Backtrace.cpp

#include <c10/util/Backtrace.h>
#include <c10/util/Optional.h>
#include <c10/util/Type.h>
#include <c10/util/irange.h>

#include <functional>
#include <memory>
#include <sstream>
#include <string>
#include <vector>

#ifdef _MSC_VER
#include <c10/util/win32-headers.h>
#include <iomanip>
#pragma comment(lib, "Dbghelp.lib")
#endif

#if SUPPORTS_BACKTRACE
#include <cxxabi.h>
#ifdef C10_ANDROID
#include <dlfcn.h>
#include <unwind.h>
#else
#include <execinfo.h>
#endif
#endif

#ifdef FBCODE_CAFFE2
#include <common/process/StackTrace.h>
#endif

namespace c10 {

#if SUPPORTS_BACKTRACE && defined(C10_ANDROID)

struct AndroidBacktraceState {
  std::vector<void*> buffer;
};

_Unwind_Reason_Code android_unwind_callback(
    struct _Unwind_Context* context,
    void* arg) {
  AndroidBacktraceState* state = (AndroidBacktraceState*)arg;
  uintptr_t pc = _Unwind_GetIP(context);
  if (pc) {
    state->buffer.emplace_back(reinterpret_cast<void*>(pc));
  }
  return _URC_NO_REASON;
}

void dump_stack(
    std::ostream& os,
    size_t frames_to_skip,
    size_t maximum_number_of_frames) {
  AndroidBacktraceState state;

  _Unwind_Backtrace(android_unwind_callback, &state);

  int idx = 0;
  char* demangled = nullptr;
  size_t length = 0;

  for (const void* addr : state.buffer) {
    const char* symbol = "";

    Dl_info info;
    if (dladdr(addr, &info) && info.dli_sname) {
      symbol = info.dli_sname;
    }

    int status = 0;
    demangled = __cxxabiv1::__cxa_demangle(
        /*mangled_name*/ symbol,
        /*output_buffer*/ demangled,
        /*length*/ &length,
        /*status*/ &status);

    os << " frame #" << idx++ << "\t"
       << ((demangled != NULL && status == 0) ? demangled : symbol) << "["
       << addr << "]\t" << std::endl;
  }
  free(demangled);
}

#endif /* SUPPORTS_BACKTRACE && defined(C10_ANDROID) */

#if SUPPORTS_BACKTRACE
namespace {

struct FrameInformation {
  /// If available, the demangled name of the function at this frame, else
  /// whatever (possibly mangled) name we got from `backtrace()`.
  std::string function_name;
  /// This is a number in hexadecimal form (e.g. "0xdead") representing the
  /// offset into the function's machine code at which the function's body
  /// starts, i.e. skipping the "prologue" that handles stack manipulation and
  /// other calling convention things.
  std::string offset_into_function;
  /// NOTE: In debugger parlance, the "object file" refers to the ELF file that
  /// the symbol originates from, i.e. either an executable or a library.
  std::string object_file;
};

#ifndef C10_ANDROID
bool is_python_frame(const FrameInformation& frame) {
  return frame.object_file == "python" || frame.object_file == "python3" ||
      (frame.object_file.find("libpython") != std::string::npos);
}

c10::optional<FrameInformation> parse_frame_information(
    const std::string& frame_string) {
  FrameInformation frame;

  // This is the function name in the CXX ABI mangled format, e.g. something
  // like _Z1gv. Reference:
  // https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling
  std::string mangled_function_name;

#if defined(__GLIBCXX__)
  // In GLIBCXX, `frame_string` follows the pattern
  // `<object-file>(<mangled-function-name>+<offset-into-function>)
  // [<return-address>]`

  auto function_name_start = frame_string.find('(');
  if (function_name_start == std::string::npos) {
    return c10::nullopt;
  }
  function_name_start += 1;

  auto offset_start = frame_string.find('+', function_name_start);
  if (offset_start == std::string::npos) {
    return c10::nullopt;
  }
  offset_start += 1;

  const auto offset_end = frame_string.find(')', offset_start);
  if (offset_end == std::string::npos) {
    return c10::nullopt;
  }

  frame.object_file = frame_string.substr(0, function_name_start - 1);
  frame.offset_into_function =
      frame_string.substr(offset_start, offset_end - offset_start);

  // NOTE: We don't need to parse the return address because
  // we already have it from the call to `backtrace()`.

