PrettyStackTrace.cpp

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//===- PrettyStackTrace.cpp - Pretty Crash Handling -----------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines some helpful functions for dealing with the possibility of
// Unix signals occurring while your program is running.
//
//===----------------------------------------------------------------------===//

#include "llvm/Support/PrettyStackTrace.h"
#include "llvm-c/ErrorHandling.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Config/config.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/SaveAndRestore.h"
#include "llvm/Support/Signals.h"
#include "llvm/Support/Watchdog.h"
#include "llvm/Support/raw_ostream.h"

#include <atomic>
#include <cstdarg>
#include <cstdio>
#include <tuple>

#ifdef HAVE_CRASHREPORTERCLIENT_H
#include <CrashReporterClient.h>
#endif

using namespace llvm;

// If backtrace support is not enabled, compile out support for pretty stack
// traces.  This has the secondary effect of not requiring thread local storage
// when backtrace support is disabled.
#if ENABLE_BACKTRACES

// We need a thread local pointer to manage the stack of our stack trace
// objects, but we *really* cannot tolerate destructors running and do not want
// to pay any overhead of synchronizing. As a consequence, we use a raw
// thread-local variable.
static LLVM_THREAD_LOCAL PrettyStackTraceEntry *PrettyStackTraceHead = nullptr;

// The use of 'volatile' here is to ensure that any particular thread always
// reloads the value of the counter. The 'std::atomic' allows us to specify that
// this variable is accessed in an unsychronized way (it's not actually
// synchronizing). This does technically mean that the value may not appear to
// be the same across threads running simultaneously on different CPUs, but in
// practice the worst that will happen is that we won't print a stack trace when
// we could have.
//
// This is initialized to 1 because 0 is used as a sentinel for "not enabled on
// the current thread". If the user happens to overflow an 'unsigned' with
// SIGINFO requests, it's possible that some threads will stop responding to it,
// but the program won't crash.
static volatile std::atomic<unsigned> GlobalSigInfoGenerationCounter =
    ATOMIC_VAR_INIT(1);
static LLVM_THREAD_LOCAL unsigned ThreadLocalSigInfoGenerationCounter = 0;

namespace llvm {
PrettyStackTraceEntry *ReverseStackTrace(PrettyStackTraceEntry *Head) {
  PrettyStackTraceEntry *Prev = nullptr;
  while (Head)
    std::tie(Prev, Head, Head->NextEntry) =
        std::make_tuple(Head, Head->NextEntry, Prev);
  return Prev;
}
}

static void PrintStack(raw_ostream &OS) {
  // Print out the stack in reverse order. To avoid recursion (which is likely
  // to fail if we crashed due to stack overflow), we do an up-front pass to
  // reverse the stack, then print it, then reverse it again.
  unsigned ID = 0;
  SaveAndRestore<PrettyStackTraceEntry *> SavedStack{PrettyStackTraceHead,
                                                     nullptr};
  PrettyStackTraceEntry *ReversedStack = ReverseStackTrace(SavedStack.get());
  for (const PrettyStackTraceEntry *Entry = ReversedStack; Entry;
       Entry = Entry->getNextEntry()) {
    OS << ID++ << ".\t";
    sys::Watchdog W(5);
    Entry->print(OS);
  }
  llvm::ReverseStackTrace(ReversedStack);
}

/// Print the current stack trace to the specified stream.
///
/// Marked NOINLINE so it can be called from debuggers.
LLVM_ATTRIBUTE_NOINLINE
static void PrintCurStackTrace(raw_ostream &OS) {
  // Don't print an empty trace.
  if (!PrettyStackTraceHead) return;

  // If there are pretty stack frames registered, walk and emit them.
  OS << "Stack dump:\n";

  PrintStack(OS);
  OS.flush();
}

// Integrate with crash reporter libraries.
#if defined (__APPLE__) && defined(HAVE_CRASHREPORTERCLIENT_H)
//  If any clients of llvm try to link to libCrashReporterClient.a themselves,
//  only one crash info struct will be used.
extern "C" {
CRASH_REPORTER_CLIENT_HIDDEN
struct crashreporter_annotations_t gCRAnnotations
        __attribute__((section("__DATA," CRASHREPORTER_ANNOTATIONS_SECTION)))
#if CRASHREPORTER_ANNOTATIONS_VERSION < 5
        = { CRASHREPORTER_ANNOTATIONS_VERSION, 0, 0, 0, 0, 0, 0 };
#else
        = { CRASHREPORTER_ANNOTATIONS_VERSION, 0, 0, 0, 0, 0, 0, 0 };
#endif
}
#elif defined(__APPLE__) && HAVE_CRASHREPORTER_INFO
extern "C" const char *__crashreporter_info__
    __attribute__((visibility("hidden"))) = 0;
asm(".desc ___crashreporter_info__, 0x10");
#endif

static void setCrashLogMessage(const char *msg) LLVM_ATTRIBUTE_UNUSED;
static void setCrashLogMessage(const char *msg) {
#ifdef HAVE_CRASHREPORTERCLIENT_H
  (void)CRSetCrashLogMessage(msg);
#elif HAVE_CRASHREPORTER_INFO
  __crashreporter_info__ = msg;
#endif
  // Don't reorder subsequent operations: whatever comes after might crash and
  // we want the system crash handling to see the message we just set.
  std::atomic_signal_fence(std::memory_order_seq_cst);
}

#ifdef __APPLE__
using CrashHandlerString = SmallString<2048>;
using CrashHandlerStringStorage =
    std::aligned_storage<sizeof(CrashHandlerString),
                         alignof(CrashHandlerString)>::type;
static CrashHandlerStringStorage crashHandlerStringStorage;
#endif

