on
 * success.
 */
template <class Lambda>
static bool RunWithSystemLinkerLock(Lambda&& aLambda) {
  if (!dl_iterate_phdr) {
    // No dl_iterate_phdr support.
    return false;
  }

#if defined(ANDROID)
  if (GetAndroidSDKVersion() < 23) {
    // dl_iterate_phdr is _not_ protected by a lock on Android < 23.
    // Also return false here if we failed to get the version.
    return false;
  }
#endif

  dl_iterate_phdr(
      [](dl_phdr_info*, size_t, void* lambda) -> int {
        (*static_cast<Lambda*>(lambda))();
        // Return 1 to stop iterating.
        return 1;
      },
      &aLambda);
  return true;
}

} /* Anonymous namespace */

/**
 * LibHandle
 */
LibHandle::~LibHandle() { free(path); }

const char* LibHandle::GetName() const {
  return path ? LeafName(path) : nullptr;
}

size_t LibHandle::GetMappableLength() const {
  if (!mappable) mappable = GetMappable();
  if (!mappable) return 0;
  return mappable->GetLength();
}

void* LibHandle::MappableMMap(void* addr, size_t length, off_t offset) const {
  if (!mappable) mappable = GetMappable();
  if (!mappable) return MAP_FAILED;
  void* mapped = mappable->mmap(addr, length, PROT_READ, MAP_PRIVATE, offset);
  return mapped;
}

void LibHandle::MappableMUnmap(void* addr, size_t length) const {
  if (mappable) mappable->munmap(addr, length);
}

/**
 * SystemElf
 */
already_AddRefed<LibHandle> SystemElf::Load(const char* path, int flags) {
  /* The Android linker returns a handle when the file name matches an
   * already loaded library, even when the full path doesn't exist */
  if (path && path[0] == '/' && (access(path, F_OK) == -1)) {
    DEBUG_LOG("dlopen(\"%s\", 0x%x) = %p", path, flags, (void*)nullptr);
    ElfLoader::Singleton.lastError = "Specified file does not exist";
    return nullptr;
  }

  ElfLoader::Singleton.lastError = nullptr;  // Use system dlerror.
  void* handle = dlopen(path, flags);
  DEBUG_LOG("dlopen(\"%s\", 0x%x) = %p", path, flags, handle);
  if (handle) {
    SystemElf* elf = new SystemElf(path, handle);
    ElfLoader::Singleton.Register(elf);
    RefPtr<LibHandle> lib(elf);
    return lib.forget();
  }
  return nullptr;
}

SystemElf::~SystemElf() {
  if (!dlhandle) return;
  DEBUG_LOG("dlclose(%p [\"%s\"])", dlhandle, GetPath());
  ElfLoader::Singleton.lastError = nullptr;  // Use system dlerror.
  dlclose(dlhandle);
  ElfLoader::Singleton.Forget(this);
}

void* SystemElf::GetSymbolPtr(const char* symbol) const {
  ElfLoader::Singleton.lastError = nullptr;  // Use system dlerror.
  void* sym = dlsym(dlhandle, symbol);
  DEBUG_LOG("dlsym(%p [\"%s\"], \"%s\") = %p", dlhandle, GetPath(), symbol,
            sym);
  return sym;
}

Mappable* SystemElf::GetMappable() const {
  const char* path = GetPath();
  if (!path) return nullptr;
#ifdef ANDROID
  /* On Android, if we don't have the full path, try in /system/lib */
  const char* name = LeafName(path);
  std::string systemPath;
  if (name == path) {
    systemPath = "/system/lib/";
    systemPath += path;
    path = systemPath.c_str();
  }
#endif

  return MappableFile::Create(path);
}

#ifdef __ARM_EABI__
const void* SystemElf::FindExidx(int* pcount) const {
  /* TODO: properly implement when ElfLoader::GetHandleByPtr
     does return SystemElf handles */
  *pcount = 0;
  return nullptr;
}
#endif

/**
 * ElfLoader
 */

/* Unique ElfLoader instance */
ElfLoader ElfLoader::Singleton;

already_AddRefed<LibHandle> ElfLoader::Load(const char* path, int flags,
                                            LibHandle* parent) {
  /* Ensure logging is initialized or refresh if environment changed. */
  Logging::Init();

  /* Ensure self_elf initialization. */
  if (!self_elf) Init();

  RefPtr<LibHandle> handle;

  /* Handle dlopen(nullptr) directly. */
  if (!path) {
    handle = SystemElf::Load(nullptr, flags);
    return handle.forget();
  }

  /* TODO: Handle relative paths correctly */
  const char* name = LeafName(path);

