text: /*
   * Copyright (c) Facebook, Inc. and its affiliates.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  // This is heavily inspired by the signal handler from google-glog
  
  #include <folly/experimental/symbolizer/SignalHandler.h>
  
  #include <signal.h>
  #include <sys/types.h>
  
  #include <algorithm>
  #include <atomic>
  #include <cerrno>
  #include <ctime>
  #include <mutex>
  #include <vector>
  
  #include <glog/logging.h>
  
  #include <folly/Conv.h>
  #include <folly/ScopeGuard.h>
  #include <folly/experimental/symbolizer/Symbolizer.h>
  #include <folly/portability/SysSyscall.h>
  #include <folly/portability/Unistd.h>
  
  namespace folly {
  namespace symbolizer {
  
  #ifndef _WIN32
  
  const unsigned long kAllFatalSignals = (1UL << SIGSEGV) | (1UL << SIGILL) |
      (1UL << SIGFPE) | (1UL << SIGABRT) | (1UL << SIGBUS) | (1UL << SIGTERM) |
      (1UL << SIGQUIT);
  
  #endif
  
  namespace {
  
  /**
   * Fatal signal handler registry.
   */
  class FatalSignalCallbackRegistry {
   public:
    FatalSignalCallbackRegistry();
  
    void add(SignalCallback func);
    void markInstalled();
    void run();
  
   private:
    std::atomic<bool> installed_;
    std::mutex mutex_;
    std::vector<SignalCallback> handlers_;
  };
  
  FatalSignalCallbackRegistry::FatalSignalCallbackRegistry()
      : installed_(false) {}
  
  void FatalSignalCallbackRegistry::add(SignalCallback func) {
    std::lock_guard<std::mutex> lock(mutex_);
    CHECK(!installed_) << "FatalSignalCallbackRegistry::add may not be used "
                          "after installing the signal handlers.";
    handlers_.push_back(func);
  }
  
  void FatalSignalCallbackRegistry::markInstalled() {
    std::lock_guard<std::mutex> lock(mutex_);
    CHECK(!installed_.exchange(true))
        << "FatalSignalCallbackRegistry::markInstalled must be called "
        << "at most once";
  }
  
  void FatalSignalCallbackRegistry::run() {
    if (!installed_) {
      return;
    }
  
    for (auto& fn : handlers_) {
      fn();
    }
  }
  
  std::atomic<FatalSignalCallbackRegistry*> gFatalSignalCallbackRegistry{};
  
  FatalSignalCallbackRegistry* getFatalSignalCallbackRegistry() {
    // Leak it so we don't have to worry about destruction order
    static FatalSignalCallbackRegistry* fatalSignalCallbackRegistry =
        new FatalSignalCallbackRegistry();
  
    return fatalSignalCallbackRegistry;
  }
  
  } // namespace
  
  void addFatalSignalCallback(SignalCallback cb) {
    getFatalSignalCallbackRegistry()->add(cb);
  }
  
  void installFatalSignalCallbacks() {
    getFatalSignalCallbackRegistry()->markInstalled();
  }
  
  #ifndef _WIN32
  
  namespace {
  
  struct {
    int number;
    const char* name;
    struct sigaction oldAction;
  } kFatalSignals[] = {
      {SIGSEGV, "SIGSEGV", {}},
      {SIGILL, "SIGILL", {}},
      {SIGFPE, "SIGFPE", {}},
      {SIGABRT, "SIGABRT", {}},
      {SIGBUS, "SIGBUS", {}},
      {SIGTERM, "SIGTERM", {}},
      {SIGQUIT, "SIGQUIT", {}},
      {0, nullptr, {}},
  };
  
  FOLLY_MAYBE_UNUSED void callPreviousSignalHandler(int signum) {
    // Restore disposition to old disposition, then kill ourselves with the same
    // signal. The signal will be blocked until we return from our handler,
    // then it will invoke the default handler and abort.
    for (auto p = kFatalSignals; p->name; ++p) {
      if (p->number == signum) {
        sigaction(signum, &p->oldAction, nullptr);
        raise(signum);
        return;
      }
    }
  
    // Not one of the signals we know about. Oh well. Reset to default.
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    sa.sa_handler = SIG_DFL;
    sigaction(signum, &sa, nullptr);
    raise(signum);
  }
  