/// This callback is run if a fatal signal is delivered to the process, it
/// prints the pretty stack trace.
static void CrashHandler(void *) {
#ifndef __APPLE__
  // On non-apple systems, just emit the crash stack trace to stderr.
  PrintCurStackTrace(errs());
#else
  // Emit the crash stack trace to a SmallString, put it where the system crash
  // handling will find it, and also send it to stderr.
  //
  // The SmallString is fairly large in the hope that we don't allocate (we're
  // handling a fatal signal, something is already pretty wrong, allocation
  // might not work). Further, we don't use a magic static in case that's also
  // borked. We leak any allocation that does occur because the program is about
  // to die anyways. This is technically racy if we were handling two fatal
  // signals, however if we're in that situation a race is the least of our
  // worries.
  auto &crashHandlerString =
      *new (&crashHandlerStringStorage) CrashHandlerString;

  // If we crash while trying to print the stack trace, we still want the system
  // crash handling to have some partial information. That'll work out as long
  // as the SmallString doesn't allocate. If it does allocate then the system
  // crash handling will see some garbage because the inline buffer now contains
  // a pointer.
  setCrashLogMessage(crashHandlerString.c_str());

  {
    raw_svector_ostream Stream(crashHandlerString);
    PrintCurStackTrace(Stream);
  }

  if (!crashHandlerString.empty()) {
    setCrashLogMessage(crashHandlerString.c_str());
    errs() << crashHandlerString.str();
  } else
    setCrashLogMessage("No crash information.");
#endif
}

static void printForSigInfoIfNeeded() {
  unsigned CurrentSigInfoGeneration =
      GlobalSigInfoGenerationCounter.load(std::memory_order_relaxed);
  if (ThreadLocalSigInfoGenerationCounter == 0 ||
      ThreadLocalSigInfoGenerationCounter == CurrentSigInfoGeneration) {
    return;
  }

  PrintCurStackTrace(errs());
  ThreadLocalSigInfoGenerationCounter = CurrentSigInfoGeneration;
}

#endif // ENABLE_BACKTRACES

PrettyStackTraceEntry::PrettyStackTraceEntry() {
#if ENABLE_BACKTRACES
  // Handle SIGINFO first, because we haven't finished constructing yet.
  printForSigInfoIfNeeded();
  // Link ourselves.
  NextEntry = PrettyStackTraceHead;
  PrettyStackTraceHead = this;
#endif
}

PrettyStackTraceEntry::~PrettyStackTraceEntry() {
#if ENABLE_BACKTRACES
  assert(PrettyStackTraceHead == this &&
         "Pretty stack trace entry destruction is out of order");
  PrettyStackTraceHead = NextEntry;
  // Handle SIGINFO first, because we already started destructing.
  printForSigInfoIfNeeded();
#endif
}

void PrettyStackTraceString::print(raw_ostream &OS) const { OS << Str << "\n"; }

PrettyStackTraceFormat::PrettyStackTraceFormat(const char *Format, ...) {
  va_list AP;
  va_start(AP, Format);
  const int SizeOrError = vsnprintf(nullptr, 0, Format, AP);
  va_end(AP);
  if (SizeOrError < 0) {
    return;
  }

  const int Size = SizeOrError + 1; // '\0'
  Str.resize(Size);
  va_start(AP, Format);
  vsnprintf(Str.data(), Size, Format, AP);
  va_end(AP);
}

void PrettyStackTraceFormat::print(raw_ostream &OS) const { OS << Str << "\n"; }

void PrettyStackTraceProgram::print(raw_ostream &OS) const {
  OS << "Program arguments: ";
  // Print the argument list.
  for (unsigned i = 0, e = ArgC; i != e; ++i)
    OS << ArgV[i] << ' ';
  OS << '\n';
}

#if ENABLE_BACKTRACES
static bool RegisterCrashPrinter() {
  sys::AddSignalHandler(CrashHandler, nullptr);
  return false;
}
#endif

void llvm::EnablePrettyStackTrace() {
#if ENABLE_BACKTRACES
  // The first time this is called, we register the crash printer.
  static bool HandlerRegistered = RegisterCrashPrinter();
  (void)HandlerRegistered;
#endif
}

void llvm::EnablePrettyStackTraceOnSigInfoForThisThread(bool ShouldEnable) {
#if ENABLE_BACKTRACES
  if (!ShouldEnable) {
    ThreadLocalSigInfoGenerationCounter = 0;
    return;
  }

  // The first time this is called, we register the SIGINFO handler.
  static bool HandlerRegistered = []{
    sys::SetInfoSignalFunction([]{
      GlobalSigInfoGenerationCounter.fetch_add(1, std::memory_order_relaxed);
    });
    return false;
  }();
  (void)HandlerRegistered;

  // Next, enable it for the current thread.
  ThreadLocalSigInfoGenerationCounter =
      GlobalSigInfoGenerationCounter.load(std::memory_order_relaxed);
#endif
}

const void *llvm::SavePrettyStackState() {
#if ENABLE_BACKTRACES
  return PrettyStackTraceHead;
#else
  return nullptr;
#endif
}

void llvm::RestorePrettyStackState(const void *Top) {
#if ENABLE_BACKTRACES
  PrettyStackTraceHead =
      static_cast<PrettyStackTraceEntry *>(const_cast<void *>(Top));
#endif
}

void LLVMEnablePrettyStackTrace() {
  EnablePrettyStackTrace();
}