  /* Search the list of handles we already have for a match. When the given
   * path is not absolute, compare file names, otherwise compare full paths. */
  if (name == path) {
    AutoLock lock(&handlesMutex);
    for (LibHandleList::iterator it = handles.begin(); it < handles.end(); ++it)
      if ((*it)->GetName() && (strcmp((*it)->GetName(), name) == 0)) {
        handle = *it;
        return handle.forget();
      }
  } else {
    AutoLock lock(&handlesMutex);
    for (LibHandleList::iterator it = handles.begin(); it < handles.end(); ++it)
      if ((*it)->GetPath() && (strcmp((*it)->GetPath(), path) == 0)) {
        handle = *it;
        return handle.forget();
      }
  }

  char* abs_path = nullptr;
  const char* requested_path = path;

  /* When the path is not absolute and the library is being loaded for
   * another, first try to load the library from the directory containing
   * that parent library. */
  if ((name == path) && parent) {
    const char* parentPath = parent->GetPath();
    abs_path = new char[strlen(parentPath) + strlen(path)];
    strcpy(abs_path, parentPath);
    char* slash = strrchr(abs_path, '/');
    strcpy(slash + 1, path);
    path = abs_path;
  }

  Mappable* mappable = GetMappableFromPath(path);

  /* Try loading with the custom linker if we have a Mappable */
  if (mappable) handle = CustomElf::Load(mappable, path, flags);

  /* Try loading with the system linker if everything above failed */
  if (!handle) handle = SystemElf::Load(path, flags);

  /* If we didn't have an absolute path and haven't been able to load
   * a library yet, try in the system search path */
  if (!handle && abs_path) handle = SystemElf::Load(name, flags);

  delete[] abs_path;
  DEBUG_LOG("ElfLoader::Load(\"%s\", 0x%x, %p [\"%s\"]) = %p", requested_path,
            flags, reinterpret_cast<void*>(parent),
            parent ? parent->GetPath() : "", static_cast<void*>(handle));

  return handle.forget();
}

already_AddRefed<LibHandle> ElfLoader::GetHandleByPtr(void* addr) {
  AutoLock lock(&handlesMutex);
  /* Scan the list of handles we already have for a match */
  for (LibHandleList::iterator it = handles.begin(); it < handles.end(); ++it) {
    if ((*it)->Contains(addr)) {
      RefPtr<LibHandle> lib = *it;
      return lib.forget();
    }
  }
  return nullptr;
}

Mappable* ElfLoader::GetMappableFromPath(const char* path) {
  const char* name = LeafName(path);
  Mappable* mappable = nullptr;
  RefPtr<Zip> zip;
  const char* subpath;
  if ((subpath = strchr(path, '!'))) {
    char* zip_path = strndup(path, subpath - path);
    while (*(++subpath) == '/') {
    }
    zip = ZipCollection::GetZip(zip_path);
    free(zip_path);
    Zip::Stream s;
    if (zip && zip->GetStream(subpath, &s)) {
      /* When the MOZ_LINKER_EXTRACT environment variable is set to "1",
       * compressed libraries are going to be (temporarily) extracted as
       * files, in the directory pointed by the MOZ_LINKER_CACHE
       * environment variable. */
      const char* extract = getenv("MOZ_LINKER_EXTRACT");
      if (extract && !strncmp(extract, "1", 2 /* Including '\0' */))
        mappable = MappableExtractFile::Create(name, zip, &s);
      if (!mappable) {
        if (s.GetType() == Zip::Stream::DEFLATE) {
          mappable = MappableDeflate::Create(name, zip, &s);
        }
      }
    }
  }
  /* If we couldn't load above, try with a MappableFile */
  if (!mappable && !zip) mappable = MappableFile::Create(path);

  return mappable;
}

void ElfLoader::Register(LibHandle* handle) {
  AutoLock lock(&handlesMutex);
  handles.push_back(handle);
}

void ElfLoader::Register(CustomElf* handle) {
  Register(static_cast<LibHandle*>(handle));
  if (dbg) {
    // We could race with the system linker when modifying the debug map, so
    // only do so while holding the system linker's internal lock.
    RunWithSystemLinkerLock([this, handle] { dbg.Add(handle); });
  }
}

void ElfLoader::Forget(LibHandle* handle) {
  /* Ensure logging is initialized or refresh if environment changed. */
  Logging::Init();

  AutoLock lock(&handlesMutex);
  LibHandleList::iterator it =
      std::find(handles.begin(), handles.end(), handle);
  if (it != handles.end()) {
    DEBUG_LOG("ElfLoader::Forget(%p [\"%s\"])", reinterpret_cast<void*>(handle),
              handle->GetPath());
    handles.erase(it);
  } else {
    DEBUG_LOG("ElfLoader::Forget(%p [\"%s\"]): Handle not found",
              reinterpret_cast<void*>(handle), handle->GetPath());
  }
}

void ElfLoader::Forget(CustomElf* handle) {
  Forget(static_cast<LibHandle*>(handle));
  if (dbg) {
    // We could race with the system linker when modifying the debug map, so
    // only do so while holding the system linker's internal lock.
    RunWithSystemLinkerLock([this, handle] { dbg.Remove(handle); });
  }
}