  #if FOLLY_USE_SYMBOLIZER
  
  // Note: not thread-safe, but that's okay, as we only let one thread
  // in our signal handler at a time.
  //
  // Leak it so we don't have to worry about destruction order
  //
  // Initialized by installFatalSignalHandler
  SafeStackTracePrinter* gStackTracePrinter;
  
  void print(StringPiece sp) {
    gStackTracePrinter->print(sp);
  }
  
  void flush() {
    gStackTracePrinter->flush();
  }
  
  void printDec(uint64_t val) {
    char buf[20];
    uint32_t n = uint64ToBufferUnsafe(val, buf);
    gStackTracePrinter->print(StringPiece(buf, n));
  }
  
  const char kHexChars[] = "0123456789abcdef";
  void printHex(uint64_t val) {
    // TODO(tudorb): Add this to folly/Conv.h
    char buf[2 + 2 * sizeof(uint64_t)]; // "0x" prefix, 2 digits for each byte
  
    char* end = buf + sizeof(buf);
    char* p = end;
    do {
      *--p = kHexChars[val & 0x0f];
      val >>= 4;
    } while (val != 0);
    *--p = 'x';
    *--p = '0';
  
    gStackTracePrinter->print(StringPiece(p, end));
  }
  
  void dumpTimeInfo() {
    SCOPE_EXIT { flush(); };
    time_t now = time(nullptr);
    print("*** Aborted at ");
    printDec(now);
    print(" (Unix time, try 'date -d @");
    printDec(now);
    print("') ***\n");
  }
  
  const char* sigill_reason(int si_code) {
    switch (si_code) {
      case ILL_ILLOPC:
        return "illegal opcode";
      case ILL_ILLOPN:
        return "illegal operand";
      case ILL_ILLADR:
        return "illegal addressing mode";
      case ILL_ILLTRP:
        return "illegal trap";
      case ILL_PRVOPC:
        return "privileged opcode";
      case ILL_PRVREG:
        return "privileged register";
      case ILL_COPROC:
        return "coprocessor error";
      case ILL_BADSTK:
        return "internal stack error";
  
      default:
        return nullptr;
    }
  }
  
  const char* sigfpe_reason(int si_code) {
    switch (si_code) {
      case FPE_INTDIV:
        return "integer divide by zero";
      case FPE_INTOVF:
        return "integer overflow";
      case FPE_FLTDIV:
        return "floating-point divide by zero";
      case FPE_FLTOVF:
        return "floating-point overflow";
      case FPE_FLTUND:
        return "floating-point underflow";
      case FPE_FLTRES:
        return "floating-point inexact result";
      case FPE_FLTINV:
        return "floating-point invalid operation";
      case FPE_FLTSUB:
        return "subscript out of range";
  
      default:
        return nullptr;
    }
  }
  
  const char* sigsegv_reason(int si_code) {
    switch (si_code) {
      case SEGV_MAPERR:
        return "address not mapped to object";
      case SEGV_ACCERR:
        return "invalid permissions for mapped object";
  
      default:
        return nullptr;
    }
  }
  
  const char* sigbus_reason(int si_code) {
    switch (si_code) {
      case BUS_ADRALN:
        return "invalid address alignment";
      case BUS_ADRERR:
        return "nonexistent physical address";
      case BUS_OBJERR:
        return "object-specific hardware error";
  
        // MCEERR_AR and MCEERR_AO: in sigaction(2) but not in headers.
  
      default:
        return nullptr;
    }
  }
  
  const char* sigtrap_reason(int si_code) {
    switch (si_code) {
      case TRAP_BRKPT:
        return "process breakpoint";
      case TRAP_TRACE:
        return "process trace trap";
  
        // TRAP_BRANCH and TRAP_HWBKPT: in sigaction(2) but not in headers.
  
