libbacktrace/BacktraceThread.cpp

/*
 * Copyright (C) 2013 The Android Open Source Project
 *
 * 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.
 */

#include <errno.h>
#include <inttypes.h>
#include <limits.h>
#include <linux/futex.h>
#include <pthread.h>
#include <signal.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <sys/types.h>
#include <ucontext.h>

#include <cutils/atomic.h>

#include "BacktraceLog.h"
#include "BacktraceThread.h"
#include "thread_utils.h"

static inline int futex(volatile int* uaddr, int op, int val, const struct timespec* ts, volatile int* uaddr2, int val3) {
  return syscall(__NR_futex, uaddr, op, val, ts, uaddr2, val3);
}

//-------------------------------------------------------------------------
// ThreadEntry implementation.
//-------------------------------------------------------------------------
ThreadEntry* ThreadEntry::list_ = NULL;
pthread_mutex_t ThreadEntry::list_mutex_ = PTHREAD_MUTEX_INITIALIZER;

// Assumes that ThreadEntry::list_mutex_ has already been locked before
// creating a ThreadEntry object.
ThreadEntry::ThreadEntry(pid_t pid, pid_t tid)
    : pid_(pid), tid_(tid), futex_(0), ref_count_(1), mutex_(PTHREAD_MUTEX_INITIALIZER), next_(ThreadEntry::list_), prev_(NULL) {
  // Add ourselves to the list.
  if (ThreadEntry::list_) {
    ThreadEntry::list_->prev_ = this;
  }
  ThreadEntry::list_ = this;
}

ThreadEntry* ThreadEntry::Get(pid_t pid, pid_t tid, bool create) {
  pthread_mutex_lock(&ThreadEntry::list_mutex_);
  ThreadEntry* entry = list_;
  while (entry != NULL) {
    if (entry->Match(pid, tid)) {
      break;
    }
    entry = entry->next_;
  }

  if (!entry) {
    if (create) {
      entry = new ThreadEntry(pid, tid);
    }
  } else {
    entry->ref_count_++;
  }
  pthread_mutex_unlock(&ThreadEntry::list_mutex_);

  return entry;
}

void ThreadEntry::Remove(ThreadEntry* entry) {
  pthread_mutex_unlock(&entry->mutex_);

  pthread_mutex_lock(&ThreadEntry::list_mutex_);
  if (--entry->ref_count_ == 0) {
    delete entry;
  }
  pthread_mutex_unlock(&ThreadEntry::list_mutex_);
}

// Assumes that ThreadEntry::list_mutex_ has already been locked before
// deleting a ThreadEntry object.
ThreadEntry::~ThreadEntry() {
  if (list_ == this) {
    list_ = next_;
  } else {
    if (next_) {
      next_->prev_ = prev_;
    }
    prev_->next_ = next_;
  }

  next_ = NULL;
  prev_ = NULL;
}

void ThreadEntry::Wait(int value) {
  timespec ts;
  ts.tv_sec = 10;
  ts.tv_nsec = 0;
  errno = 0;
  futex(&futex_, FUTEX_WAIT, value, &ts, NULL, 0);
  if (errno != 0 && errno != EWOULDBLOCK) {
    BACK_LOGW("futex wait failed, futex = %d: %s", futex_, strerror(errno));
  }
}

void ThreadEntry::Wake() {
  futex_++;
  futex(&futex_, FUTEX_WAKE, INT_MAX, NULL, NULL, 0);
}

void ThreadEntry::CopyUcontextFromSigcontext(void* sigcontext) {
  ucontext_t* ucontext = reinterpret_cast<ucontext_t*>(sigcontext);
  // The only thing the unwinder cares about is the mcontext data.
  memcpy(&ucontext_.uc_mcontext, &ucontext->uc_mcontext, sizeof(ucontext->uc_mcontext));
}

//-------------------------------------------------------------------------
// BacktraceThread functions.
//-------------------------------------------------------------------------
static pthread_mutex_t g_sigaction_mutex = PTHREAD_MUTEX_INITIALIZER;

static void SignalHandler(int, siginfo_t*, void* sigcontext) {
  ThreadEntry* entry = ThreadEntry::Get(getpid(), gettid(), false);
  if (!entry) {
    BACK_LOGW("Unable to find pid %d tid %d information", getpid(), gettid());
    return;
  }

  entry->CopyUcontextFromSigcontext(sigcontext);

  // Indicate the ucontext is now valid.
  entry->Wake();

  // Pause the thread until the unwind is complete. This avoids having
  // the thread run ahead causing problems.
  entry->Wait(1);

  ThreadEntry::Remove(entry);
}

BacktraceThread::BacktraceThread(BacktraceImpl* impl, pid_t tid, BacktraceMap* map)
    : BacktraceCurrent(impl, map) {
  tid_ = tid;
}

BacktraceThread::~BacktraceThread() {
}

bool BacktraceThread::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
  if (ucontext) {
    // Unwind using an already existing ucontext.
    return impl_->Unwind(num_ignore_frames, ucontext);
  }

  // Prevent multiple threads trying to set the trigger action on different
  // threads at the same time.
  if (pthread_mutex_lock(&g_sigaction_mutex) < 0) {
    BACK_LOGW("sigaction failed: %s", strerror(errno));
    return false;
  }

  ThreadEntry* entry = ThreadEntry::Get(Pid(), Tid());
  entry->Lock();

  struct sigaction act, oldact;
  memset(&act, 0, sizeof(act));
  act.sa_sigaction = SignalHandler;
  act.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK;
  sigemptyset(&act.sa_mask);
  if (sigaction(THREAD_SIGNAL, &act, &oldact) != 0) {
    BACK_LOGW("sigaction failed %s", strerror(errno));
    entry->Unlock();
    ThreadEntry::Remove(entry);
    pthread_mutex_unlock(&g_sigaction_mutex);
    return false;
  }

  if (tgkill(Pid(), Tid(), THREAD_SIGNAL) != 0) {
    BACK_LOGW("tgkill %d failed: %s", Tid(), strerror(errno));
    sigaction(THREAD_SIGNAL, &oldact, NULL);
    entry->Unlock();
    ThreadEntry::Remove(entry);
    pthread_mutex_unlock(&g_sigaction_mutex);
    return false;
  }

  // Wait for the thread to get the ucontext.
  entry->Wait(0);

  // After the thread has received the signal, allow other unwinders to
  // continue.
  sigaction(THREAD_SIGNAL, &oldact, NULL);
  pthread_mutex_unlock(&g_sigaction_mutex);

  bool unwind_done = impl_->Unwind(num_ignore_frames, entry->GetUcontext());

  // Tell the signal handler to exit and release the entry.
  entry->Wake();

  return unwind_done;
}