void ElfLoader::Init() {
  Dl_info info;
  /* On Android < 4.1 can't reenter dl* functions. So when the library
   * containing this code is dlopen()ed, it can't call dladdr from a
   * static initializer. */
  if (dladdr(_DYNAMIC, &info) != 0) {
    self_elf = LoadedElf::Create(info.dli_fname, info.dli_fbase);
  }
#if defined(ANDROID)
  // On Android < 5.0, resolving weak symbols via dlsym doesn't work.
  // The weak symbols Gecko uses are in either libc or libm, so we
  // wrap those such that this linker does symbol resolution for them.
  if (GetAndroidSDKVersion() < 21) {
    if (dladdr(FunctionPtr(syscall), &info) != 0) {
      libc = LoadedElf::Create(info.dli_fname, info.dli_fbase);
    }
    if (dladdr(FunctionPtr<int (*)(double)>(isnan), &info) != 0) {
      libm = LoadedElf::Create(info.dli_fname, info.dli_fbase);
    }
  }
#endif
}

ElfLoader::~ElfLoader() {
  LibHandleList list;

  if (!Singleton.IsShutdownExpected()) {
    MOZ_CRASH("Unexpected shutdown");
  }

  /* Release self_elf and libc */
  self_elf = nullptr;
#if defined(ANDROID)
  libc = nullptr;
  libm = nullptr;
#endif

  AutoLock lock(&handlesMutex);
  /* Build up a list of all library handles with direct (external) references.
   * We actually skip system library handles because we want to keep at least
   * some of these open. Most notably, Mozilla codebase keeps a few libgnome
   * libraries deliberately open because of the mess that libORBit destruction
   * is. dlclose()ing these libraries actually leads to problems. */
  for (LibHandleList::reverse_iterator it = handles.rbegin();
       it < handles.rend(); ++it) {
    if ((*it)->DirectRefCount()) {
      if (SystemElf* se = (*it)->AsSystemElf()) {
        se->Forget();
      } else {
        list.push_back(*it);
      }
    }
  }
  /* Force release all external references to the handles collected above */
  for (LibHandleList::iterator it = list.begin(); it < list.end(); ++it) {
    while ((*it)->ReleaseDirectRef()) {
    }
  }
  /* Remove the remaining system handles. */
  if (handles.size()) {
    list = handles;
    for (LibHandleList::reverse_iterator it = list.rbegin(); it < list.rend();
         ++it) {
      if ((*it)->AsSystemElf()) {
        DEBUG_LOG(
            "ElfLoader::~ElfLoader(): Remaining handle for \"%s\" "
            "[%" PRIdPTR " direct refs, %" PRIdPTR " refs total]",
            (*it)->GetPath(), (*it)->DirectRefCount(), (*it)->refCount());
      } else {
        DEBUG_LOG(
            "ElfLoader::~ElfLoader(): Unexpected remaining handle for \"%s\" "
            "[%" PRIdPTR " direct refs, %" PRIdPTR " refs total]",
            (*it)->GetPath(), (*it)->DirectRefCount(), (*it)->refCount());
        /* Not removing, since it could have references to other libraries,
         * destroying them as a side effect, and possibly leaving dangling
         * pointers in the handle list we're scanning */
      }
    }
  }
  pthread_mutex_destroy(&handlesMutex);
}

#ifdef __ARM_EABI__
int ElfLoader::__wrap_aeabi_atexit(void* that, ElfLoader::Destructor destructor,
                                   void* dso_handle) {
  Singleton.destructors.push_back(
      DestructorCaller(destructor, that, dso_handle));
  return 0;
}
#else
int ElfLoader::__wrap_cxa_atexit(ElfLoader::Destructor destructor, void* that,
                                 void* dso_handle) {
  Singleton.destructors.push_back(
      DestructorCaller(destructor, that, dso_handle));
  return 0;
}
#endif

void ElfLoader::__wrap_cxa_finalize(void* dso_handle) {
  /* Call all destructors for the given DSO handle in reverse order they were
   * registered. */
  std::vector<DestructorCaller>::reverse_iterator it;
  for (it = Singleton.destructors.rbegin(); it < Singleton.destructors.rend();
       ++it) {
    if (it->IsForHandle(dso_handle)) {
      it->Call();
    }
  }
}

void ElfLoader::DestructorCaller::Call() {
  if (destructor) {
    DEBUG_LOG("ElfLoader::DestructorCaller::Call(%p, %p, %p)",
              FunctionPtr(destructor), object, dso_handle);
    destructor(object);
    destructor = nullptr;
  }
}