      default:
        return nullptr;
    }
  }
  
  const char* sigchld_reason(int si_code) {
    switch (si_code) {
      case CLD_EXITED:
        return "child has exited";
      case CLD_KILLED:
        return "child was killed";
      case CLD_DUMPED:
        return "child terminated abnormally";
      case CLD_TRAPPED:
        return "traced child has trapped";
      case CLD_STOPPED:
        return "child has stopped";
      case CLD_CONTINUED:
        return "stopped child has continued";
  
      default:
        return nullptr;
    }
  }
  
  const char* sigio_reason(int si_code) {
    switch (si_code) {
      case POLL_IN:
        return "data input available";
      case POLL_OUT:
        return "output buffers available";
      case POLL_MSG:
        return "input message available";
      case POLL_ERR:
        return "I/O error";
      case POLL_PRI:
        return "high priority input available";
      case POLL_HUP:
        return "device disconnected";
  
      default:
        return nullptr;
    }
  }
  
  const char* signal_reason(int signum, int si_code) {
    switch (signum) {
      case SIGILL:
        return sigill_reason(si_code);
      case SIGFPE:
        return sigfpe_reason(si_code);
      case SIGSEGV:
        return sigsegv_reason(si_code);
      case SIGBUS:
        return sigbus_reason(si_code);
      case SIGTRAP:
        return sigtrap_reason(si_code);
      case SIGCHLD:
        return sigchld_reason(si_code);
      case SIGIO:
        return sigio_reason(si_code); // aka SIGPOLL
  
      default:
        return nullptr;
    }
  }
  
  void dumpSignalInfo(int signum, siginfo_t* siginfo) {
    SCOPE_EXIT { flush(); };
    // Get the signal name, if possible.
    const char* name = nullptr;
    for (auto p = kFatalSignals; p->name; ++p) {
      if (p->number == signum) {
        name = p->name;
        break;
      }
    }
  
    print("*** Signal ");
    printDec(signum);
    if (name) {
      print(" (");
      print(name);
      print(")");
    }
  
    print(" (");
    printHex(reinterpret_cast<uint64_t>(siginfo->si_addr));
    print(") received by PID ");
    printDec(getpid());
    print(" (pthread TID ");
    printHex((uint64_t)pthread_self());
  #if defined(__linux__)
    print(") (linux TID ");
    printDec(syscall(__NR_gettid));
  #elif defined(__FreeBSD__)
    long tid = 0;
    syscall(432, &tid);
    print(") (freebsd TID ");
    printDec(tid);
  #endif
  
    // Kernel-sourced signals don't give us useful info for pid/uid.
    if (siginfo->si_code <= 0) {
      print(") (maybe from PID ");
      printDec(siginfo->si_pid);
      print(", UID ");
      printDec(siginfo->si_uid);
    }
  
    auto reason = signal_reason(signum, siginfo->si_code);
  
    print(") (code: ");
    // If we can't find a reason code make a best effort to print the (int) code.
    if (reason != nullptr) {
      print(reason);
    } else {
      if (siginfo->si_code < 0) {
        print("-");
        printDec(-siginfo->si_code);
      } else {
        printDec(siginfo->si_code);
      }
    }
  
    print("), stack trace: ***\n");
  }
  
  // On Linux, pthread_t is a pointer, so 0 is an invalid value, which we
  // take to indicate "no thread in the signal handler".
  //
  // POSIX defines PTHREAD_NULL for this purpose, but that's not available.
  constexpr pthread_t kInvalidThreadId = 0;
  
  std::atomic<pthread_t> gSignalThread(kInvalidThreadId);
  std::atomic<bool> gInRecursiveSignalHandler(false);
  
  // Here be dragons.
  void innerSignalHandler(int signum, siginfo_t* info, void* /* uctx */) {
    // First, let's only let one thread in here at a time.
    pthread_t myId = pthread_self();
  
    pthread_t prevSignalThread = kInvalidThreadId;
    while (!gSignalThread.compare_exchange_strong(prevSignalThread, myId)) {
      if (pthread_equal(prevSignalThread, myId)) {
        // First time here. Try to dump the stack trace without symbolization.
        // If we still fail, well, we're mightily screwed, so we do nothing the
        // next time around.
        if (!gInRecursiveSignalHandler.exchange(true)) {
          print("Entered fatal signal handler recursively. We're in trouble.\n");
          gStackTracePrinter->printStackTrace(false); // no symbolization
        }
        return;
      }
  
