from_exception.cpp

// Copyright Antony Polukhin, 2023-2025.
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

#if defined(__MINGW32__) || defined(_MSC_VER)

#include <boost/stacktrace/safe_dump_to.hpp>
#include <windows.h>

extern "C" void** __cdecl __current_exception(); // exported from vcruntime.dll
#define _pCurrentException static_cast<PEXCEPTION_RECORD>(*__current_exception())

namespace {

constexpr std::size_t kStacktraceDumpSize = 4096;

struct thrown_info {
  ULONG_PTR object;
  char* dump;
};

struct exception_data {
  bool capture_stacktraces_at_throw = true;
  unsigned count = 0;
  thrown_info* info = nullptr;

  ~exception_data() noexcept {
    HANDLE hHeap = GetProcessHeap();
    for (unsigned i = 0; i < count; ++i) {
      HeapFree(hHeap, 0, info[i].dump);
    }
    HeapFree(hHeap, 0, info);
  }
};

thread_local exception_data data;

inline bool PER_IS_MSVC_EH(PEXCEPTION_RECORD p) noexcept {
  const DWORD EH_EXCEPTION_NUMBER = 0xE06D7363;
  const ULONG_PTR EH_MAGIC_NUMBER1 = 0x19930520;

  return p->ExceptionCode == EH_EXCEPTION_NUMBER &&
    (p->NumberParameters == 3 || p->NumberParameters == 4) &&
    p->ExceptionInformation[0] == EH_MAGIC_NUMBER1;
}

inline ULONG_PTR PER_PEXCEPTOBJ(PEXCEPTION_RECORD p) noexcept {
  return p->ExceptionInformation[1];
}

unsigned current_cxx_exception_index() noexcept {
  if (PEXCEPTION_RECORD current_cxx_exception = _pCurrentException) {
    for (unsigned i = data.count; i > 0;) {
      --i;
      if (data.info[i].object == PER_PEXCEPTOBJ(current_cxx_exception)) {
        return i;
      }
    }
  }
  return data.count;
}

bool is_processing_rethrow(PEXCEPTION_RECORD p) noexcept {
  // Processing flow:
  // 0. throw;
  // 1. _CxxThrowException(pExceptionObject = nullptr)
  // 2. VEH & SEH (may throw new c++ exceptions!)
  // 3. __RethrowException(_pCurrentException)
  // 4. VEH
  if (PER_PEXCEPTOBJ(p) == 0) return true;
  PEXCEPTION_RECORD current_cxx_exception = _pCurrentException;
  if (current_cxx_exception == nullptr) return false;
  return PER_PEXCEPTOBJ(p) == PER_PEXCEPTOBJ(current_cxx_exception);
}

LONG NTAPI veh(PEXCEPTION_POINTERS p) {
  if (data.capture_stacktraces_at_throw &&
      PER_IS_MSVC_EH(p->ExceptionRecord) &&
      !is_processing_rethrow(p->ExceptionRecord)) {
    HANDLE hHeap = GetProcessHeap();
    unsigned index = current_cxx_exception_index();
    unsigned new_count = 1 + (index < data.count ? index + 1 : 0);

    for (unsigned i = new_count; i < data.count; ++i) {
      HeapFree(hHeap, 0, data.info[i].dump);
      data.info[i].dump = nullptr;
    }

    void* new_info;
    if (data.info) {
      new_info = HeapReAlloc(hHeap, HEAP_ZERO_MEMORY, data.info, sizeof(thrown_info) * new_count);
    } else {
      new_info = HeapAlloc(hHeap, HEAP_ZERO_MEMORY, sizeof(thrown_info) * new_count);
    }
    if (new_info) {
      data.count = new_count;
      data.info = static_cast<thrown_info*>(new_info);
      data.info[data.count - 1].object = PER_PEXCEPTOBJ(p->ExceptionRecord);
      char*& dump_ptr = data.info[data.count - 1].dump;
      if (dump_ptr == nullptr) {
        dump_ptr = static_cast<char*>(HeapAlloc(hHeap, 0, kStacktraceDumpSize));
      }
      if (dump_ptr != nullptr) {
        const std::size_t kSkip = 4;
        boost::stacktrace::safe_dump_to(kSkip, dump_ptr, kStacktraceDumpSize);
      }
    } else if (new_count <= data.count) {
      data.count = new_count - 1;
      HeapFree(hHeap, 0, data.info[data.count - 1].dump);
      data.info[data.count - 1].dump = nullptr;
    }
  }
  return EXCEPTION_CONTINUE_SEARCH;
}

struct veh_installer {
  PVOID h;
  veh_installer() noexcept : h(AddVectoredExceptionHandler(1, veh)) {}
  ~veh_installer() noexcept { RemoveVectoredExceptionHandler(h); }
} installer;