ElfLoader::DebuggerHelper::DebuggerHelper()
    : dbg(nullptr), firstAdded(nullptr) {
  /* Find ELF auxiliary vectors.
   *
   * The kernel stores the following data on the stack when starting a
   * program:
   *   argc
   *   argv[0] (pointer into argv strings defined below)
   *   argv[1] (likewise)
   *   ...
   *   argv[argc - 1] (likewise)
   *   nullptr
   *   envp[0] (pointer into environment strings defined below)
   *   envp[1] (likewise)
   *   ...
   *   envp[n] (likewise)
   *   nullptr
   *   ... (more NULLs on some platforms such as Android 4.3)
   *   auxv[0] (first ELF auxiliary vector)
   *   auxv[1] (second ELF auxiliary vector)
   *   ...
   *   auxv[p] (last ELF auxiliary vector)
   *   (AT_NULL, nullptr)
   *   padding
   *   argv strings, separated with '\0'
   *   environment strings, separated with '\0'
   *   nullptr
   *
   * What we are after are the auxv values defined by the following struct.
   */
  struct AuxVector {
    Elf::Addr type;
    Elf::Addr value;
  };

  /* Pointer to the environment variables list */
  extern char** environ;

  /* The environment may have changed since the program started, in which
   * case the environ variables list isn't the list the kernel put on stack
   * anymore. But in this new list, variables that didn't change still point
   * to the strings the kernel put on stack. It is quite unlikely that two
   * modified environment variables point to two consecutive strings in memory,
   * so we assume that if two consecutive environment variables point to two
   * consecutive strings, we found strings the kernel put on stack. */
  char** env;
  for (env = environ; *env; env++)
    if (*env + strlen(*env) + 1 == env[1]) break;
  if (!*env) return;

  /* Next, we scan the stack backwards to find a pointer to one of those
   * strings we found above, which will give us the location of the original
   * envp list. As we are looking for pointers, we need to look at 32-bits or
   * 64-bits aligned values, depening on the architecture. */
  char** scan = reinterpret_cast<char**>(reinterpret_cast<uintptr_t>(*env) &
                                         ~(sizeof(void*) - 1));
  while (*env != *scan) scan--;

  /* Finally, scan forward to find the last environment variable pointer and
   * thus the first auxiliary vector. */
  while (*scan++)
    ;

  /* Some platforms have more NULLs here, so skip them if we encounter them */
  while (!*scan) scan++;

  AuxVector* auxv = reinterpret_cast<AuxVector*>(scan);

  /* The two values of interest in the auxiliary vectors are AT_PHDR and
   * AT_PHNUM, which gives us the the location and size of the ELF program
   * headers. */
  Array<Elf::Phdr> phdrs;
  char* base = nullptr;
  while (auxv->type) {
    if (auxv->type == AT_PHDR) {
      phdrs.Init(reinterpret_cast<Elf::Phdr*>(auxv->value));
      /* Assume the base address is the first byte of the same page */
      base = reinterpret_cast<char*>(PageAlignedPtr(auxv->value));
    }
    if (auxv->type == AT_PHNUM) phdrs.Init(auxv->value);
    auxv++;
  }

  if (!phdrs) {
    DEBUG_LOG("Couldn't find program headers");
    return;
  }

  /* In some cases, the address for the program headers we get from the
   * auxiliary vectors is not mapped, because of the PT_LOAD segments
   * definitions in the program executable. Trying to map anonymous memory
   * with a hint giving the base address will return a different address
   * if something is mapped there, and the base address otherwise. */
  MappedPtr mem(MemoryRange::mmap(base, PageSize(), PROT_NONE,
                                  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0));
  if (mem == base) {
    /* If program headers aren't mapped, try to map them */
    int fd = open("/proc/self/exe", O_RDONLY);
    if (fd == -1) {
      DEBUG_LOG("Failed to open /proc/self/exe");
      return;
    }
    mem.Assign(
        MemoryRange::mmap(base, PageSize(), PROT_READ, MAP_PRIVATE, fd, 0));
    /* If we don't manage to map at the right address, just give up. */
    if (mem != base) {
      DEBUG_LOG("Couldn't read program headers");
      return;
    }
  }
  /* Sanity check: the first bytes at the base address should be an ELF
   * header. */
  if (!Elf::Ehdr::validate(base)) {
    DEBUG_LOG("Couldn't find program base");
    return;
  }

  /* Search for the program PT_DYNAMIC segment */
  Array<Elf::Dyn> dyns;
  for (Array<Elf::Phdr>::iterator phdr = phdrs.begin(); phdr < phdrs.end();
       ++phdr) {
    /* While the program headers are expected within the first mapped page of
     * the program executable, the executable PT_LOADs may actually make them
     * loaded at an address that is not the wanted base address of the
     * library. We thus need to adjust the base address, compensating for the
     * virtual address of the PT_LOAD segment corresponding to offset 0. */
    if (phdr->p_type == PT_LOAD && phdr->p_offset == 0) base -= phdr->p_vaddr;
    if (phdr->p_type == PT_DYNAMIC)
      dyns.Init(base + phdr->p_vaddr, phdr->p_filesz);
  }
  if (!dyns) {
    DEBUG_LOG("Failed to find PT_DYNAMIC section in program");
    return;
  }