      // Wait a while, try again.
      timespec ts;
      ts.tv_sec = 0;
      ts.tv_nsec = 100L * 1000 * 1000; // 100ms
      nanosleep(&ts, nullptr);
  
      prevSignalThread = kInvalidThreadId;
    }
  
    dumpTimeInfo();
    dumpSignalInfo(signum, info);
    gStackTracePrinter->printStackTrace(true); // with symbolization
  
    // Run user callbacks
    auto callbacks = gFatalSignalCallbackRegistry.load(std::memory_order_acquire);
    if (callbacks) {
      callbacks->run();
    }
  }
  
  void signalHandler(int signum, siginfo_t* info, void* uctx) {
    int savedErrno = errno;
    SCOPE_EXIT {
      flush();
      errno = savedErrno;
    };
    innerSignalHandler(signum, info, uctx);
  
    gSignalThread = kInvalidThreadId;
    // Kill ourselves with the previous handler.
    callPreviousSignalHandler(signum);
  }
  
  #endif // FOLLY_USE_SYMBOLIZER
  
  // Small sigaltstack size threshold.
  // 8931 is known to cause the signal handler to stack overflow during
  // symbolization even for a simple one-liner "kill(getpid(), SIGTERM)".
  constexpr size_t kSmallSigAltStackSize = 8931;
  
  FOLLY_MAYBE_UNUSED bool isSmallSigAltStackEnabled() {
    stack_t ss;
    if (sigaltstack(nullptr, &ss) != 0) {
      return false;
    }
    if ((ss.ss_flags & SS_DISABLE) != 0) {
      return false;
    }
    return ss.ss_size <= kSmallSigAltStackSize;
  }
  
  } // namespace
  
  #endif // _WIN32
  
  namespace {
  std::atomic<bool> gAlreadyInstalled;
  }
  
  void installFatalSignalHandler(std::bitset<64> signals) {
    if (gAlreadyInstalled.exchange(true)) {
      // Already done.
      return;
    }
  
    // make sure gFatalSignalCallbackRegistry is created before we
    // install the fatal signal handler
    gFatalSignalCallbackRegistry.store(
        getFatalSignalCallbackRegistry(), std::memory_order_release);
  
  #if FOLLY_USE_SYMBOLIZER
    // If a small sigaltstack is enabled (ex. Rust stdlib might use sigaltstack
    // to set a small stack), the default SafeStackTracePrinter would likely
    // stack overflow. Replace it with the unsafe self-allocate printer.
    bool useUnsafePrinter = kIsLinux && isSmallSigAltStackEnabled();
    if (useUnsafePrinter) {
  #if FOLLY_HAVE_SWAPCONTEXT
      gStackTracePrinter = new UnsafeSelfAllocateStackTracePrinter();
  #else
      // This environment does not support swapcontext, so always use
      // SafeStackTracePrinter.
      gStackTracePrinter = new SafeStackTracePrinter();
  #endif // FOLLY_HAVE_SWAPCONTEXT
    } else {
      gStackTracePrinter = new SafeStackTracePrinter();
    }
  
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    if (useUnsafePrinter) {
      // The signal handler is not async-signal-safe. Block all signals to
      // make it safer. But it's still unsafe.
      sigfillset(&sa.sa_mask);
    } else {
      sigemptyset(&sa.sa_mask);
    }
    // By default signal handlers are run on the signaled thread's stack.
    // In case of stack overflow running the SIGSEGV signal handler on
    // the same stack leads to another SIGSEGV and crashes the program.
    // Use SA_ONSTACK, so alternate stack is used (only if configured via
    // sigaltstack).
    // Golang also requires SA_ONSTACK. See:
    // https://golang.org/pkg/os/signal/#hdr-Go_programs_that_use_cgo_or_SWIG
    sa.sa_flags |= SA_SIGINFO | SA_ONSTACK;
    sa.sa_sigaction = &signalHandler;
  
    for (auto p = kFatalSignals; p->name; ++p) {
      if ((p->number < static_cast<int>(signals.size())) &&
          signals.test(p->number)) {
        CHECK_ERR(sigaction(p->number, &sa, &p->oldAction));
      }
    }
  #endif // FOLLY_USE_SYMBOLIZER
  }
  } // namespace symbolizer
  } // namespace folly
  