sigchainlib/sigchain.cc

/*
 * Copyright (C) 2014 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.
 */

#ifdef ART_TARGET_ANDROID
#include <android/log.h>
#else
#include <stdarg.h>
#include <iostream>
#endif

#include <dlfcn.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

#include <utility>

#include "sigchain.h"

#if defined(__APPLE__)
#define _NSIG NSIG
#define sighandler_t sig_t

// Darwin has an #error when ucontext.h is included without _XOPEN_SOURCE defined.
#define _XOPEN_SOURCE
#endif

#include <ucontext.h>

// libsigchain provides an interception layer for signal handlers, to allow ART and others to give
// their signal handlers the first stab at handling signals before passing them on to user code.
//
// It implements wrapper functions for signal, sigaction, and sigprocmask, and a handler that
// forwards signals appropriately.
//
// In our handler, we start off with all signals blocked, fetch the original signal mask from the
// passed in ucontext, and then adjust our signal mask appropriately for the user handler.
//
// It's somewhat tricky for us to properly handle some flag cases:
//   SA_NOCLDSTOP and SA_NOCLDWAIT: shouldn't matter, we don't have special handlers for SIGCHLD.
//   SA_NODEFER: unimplemented, we can manually change the signal mask appropriately.
//  ~SA_ONSTACK: always silently enable this
//   SA_RESETHAND: unimplemented, but we can probably do this?
//  ~SA_RESTART: unimplemented, maybe we can reserve an RT signal, register an empty handler that
//               doesn't have SA_RESTART, and raise the signal to avoid restarting syscalls that are
//               expected to be interrupted?


static void log(const char* format, ...) {
  char buf[256];
  va_list ap;
  va_start(ap, format);
  vsnprintf(buf, sizeof(buf), format, ap);
#ifdef ART_TARGET_ANDROID
  __android_log_write(ANDROID_LOG_ERROR, "libsigchain", buf);
#else
  std::cout << buf << "\n";
#endif
  va_end(ap);
}

#define fatal(...) log(__VA_ARGS__); abort()

static int sigorset(sigset_t* dest, sigset_t* left, sigset_t* right) {
  sigemptyset(dest);
  for (size_t i = 0; i < sizeof(sigset_t) * CHAR_BIT; ++i) {
    if (sigismember(left, i) == 1 || sigismember(right, i) == 1) {
      sigaddset(dest, i);
    }
  }
  return 0;
}

namespace art {

static decltype(&sigaction) linked_sigaction;
static decltype(&sigprocmask) linked_sigprocmask;
__thread bool handling_signal;

class SignalChain {
 public:
  SignalChain() : claimed_(false) {
  }

  bool IsClaimed() {
    return claimed_;
  }

  void Claim(int signo) {
    if (!claimed_) {
      Register(signo);
      claimed_ = true;
    }
  }

  // Register the signal chain with the kernel if needed.
  void Register(int signo) {
    struct sigaction handler_action = {};
    handler_action.sa_sigaction = SignalChain::Handler;
    handler_action.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK;
    sigfillset(&handler_action.sa_mask);
    linked_sigaction(signo, &handler_action, &action_);
  }

  void SetAction(const struct sigaction* action) {
    action_ = *action;
  }

  struct sigaction GetAction() {
    return action_;
  }

  void AddSpecialHandler(SpecialSignalHandlerFn fn) {
    for (SpecialSignalHandlerFn& slot : special_handlers_) {
      if (slot == nullptr) {
        slot = fn;
        return;
      }
    }

    fatal("too many special signal handlers");
  }

  void RemoveSpecialHandler(SpecialSignalHandlerFn fn) {
    // This isn't thread safe, but it's unlikely to be a real problem.
    size_t len = sizeof(special_handlers_)/sizeof(*special_handlers_);
    for (size_t i = 0; i < len; ++i) {
      if (special_handlers_[i] == fn) {
        for (size_t j = i; j < len - 1; ++j) {
          special_handlers_[j] = special_handlers_[j + 1];
        }
        special_handlers_[len - 1] = nullptr;
        return;
      }
    }

    fatal("failed to find special handler to remove");
  }


  static void Handler(int signo, siginfo_t* siginfo, void*);

 private:
  bool claimed_;
  struct sigaction action_;
  SpecialSignalHandlerFn special_handlers_[2];
};

static SignalChain chains[_NSIG];

class ScopedFlagSetter {
 public:
  explicit ScopedFlagSetter(bool* flag) : flag_(flag) {
    *flag_ = true;
  }

  ~ScopedFlagSetter() {
    *flag_ = false;
  }

 private:
  bool* flag_;
};

class ScopedSignalUnblocker {
 public:
  explicit ScopedSignalUnblocker(const std::initializer_list<int>& signals) {
    sigset_t new_mask;
    sigemptyset(&new_mask);
    for (int signal : signals) {
      sigaddset(&new_mask, signal);
    }
    if (sigprocmask(SIG_UNBLOCK, &new_mask, &previous_mask_) != 0) {
      fatal("failed to unblock signals: %s", strerror(errno));
    }
  }