  /* Search for the DT_DEBUG information */
  for (Array<Elf::Dyn>::iterator dyn = dyns.begin(); dyn < dyns.end(); ++dyn) {
    if (dyn->d_tag == DT_DEBUG) {
      dbg = reinterpret_cast<r_debug*>(dyn->d_un.d_ptr);
      break;
    }
  }
  DEBUG_LOG("DT_DEBUG points at %p", static_cast<void*>(dbg));
}

/**
 * Helper class to ensure the given pointer is writable within the scope of
 * an instance. Permissions to the memory page where the pointer lies are
 * restored to their original value when the instance is destroyed.
 */
class EnsureWritable {
 public:
  template <typename T>
  explicit EnsureWritable(T* ptr, size_t length_ = sizeof(T)) {
    MOZ_ASSERT(length_ < PageSize());
    prot = -1;
    page = MAP_FAILED;

    char* firstPage = PageAlignedPtr(reinterpret_cast<char*>(ptr));
    char* lastPageEnd =
        PageAlignedEndPtr(reinterpret_cast<char*>(ptr) + length_);
    length = lastPageEnd - firstPage;
    uintptr_t start = reinterpret_cast<uintptr_t>(firstPage);
    uintptr_t end;

    prot = getProt(start, &end);
    if (prot == -1 || (start + length) > end) MOZ_CRASH();

    if (prot & PROT_WRITE) {
      success = true;
      return;
    }

    page = firstPage;
    int ret = mprotect(page, length, prot | PROT_WRITE);
    success = ret == 0;
    if (!success) {
      ERROR("mprotect(%p, %zu, %d) = %d (errno=%d; %s)", page, length,
            prot | PROT_WRITE, ret, errno, strerror(errno));
    }
  }

  bool IsWritable() const { return success; }

  ~EnsureWritable() {
    if (success && page != MAP_FAILED) {
      mprotect(page, length, prot);
    }
  }

 private:
  int getProt(uintptr_t addr, uintptr_t* end) {
    /* The interesting part of the /proc/self/maps format looks like:
     * startAddr-endAddr rwxp */
    int result = 0;
    AutoCloseFILE f(fopen("/proc/self/maps", "r"));
    while (f) {
      unsigned long long startAddr, endAddr;
      char perms[5];
      if (fscanf(f, "%llx-%llx %4s %*1024[^\n] ", &startAddr, &endAddr,
                 perms) != 3)
        return -1;
      if (addr < startAddr || addr >= endAddr) continue;
      if (perms[0] == 'r')
        result |= PROT_READ;
      else if (perms[0] != '-')
        return -1;
      if (perms[1] == 'w')
        result |= PROT_WRITE;
      else if (perms[1] != '-')
        return -1;
      if (perms[2] == 'x')
        result |= PROT_EXEC;
      else if (perms[2] != '-')
        return -1;
      *end = endAddr;
      return result;
    }
    return -1;
  }

  int prot;
  void* page;
  size_t length;
  bool success;
};

/**
 * The system linker maintains a doubly linked list of library it loads
 * for use by the debugger. Unfortunately, it also uses the list pointers
 * in a lot of operations and adding our data in the list is likely to
 * trigger crashes when the linker tries to use data we don't provide or
 * that fall off the amount data we allocated. Fortunately, the linker only
 * traverses the list forward and accesses the head of the list from a
 * private pointer instead of using the value in the r_debug structure.
 * This means we can safely add members at the beginning of the list.
 * Unfortunately, gdb checks the coherency of l_prev values, so we have
 * to adjust the l_prev value for the first element the system linker
 * knows about. Fortunately, it doesn't use l_prev, and the first element
 * is not ever going to be released before our elements, since it is the
 * program executable, so the system linker should not be changing
 * r_debug::r_map.
 */
void ElfLoader::DebuggerHelper::Add(ElfLoader::link_map* map) {
  if (!dbg->r_brk) return;

  dbg->r_state = r_debug::RT_ADD;
  dbg->r_brk();

  if (!firstAdded) {
    /* When adding a library for the first time, r_map points to data
     * handled by the system linker, and that data may be read-only */
    EnsureWritable w(&dbg->r_map->l_prev);
    if (!w.IsWritable()) {
      dbg->r_state = r_debug::RT_CONSISTENT;
      dbg->r_brk();
      return;
    }

    firstAdded = map;
    dbg->r_map->l_prev = map;
  } else
    dbg->r_map->l_prev = map;

  map->l_prev = nullptr;
  map->l_next = dbg->r_map;

