Adding upstream version 115.8.0esr.upstream/115.8.0esr

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1360 insertions, 0 deletions

diff --git a/mozglue/linker/ElfLoader.cpp b/mozglue/linker/ElfLoader.cpp
new file mode 100644
index 0000000000..55b113467a
--- /dev/null
+++ b/mozglue/linker/ElfLoader.cpp
@@ -0,0 +1,1360 @@
+/* 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>
+
+/**
+ * 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;
+}
+
+#  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) {
+#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.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) {
+#if defined(ANDROID)
+  if (GetAndroidSDKVersion() >= 23) {
+    return dl_iterate_phdr(callback, data);
+  }
+#endif
+
+  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;
+}
+
+/**
+ * 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 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;
+}