}

extern "C" {

BOOST_SYMBOL_EXPORT const char* boost_stacktrace_impl_current_exception_stacktrace() {
  unsigned index = current_cxx_exception_index();
  return index < data.count ? data.info[index].dump : nullptr;
}

BOOST_SYMBOL_EXPORT bool* boost_stacktrace_impl_ref_capture_stacktraces_at_throw() {
  return &data.capture_stacktraces_at_throw;
}

}

namespace boost { namespace stacktrace { namespace impl {

BOOST_SYMBOL_EXPORT void assert_no_pending_traces() noexcept {
}

}}}  // namespace boost::stacktrace::impl

#else

#include "exception_headers.h"

// At the moment the file is used only on POSIX. _Unwind_Backtrace may be
// available on some platforms only if _GNU_SOURCE is defined.
#ifndef _GNU_SOURCE
#   define _GNU_SOURCE
#endif

#include <boost/assert.hpp>
#include <boost/stacktrace/safe_dump_to.hpp>

#include <cstddef>
#include <exception>
#include <dlfcn.h>

#if !BOOST_STACKTRACE_ALWAYS_STORE_IN_PADDING
#include <mutex>
#include <unordered_map>

#ifndef BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK

#ifdef BOOST_HAS_THREADS

#error On this platform memory leaks are possible if capturing stacktrace from \
        exceptions is enabled and exceptions are thrown concurrently \
        and libc++ runtime is used. \
        \
        Define `BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK` to \
        suppress this error if the library would not be used with libc++ \
        runtime (for example, it would be only used with GCC runtime). \
        \
        Otherwise, disable the boost_stacktrace_from_exception library build \
        (for example by `./b2 boost.stacktrace.from_exception=off` option).

#endif

#endif

#endif

namespace {

constexpr std::size_t kStacktraceDumpSize = 4096;

struct decrement_on_destroy {
  std::size_t& to_decrement;

  ~decrement_on_destroy() { --to_decrement; }
};

#if !BOOST_STACKTRACE_ALWAYS_STORE_IN_PADDING
// Inspired by the coursework by Andrei Nekrashevich in the `libsfe`
/*constinit*/ std::mutex g_mapping_mutex;
std::unordered_map<void*, const char*> g_exception_to_dump_mapping;
#endif

}  // namespace

namespace boost { namespace stacktrace { namespace impl {

BOOST_SYMBOL_EXPORT bool& ref_capture_stacktraces_at_throw() noexcept {
  /*constinit*/ thread_local bool g_capture_stacktraces_at_throw{true};
  return g_capture_stacktraces_at_throw;
}

}}}  // namespace boost::stacktrace::impl

namespace __cxxabiv1 {

#if defined(__GNUC__) && defined(__ELF__)

// libc++-runtime specific function
extern "C" BOOST_NOINLINE BOOST_SYMBOL_VISIBLE __attribute__((weak))
void __cxa_increment_exception_refcount(void *primary_exception) throw();

static bool is_libcpp_runtime() noexcept {
  return __cxa_increment_exception_refcount;
}

#else

static bool is_libcpp_runtime() noexcept { return false; }

#endif

static const char*& reference_to_empty_padding(void* ptr) noexcept {
  if (is_libcpp_runtime()) {
    // libc++-runtime
    BOOST_ASSERT_MSG(
      sizeof(void*) != 4,
      "32bit platforms are unsupported with libc++ runtime padding reusage. "
      "Please report this issue to the library maintainters."
    );
    return exception_begin_llvm_ptr(ptr)->reserve;
  }

  return exception_begin_gcc_ptr(ptr)->reserve;
}

extern "C" BOOST_SYMBOL_EXPORT
void* __cxa_allocate_exception(size_t thrown_size) throw() {
  static const auto orig_allocate_exception = []() {
    void* const ptr = ::dlsym(RTLD_NEXT, "__cxa_allocate_exception");
    BOOST_ASSERT_MSG(ptr, "Failed to find '__cxa_allocate_exception'");
    return reinterpret_cast<void*(*)(size_t)>(ptr);
  }();

  if (!boost::stacktrace::impl::ref_capture_stacktraces_at_throw()) {
    return orig_allocate_exception(thrown_size);
  }