  mangled_function_name = frame_string.substr(
      function_name_start, (offset_start - 1) - function_name_start);
#elif defined(_LIBCPP_VERSION)
  // In LIBCXX, The pattern is
  // `<frame number> <object-file> <return-address> <mangled-function-name> +
  // <offset-into-function>`
  std::string skip;
  std::istringstream input_stream(frame_string);
  // operator>>() does not fail -- if the input stream is corrupted, the
  // strings will simply be empty.
  input_stream >> skip >> frame.object_file >> skip >> mangled_function_name >>
      skip >> frame.offset_into_function;
#else
#warning Unknown standard library, backtraces may have incomplete debug information
  return c10::nullopt;
#endif // defined(__GLIBCXX__)

  // Some system-level functions don't have sufficient debug information, so
  // we'll display them as "<unknown function>". They'll still have a return
  // address and other pieces of information.
  if (mangled_function_name.empty()) {
    frame.function_name = "<unknown function>";
    return frame;
  }

  frame.function_name = demangle(mangled_function_name.c_str());
  return frame;
}
#endif /* !defined(C10_ANDROID) */
} // anonymous namespace
#elif defined(_MSC_VER)
namespace {
const int max_name_len = 256;
std::string get_module_base_name(void* addr) {
  HMODULE h_module;
  char module[max_name_len];
  strcpy(module, "");
  GetModuleHandleEx(
      GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS |
          GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT,
      (LPCTSTR)addr,
      &h_module);
  if (h_module != NULL) {
    GetModuleFileNameA(h_module, module, max_name_len);
  }
  char* last_slash_pos = strrchr(module, '\\');
  if (last_slash_pos) {
    std::string module_base_name(last_slash_pos + 1);
    return module_base_name;
  } else {
    std::string module_base_name(module);
    return module_base_name;
  }
}
class SymbolHelper {
 public:
  static SymbolHelper& getInstance() {
    static SymbolHelper instance;
    return instance;
  }
  bool inited = false;
  HANDLE process;

 private:
  SymbolHelper() {
    process = GetCurrentProcess();
    DWORD flags = SymGetOptions();
    SymSetOptions(flags | SYMOPT_DEFERRED_LOADS);
    inited = SymInitialize(process, NULL, TRUE);
  }
  ~SymbolHelper() {
    if (inited) {
      SymCleanup(process);
    }
  }

 public:
  SymbolHelper(SymbolHelper const&) = delete;
  void operator=(SymbolHelper const&) = delete;
};
} // anonymous namespace
#endif // SUPPORTS_BACKTRACE

std::string get_backtrace(
    size_t frames_to_skip,
    size_t maximum_number_of_frames,
    bool skip_python_frames) {
#ifdef FBCODE_CAFFE2
  // For some reason, the stacktrace implementation in fbcode is
  // better than ours, see  https://github.com/pytorch/pytorch/issues/56399
  // When it's available, just use that.
  facebook::process::StackTrace st;
  return st.toString();

#elif SUPPORTS_BACKTRACE && !defined(C10_ANDROID)

  // We always skip this frame (backtrace).
  frames_to_skip += 1;

  std::vector<void*> callstack(
      frames_to_skip + maximum_number_of_frames, nullptr);
  // backtrace() gives us a list of return addresses in the current call stack.
  // NOTE: As per man (3) backtrace it can never fail
  // (http://man7.org/linux/man-pages/man3/backtrace.3.html).
  auto number_of_frames =
      ::backtrace(callstack.data(), static_cast<int>(callstack.size()));

  // Skip as many frames as requested. This is not efficient, but the sizes here
  // are small and it makes the code nicer and safer.
  for (; frames_to_skip > 0 && number_of_frames > 0;
       --frames_to_skip, --number_of_frames) {
    callstack.erase(callstack.begin());
  }

  // `number_of_frames` is strictly less than the current capacity of
  // `callstack`, so this is just a pointer subtraction and makes the subsequent
  // code safer.
  callstack.resize(static_cast<size_t>(number_of_frames));

  // `backtrace_symbols` takes the return addresses obtained from `backtrace()`
  // and fetches string representations of each stack. Unfortunately it doesn't
  // return a struct of individual pieces of information but a concatenated
  // string, so we'll have to parse the string after. NOTE: The array returned
  // by `backtrace_symbols` is malloc'd and must be manually freed, but not the
  // strings inside the array.
  std::unique_ptr<char*, std::function<void(char**)>> raw_symbols(
      ::backtrace_symbols(callstack.data(), static_cast<int>(callstack.size())),
      /*deleter=*/free);
  const std::vector<std::string> symbols(
      raw_symbols.get(), raw_symbols.get() + callstack.size());

  // The backtrace string goes into here.
  std::ostringstream stream;

