Backed out changeset b71c8c052463 (bug 1943846) for causing mass failures. CLOSED...

[gecko.git] / mozglue / linker / ElfLoader.cpp

/* 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 <link.h>
#include <optional>
#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>
#include "mozilla/ScopeExit.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>

/**
 * Return the current Android version, or 0 on failure.
 */
static 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 /* ANDROID */

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

/* 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) {
#if defined(ANDROID)
  if (GetAndroidSDKVersion() >= 23) {
    return dlopen(path, flags);
  }
#endif

  RefPtr<LibHandle> handle = ElfLoader::Singleton.Load(path, flags);
  if (handle) handle->AddDirectRef();
  return handle;
}

const char* __wrap_dlerror(void) {
#if defined(ANDROID)
  if (GetAndroidSDKVersion() >= 23) {
    return dlerror();
  }
#endif

  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 defined(ANDROID)
  if (GetAndroidSDKVersion() >= 23) {
    return dlsym(handle, symbol);
  }
#endif

  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 defined(ANDROID)
  if (GetAndroidSDKVersion() >= 23) {
    return dlclose(handle);
  }
#endif

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

int __wrap_dladdr(const void* addr, Dl_info* info) {
#if defined(ANDROID)
  if (GetAndroidSDKVersion() >= 23) {
    return dladdr(addr, info);
  }
#endif

  RefPtr<LibHandle> handle =
      ElfLoader::Singleton.GetHandleByPtr(const_cast<void*>(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.emplace(pipefd[0]);
    write_fd.emplace(pipefd[1]);
  }

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

 private:
  bool valid_pipe;
  std::optional<AutoCloseFD> read_fd;
  std::optional<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) {
#if defined(ANDROID)
  if (GetAndroidSDKVersion() >= 23) {
    return dl_iterate_phdr(callback, data);
  }
#endif

  DlIteratePhdrHelper helper;
  AutoLock lock(&ElfLoader::Singleton.handlesMutex);

  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);
}

#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

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;
}

/**
 * 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 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;
}

/**
 * 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;
}

#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 */
MOZ_RUNINIT 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) {
  return Mappable::Create(path);
}

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);
  }
}

ElfLoader::~ElfLoader() {
  LibHandleList list;

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

  /* Release self_elf and libc */
  self_elf = nullptr;

  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;
    FILE* const f = fopen("/proc/self/maps", "r");
    const auto cleanup = mozilla::MakeScopeExit([&]() {
      if (f) fclose(f);
    });
    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), 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 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 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 (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
 * the segfault handler is called quickly enough. */
void SEGVHandler::test_handler(int signum, siginfo_t* info, void* context) {
  SEGVHandler& that = ElfLoader::Singleton;
  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 µs 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;
}