#ifndef NDEBUG
  static thread_local std::size_t in_allocate_exception = 0;
  BOOST_ASSERT_MSG(in_allocate_exception < 10, "Suspicious recursion");
  ++in_allocate_exception;
  const decrement_on_destroy guard{in_allocate_exception};
#endif

  static constexpr std::size_t kAlign = alignof(std::max_align_t);
  thrown_size = (thrown_size + kAlign - 1) & (~(kAlign - 1));

  void* const ptr = orig_allocate_exception(thrown_size + kStacktraceDumpSize);
  char* const dump_ptr = static_cast<char*>(ptr) + thrown_size;

  constexpr size_t kSkip = 1;
  boost::stacktrace::safe_dump_to(kSkip, dump_ptr, kStacktraceDumpSize);

#if !BOOST_STACKTRACE_ALWAYS_STORE_IN_PADDING
  if (is_libcpp_runtime()) {
    const std::lock_guard<std::mutex> guard{g_mapping_mutex};
    g_exception_to_dump_mapping[ptr] = dump_ptr;
  } else
#endif
  {
    BOOST_ASSERT_MSG(
      reference_to_empty_padding(ptr) == nullptr,
      "Not zeroed out, unsupported implementation"
    );
    reference_to_empty_padding(ptr) = dump_ptr;
  }

  return ptr;
}

#if !BOOST_STACKTRACE_ALWAYS_STORE_IN_PADDING

// __cxa_free_exception is not called in libc++ as the
// __cxa_decrement_exception_refcount has an inlined call to
// __cxa_free_exception. Overriding libc++ specific function
extern "C" BOOST_SYMBOL_EXPORT
void __cxa_decrement_exception_refcount(void *thrown_object) throw() {
  BOOST_ASSERT(is_libcpp_runtime());
  if (!thrown_object) {
    return;
  }

  static const auto orig_decrement_refcount = []() {
    void* const ptr = ::dlsym(RTLD_NEXT, "__cxa_decrement_exception_refcount");
    BOOST_ASSERT_MSG(ptr, "Failed to find '__cxa_decrement_exception_refcount'");
    return reinterpret_cast<void(*)(void*)>(ptr);
  }();

  const auto* exception_header = exception_begin_llvm_ptr(thrown_object);

  // The following line has a race and could give false positives and false
  // negatives. In first case we remove the trace earlier, in the second case
  // we get a memory leak.
  if (exception_header->referenceCount == 1) {
    const std::lock_guard<std::mutex> guard{g_mapping_mutex};
    g_exception_to_dump_mapping.erase(thrown_object);
  }

  orig_decrement_refcount(thrown_object);
}

#endif

}  // namespace __cxxabiv1

namespace boost { namespace stacktrace { namespace impl {

BOOST_SYMBOL_EXPORT const char* current_exception_stacktrace() noexcept {
  if (!ref_capture_stacktraces_at_throw()) {
    return nullptr;
  }

  auto exc_ptr = std::current_exception();
  void* const exc_raw_ptr = get_current_exception_raw_ptr(&exc_ptr);
  if (!exc_raw_ptr) {
    return nullptr;
  }

#if !BOOST_STACKTRACE_ALWAYS_STORE_IN_PADDING
  if (__cxxabiv1::is_libcpp_runtime()) {
    const std::lock_guard<std::mutex> guard{g_mapping_mutex};
    const auto it = g_exception_to_dump_mapping.find(exc_raw_ptr);
    if (it != g_exception_to_dump_mapping.end()) {
      return it->second;
    } else {
      return nullptr;
    }
  } else
#endif
  {
    return __cxxabiv1::reference_to_empty_padding(exc_raw_ptr);
  }
}

BOOST_SYMBOL_EXPORT void assert_no_pending_traces() noexcept {
#if !BOOST_STACKTRACE_ALWAYS_STORE_IN_PADDING
  if (__cxxabiv1::is_libcpp_runtime()) {
    const std::lock_guard<std::mutex> guard{g_mapping_mutex};
    BOOST_ASSERT(g_exception_to_dump_mapping.empty());
  }
#endif
}

}}}  // namespace boost::stacktrace::impl

#endif