  dbg->r_map = map;
  dbg->r_state = r_debug::RT_CONSISTENT;
  dbg->r_brk();
}

void ElfLoader::DebuggerHelper::Remove(ElfLoader::link_map* map) {
  if (!dbg->r_brk) return;

  dbg->r_state = r_debug::RT_DELETE;
  dbg->r_brk();

  if (map == firstAdded) {
    /* When removing the first added library, its l_next is going to be
     * data handled by the system linker, and that data may be read-only */
    EnsureWritable w(&map->l_next->l_prev);
    if (!w.IsWritable()) {
      dbg->r_state = r_debug::RT_CONSISTENT;
      dbg->r_brk();
      return;
    }

    firstAdded = map->l_prev;
    map->l_next->l_prev = map->l_prev;
  } else if (map->l_next) {
    map->l_next->l_prev = map->l_prev;
  }

  if (dbg->r_map == map)
    dbg->r_map = map->l_next;
  else if (map->l_prev) {
    map->l_prev->l_next = map->l_next;
  }
  dbg->r_state = r_debug::RT_CONSISTENT;
  dbg->r_brk();
}

#if defined(ANDROID) && defined(__NR_sigaction)
/* As some system libraries may be calling signal() or sigaction() to
 * set a SIGSEGV handler, effectively breaking MappableSeekableZStream,
 * or worse, restore our SIGSEGV handler with wrong flags (which using
 * signal() will do), we want to hook into the system's sigaction() to
 * replace it with our own wrapper instead, so that our handler is never
 * replaced. We used to only do that with libraries this linker loads,
 * but it turns out at least one system library does call signal() and
 * breaks us (libsc-a3xx.so on the Samsung Galaxy S4).
 * As libc's signal (bsd_signal/sysv_signal, really) calls sigaction
 * under the hood, instead of calling the signal system call directly,
 * we only need to hook sigaction. This is true for both bionic and
 * glibc.
 */

/* libc's sigaction */
extern "C" int sigaction(int signum, const struct sigaction* act,
                         struct sigaction* oldact);

/* Simple reimplementation of sigaction. This is roughly equivalent
 * to the assembly that comes in bionic, but not quite equivalent to
 * glibc's implementation, so we only use this on Android. */
int sys_sigaction(int signum, const struct sigaction* act,
                  struct sigaction* oldact) {
  return syscall(__NR_sigaction, signum, act, oldact);
}

/* Replace the first instructions of the given function with a jump
 * to the given new function. */
template <typename T>
static bool Divert(T func, T new_func) {
  void* ptr = FunctionPtr(func);
  uintptr_t addr = reinterpret_cast<uintptr_t>(ptr);

#  if defined(__i386__)
  // A 32-bit jump is a 5 bytes instruction.
  EnsureWritable w(ptr, 5);
  *reinterpret_cast<unsigned char*>(addr) = 0xe9;  // jmp
  *reinterpret_cast<intptr_t*>(addr + 1) =
      reinterpret_cast<uintptr_t>(new_func) - addr - 5;  // target displacement
  return true;
#  elif defined(__arm__) || defined(__aarch64__)
  const unsigned char trampoline[] = {
#    ifdef __arm__
      // .thumb
      0x46, 0x04,              // nop
      0x78, 0x47,              // bx pc
      0x46, 0x04,              // nop
                               // .arm
      0x04, 0xf0, 0x1f, 0xe5,  // ldr pc, [pc, #-4]
                               // .word <new_func>
#    else  // __aarch64__
      0x50, 0x00,
      0x00, 0x58,  // ldr x16, [pc, #8]   ; x16 (aka ip0) is the first
      0x00, 0x02,
      0x1f, 0xd6,  // br x16              ; intra-procedure-call
                   // .word <new_func.lo> ; scratch register.
                   // .word <new_func.hi>
#    endif
  };
  const unsigned char* start;
#    ifdef __arm__
  if (addr & 0x01) {
    /* Function is thumb, the actual address of the code is without the
     * least significant bit. */
    addr--;
    /* The arm part of the trampoline needs to be 32-bit aligned */
    if (addr & 0x02)
      start = trampoline;
    else
      start = trampoline + 2;
  } else {
    /* Function is arm, we only need the arm part of the trampoline */
    start = trampoline + 6;
  }
#    else  // __aarch64__
  start = trampoline;
#    endif

  size_t len = sizeof(trampoline) - (start - trampoline);
  EnsureWritable w(reinterpret_cast<void*>(addr), len + sizeof(void*));
  memcpy(reinterpret_cast<void*>(addr), start, len);
  *reinterpret_cast<void**>(addr + len) = FunctionPtr(new_func);
  __builtin___clear_cache(reinterpret_cast<char*>(addr),
                          reinterpret_cast<char*>(addr + len + sizeof(void*)));
  return true;
#  else
  return false;
#  endif
}
#else
#  define sys_sigaction sigaction
template <typename T>
static bool Divert(T func, T new_func) {
  return false;
}
#endif

namespace {

/* Clock that only accounts for time spent in the current process. */
static uint64_t ProcessTimeStamp_Now() {
  struct timespec ts;
  int rv = clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);

  if (rv != 0) {
    return 0;
  }

  uint64_t baseNs = (uint64_t)ts.tv_sec * 1000000000;
  return baseNs + (uint64_t)ts.tv_nsec;
}