  // Toggles to true after the first skipped python frame.
  bool has_skipped_python_frames = false;

  for (const auto frame_number : c10::irange(callstack.size())) {
    const auto frame = parse_frame_information(symbols[frame_number]);

    if (skip_python_frames && frame && is_python_frame(*frame)) {
      if (!has_skipped_python_frames) {
        stream << "<omitting python frames>\n";
        has_skipped_python_frames = true;
      }
      continue;
    }

    // frame #<number>:
    stream << "frame #" << frame_number << ": ";

    if (frame) {
      // <function_name> + <offset> (<return-address> in <object-file>)
      stream << frame->function_name << " + " << frame->offset_into_function
             << " (" << callstack[frame_number] << " in " << frame->object_file
             << ")\n";
    } else {
      // In the edge-case where we couldn't parse the frame string, we can
      // just use it directly (it may have a different format).
      stream << symbols[frame_number] << "\n";
    }
  }

  return stream.str();

#elif SUPPORTS_BACKTRACE && defined(C10_ANDROID)

  std::ostringstream oss;
  dump_stack(oss, frames_to_skip, maximum_number_of_frames);
  return oss.str().c_str();

#elif defined(_MSC_VER) // !SUPPORTS_BACKTRACE
  // This backtrace retrieval is implemented on Windows via the Windows
  // API using `CaptureStackBackTrace`, `SymFromAddr` and
  // `SymGetLineFromAddr64`.
  // https://stackoverflow.com/questions/5693192/win32-backtrace-from-c-code
  // https://stackoverflow.com/questions/26398064/counterpart-to-glibcs-backtrace-and-backtrace-symbols-on-windows
  // https://docs.microsoft.com/en-us/windows/win32/debug/capturestackbacktrace
  // https://docs.microsoft.com/en-us/windows/win32/api/dbghelp/nf-dbghelp-symfromaddr
  // https://docs.microsoft.com/en-us/windows/win32/api/dbghelp/nf-dbghelp-symgetlinefromaddr64
  // TODO: Support skipping python frames

  // We always skip this frame (backtrace).
  frames_to_skip += 1;

  DWORD64 displacement;
  DWORD disp;
  std::unique_ptr<IMAGEHLP_LINE64> line;

  char buffer[sizeof(SYMBOL_INFO) + MAX_SYM_NAME * sizeof(TCHAR)];
  PSYMBOL_INFO p_symbol = (PSYMBOL_INFO)buffer;

  std::unique_ptr<void*[]> back_trace(new void*[maximum_number_of_frames]);
  bool with_symbol = false;
  bool with_line = false;

  // The backtrace string goes into here.
  std::ostringstream stream;

  // Get the frames
  const USHORT n_frame = CaptureStackBackTrace(
      static_cast<DWORD>(frames_to_skip),
      static_cast<DWORD>(maximum_number_of_frames),
      back_trace.get(),
      NULL);

  // Initialize symbols if necessary
  SymbolHelper& sh = SymbolHelper::getInstance();

  for (USHORT i_frame = 0; i_frame < n_frame; ++i_frame) {
    // Get the address and the name of the symbol
    if (sh.inited) {
      p_symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
      p_symbol->MaxNameLen = MAX_SYM_NAME;
      with_symbol = SymFromAddr(
          sh.process, (ULONG64)back_trace[i_frame], &displacement, p_symbol);
    }

    // Get the line number and the module
    if (sh.inited) {
      line.reset(new IMAGEHLP_LINE64());
      line->SizeOfStruct = sizeof(IMAGEHLP_LINE64);
      with_line = SymGetLineFromAddr64(
          sh.process, (ULONG64)back_trace[i_frame], &disp, line.get());
    }

    // Get the module basename
    std::string module = get_module_base_name(back_trace[i_frame]);

    // The pattern on Windows is
    // `<return-address> <symbol-address>
    // <module-name>!<demangled-function-name> [<file-name> @ <line-number>]
    stream << std::setfill('0') << std::setw(16) << std::uppercase << std::hex
           << back_trace[i_frame] << std::dec;
    if (with_symbol) {
      stream << std::setfill('0') << std::setw(16) << std::uppercase << std::hex
             << p_symbol->Address << std::dec << " " << module << "!"
             << p_symbol->Name;
    } else {
      stream << " <unknown symbol address> " << module << "!<unknown symbol>";
    }
    stream << " [";
    if (with_line) {
      stream << line->FileName << " @ " << line->LineNumber;
    } else {
      stream << "<unknown file> @ <unknown line number>";
    }
    stream << "]" << std::endl;
  }

  return stream.str();
#else // !SUPPORTS_BACKTRACE && !_WIN32
  return "(no backtrace available)";
#endif // SUPPORTS_BACKTRACE
}

} // namespace c10