}  // namespace

/* Data structure used to pass data to the temporary signal handler,
 * as well as triggering a test crash. */
struct TmpData {
  volatile int crash_int;
  volatile uint64_t crash_timestamp;
};

SEGVHandler::SEGVHandler()
    : initialized(false),
      registeredHandler(false),
      signalHandlingBroken(true),
      signalHandlingSlow(true) {
  /* Ensure logging is initialized before the DEBUG_LOG in the test_handler.
   * As this constructor runs before the ElfLoader constructor (by effect
   * of ElfLoader inheriting from this class), this also initializes on behalf
   * of ElfLoader and DebuggerHelper. */
  Logging::Init();

  /* Initialize oldStack.ss_flags to an invalid value when used to set
   * an alternative stack, meaning we haven't got information about the
   * original alternative stack and thus don't mean to restore it in
   * the destructor. */
  oldStack.ss_flags = SS_ONSTACK;

  /* Get the current segfault signal handler. */
  struct sigaction old_action;
  sys_sigaction(SIGSEGV, nullptr, &old_action);

  /* Some devices don't provide useful information to their SIGSEGV handlers,
   * making it impossible for on-demand decompression to work. To check if
   * we're on such a device, setup a temporary handler and deliberately
   * trigger a segfault. The handler will set signalHandlingBroken if the
   * provided information is bogus.
   * Some other devices have a kernel option enabled that makes SIGSEGV handler
   * have an overhead so high that it affects how on-demand decompression
   * performs. The handler will also set signalHandlingSlow if the triggered
   * SIGSEGV took too much time. */
  struct sigaction action;
  action.sa_sigaction = &SEGVHandler::test_handler;
  sigemptyset(&action.sa_mask);
  action.sa_flags = SA_SIGINFO | SA_NODEFER;
  action.sa_restorer = nullptr;
  stackPtr.Assign(MemoryRange::mmap(nullptr, PageSize(), PROT_READ | PROT_WRITE,
                                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0));
  if (stackPtr.get() == MAP_FAILED) return;
  if (sys_sigaction(SIGSEGV, &action, nullptr)) return;

  TmpData* data = reinterpret_cast<TmpData*>(stackPtr.get());
  data->crash_timestamp = ProcessTimeStamp_Now();
  mprotect(stackPtr, stackPtr.GetLength(), PROT_NONE);
  data->crash_int = 123;
  /* Restore the original segfault signal handler. */
  sys_sigaction(SIGSEGV, &old_action, nullptr);
  stackPtr.Assign(MAP_FAILED, 0);
}

void SEGVHandler::FinishInitialization() {
  /* Ideally, we'd need some locking here, but in practice, we're not
   * going to race with another thread. */
  initialized = true;

  if (signalHandlingBroken || signalHandlingSlow) return;

  typedef int (*sigaction_func)(int, const struct sigaction*,
                                struct sigaction*);

  sigaction_func libc_sigaction;

#if defined(ANDROID)
  /* Android > 4.4 comes with a sigaction wrapper in a LD_PRELOADed library
   * (libsigchain) for ART. That wrapper kind of does the same trick as we
   * do, so we need extra care in handling it.
   * - Divert the libc's sigaction, assuming the LD_PRELOADed library uses
   *   it under the hood (which is more or less true according to the source
   *   of that library, since it's doing a lookup in RTLD_NEXT)
   * - With the LD_PRELOADed library in place, all calls to sigaction from
   *   from system libraries will go to the LD_PRELOADed library.
   * - The LD_PRELOADed library calls to sigaction go to our __wrap_sigaction.
   * - The calls to sigaction from libraries faulty.lib loads are sent to
   *   the LD_PRELOADed library.
   * In practice, for signal handling, this means:
   * - The signal handler registered to the kernel is ours.
   * - Our handler redispatches to the LD_PRELOADed library's if there's a
   *   segfault we don't handle.
   * - The LD_PRELOADed library redispatches according to whatever system
   *   library or faulty.lib-loaded library set with sigaction.
   *
   * When there is no sigaction wrapper in place:
   * - Divert the libc's sigaction.
   * - Calls to sigaction from system library and faulty.lib-loaded libraries
   *   all go to the libc's sigaction, which end up in our __wrap_sigaction.
   * - The signal handler registered to the kernel is ours.
   * - Our handler redispatches according to whatever system library or
   *   faulty.lib-loaded library set with sigaction.
   */
  void* libc = dlopen("libc.so", RTLD_GLOBAL | RTLD_LAZY);
  if (libc) {
    /*
     * Lollipop bionic only has a small trampoline in sigaction, with the real
     * work happening in __sigaction. Divert there instead of sigaction if it
     * exists. Bug 1154803
     */
    libc_sigaction =
        reinterpret_cast<sigaction_func>(dlsym(libc, "__sigaction"));

    if (!libc_sigaction) {
      libc_sigaction =
          reinterpret_cast<sigaction_func>(dlsym(libc, "sigaction"));
    }
  } else
#endif
  {
    libc_sigaction = sigaction;
  }

  if (!Divert(libc_sigaction, __wrap_sigaction)) return;

  /* Setup an alternative stack if the already existing one is not big
   * enough, or if there is none. */
  if (sigaltstack(nullptr, &oldStack) == 0) {
    if (oldStack.ss_flags == SS_ONSTACK) oldStack.ss_flags = 0;
    if (!oldStack.ss_sp || oldStack.ss_size < stackSize) {
      stackPtr.Assign(MemoryRange::mmap(nullptr, stackSize,
                                        PROT_READ | PROT_WRITE,
                                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0));
      if (stackPtr.get() == MAP_FAILED) return;
      stack_t stack;
      stack.ss_sp = stackPtr;
      stack.ss_size = stackSize;
      stack.ss_flags = 0;
      if (sigaltstack(&stack, nullptr) != 0) return;
    }
  }
  /* Register our own handler, and store the already registered one in
   * SEGVHandler's struct sigaction member */
  action.sa_sigaction = &SEGVHandler::handler;
  action.sa_flags = SA_SIGINFO | SA_NODEFER | SA_ONSTACK;
  registeredHandler = !sys_sigaction(SIGSEGV, &action, &this->action);
}

SEGVHandler::~SEGVHandler() {
  /* Restore alternative stack for signals */
  if (oldStack.ss_flags != SS_ONSTACK) sigaltstack(&oldStack, nullptr);
  /* Restore original signal handler */
  if (registeredHandler) sys_sigaction(SIGSEGV, &this->action, nullptr);
}

/* Test handler for a deliberately triggered SIGSEGV that determines whether
 * useful information is provided to signal handlers, particularly whether
 * si_addr is filled in properly, and whether the segfault handler is called
 * quickly enough. */
void SEGVHandler::test_handler(int signum, siginfo_t* info, void* context) {
  SEGVHandler& that = ElfLoader::Singleton;
  if (signum == SIGSEGV && info && info->si_addr == that.stackPtr.get())
    that.signalHandlingBroken = false;
  mprotect(that.stackPtr, that.stackPtr.GetLength(), PROT_READ | PROT_WRITE);
  TmpData* data = reinterpret_cast<TmpData*>(that.stackPtr.get());
  uint64_t latency = ProcessTimeStamp_Now() - data->crash_timestamp;
  DEBUG_LOG("SEGVHandler latency: %" PRIu64, latency);
  /* See bug 886736 for timings on different devices, 150 ┬Ás is reasonably above
   * the latency on "working" devices and seems to be short enough to not incur
   * a huge overhead to on-demand decompression. */
  if (latency <= 150000) that.signalHandlingSlow = false;
}

/* TODO: "properly" handle signal masks and flags */
void SEGVHandler::handler(int signum, siginfo_t* info, void* context) {
  // ASSERT(signum == SIGSEGV);
  DEBUG_LOG("Caught segmentation fault @%p", info->si_addr);

  /* Redispatch to the registered handler */
  SEGVHandler& that = ElfLoader::Singleton;
  if (that.action.sa_flags & SA_SIGINFO) {
    DEBUG_LOG("Redispatching to registered handler @%p",
              FunctionPtr(that.action.sa_sigaction));
    that.action.sa_sigaction(signum, info, context);
  } else if (that.action.sa_handler == SIG_DFL) {
    DEBUG_LOG("Redispatching to default handler");
    /* Reset the handler to the default one, and trigger it. */
    sys_sigaction(signum, &that.action, nullptr);
    raise(signum);
  } else if (that.action.sa_handler != SIG_IGN) {
    DEBUG_LOG("Redispatching to registered handler @%p",
              FunctionPtr(that.action.sa_handler));
    that.action.sa_handler(signum);
  } else {
    DEBUG_LOG("Ignoring");
  }
}

int SEGVHandler::__wrap_sigaction(int signum, const struct sigaction* act,
                                  struct sigaction* oldact) {
  SEGVHandler& that = ElfLoader::Singleton;

  /* Use system sigaction() function for all but SIGSEGV signals. */
  if (!that.registeredHandler || (signum != SIGSEGV))
    return sys_sigaction(signum, act, oldact);

  if (oldact) *oldact = that.action;
  if (act) that.action = *act;
  return 0;
}

Logging Logging::Singleton;