  ~ScopedSignalUnblocker() {
    if (sigprocmask(SIG_SETMASK, &previous_mask_, nullptr) != 0) {
      fatal("failed to unblock signals: %s", strerror(errno));
    }
  }

 private:
  sigset_t previous_mask_;
};


void SignalChain::Handler(int signo, siginfo_t* siginfo, void* ucontext_raw) {
  // Try the special handlers first.
  // If one of them crashes, we'll reenter this handler and pass that crash onto the user handler.
  if (!handling_signal) {
    ScopedFlagSetter flag(&handling_signal);
    ScopedSignalUnblocker unblocked { SIGABRT, SIGSEGV, SIGBUS }; // NOLINT

    for (const auto& handler : chains[signo].special_handlers_) {
      if (handler != nullptr && handler(signo, siginfo, ucontext_raw)) {
        return;
      }
    }
  }

  // Forward to the user's signal handler.
  int handler_flags = chains[signo].action_.sa_flags;
  ucontext_t* ucontext = static_cast<ucontext_t*>(ucontext_raw);
  sigset_t mask;
  sigorset(&mask, &ucontext->uc_sigmask, &chains[signo].action_.sa_mask);
  if ((handler_flags & SA_NODEFER)) {
    sigdelset(&mask, signo);
  }
  sigprocmask(SIG_SETMASK, &mask, nullptr);

  if ((handler_flags & SA_SIGINFO)) {
    chains[signo].action_.sa_sigaction(signo, siginfo, ucontext_raw);
  } else {
    auto handler = chains[signo].action_.sa_handler;
    if (handler == SIG_IGN) {
      return;
    } else if (handler == SIG_DFL) {
      raise(signo);
    } else {
      handler(signo);
    }
  }
}

extern "C" int sigaction(int signal, const struct sigaction* new_action, struct sigaction* old_action) {
  // If this signal has been claimed as a signal chain, record the user's
  // action but don't pass it on to the kernel.
  // Note that we check that the signal number is in range here.  An out of range signal
  // number should behave exactly as the libc sigaction.
  if (signal < 0 || signal >= _NSIG) {
    errno = EINVAL;
    return -1;
  }

  if (chains[signal].IsClaimed() && new_action != nullptr) {
    struct sigaction saved_action = chains[signal].GetAction();
    if (new_action != nullptr) {
      chains[signal].SetAction(new_action);
    }
    if (old_action != nullptr) {
      *old_action = saved_action;
    }
    return 0;
  }

  // Will only get here if the signal chain has not been claimed.  We want
  // to pass the sigaction on to the kernel via the real sigaction in libc.
  InitializeSignalChain();
  return linked_sigaction(signal, new_action, old_action);
}

extern "C" sighandler_t signal(int signo, sighandler_t handler) {
  if (signo < 0 || signo > _NSIG) {
    errno = EINVAL;
    return SIG_ERR;
  }

  struct sigaction sa = {};
  sigemptyset(&sa.sa_mask);
  sa.sa_handler = handler;
  sa.sa_flags = SA_RESTART | SA_ONSTACK;
  sighandler_t oldhandler;

  // If this signal has been claimed as a signal chain, record the user's
  // action but don't pass it on to the kernel.
  if (chains[signo].IsClaimed()) {
    oldhandler = reinterpret_cast<sighandler_t>(chains[signo].GetAction().sa_handler);
    chains[signo].SetAction(&sa);
    return oldhandler;
  }

  // Will only get here if the signal chain has not been claimed.  We want
  // to pass the sigaction on to the kernel via the real sigaction in libc.
  InitializeSignalChain();
  if (linked_sigaction(signo, &sa, &sa) == -1) {
    return SIG_ERR;
  }

  return reinterpret_cast<sighandler_t>(sa.sa_handler);
}

#if !defined(__LP64__)
extern "C" sighandler_t bsd_signal(int signo, sighandler_t handler) {
  return signal(signo, handler);
}
#endif

extern "C" int sigprocmask(int how, const sigset_t* bionic_new_set, sigset_t* bionic_old_set) {
  const sigset_t* new_set_ptr = bionic_new_set;
  sigset_t tmpset;
  if (bionic_new_set != nullptr) {
    tmpset = *bionic_new_set;

    if (how == SIG_BLOCK) {
      // Don't allow claimed signals in the mask.  If a signal chain has been claimed
      // we can't allow the user to block that signal.
      for (int i = 0 ; i < _NSIG; ++i) {
        if (chains[i].IsClaimed() && sigismember(&tmpset, i)) {
          sigdelset(&tmpset, i);
        }
      }
    }
    new_set_ptr = &tmpset;
  }

  InitializeSignalChain();
  return linked_sigprocmask(how, new_set_ptr, bionic_old_set);
}

extern "C" void InitializeSignalChain() {
  // Warning.
  // Don't call this from within a signal context as it makes calls to
  // dlsym.  Calling into the dynamic linker will result in locks being
  // taken and if it so happens that a signal occurs while one of these
  // locks is already taken, dlsym will block trying to reenter a
  // mutex and we will never get out of it.
  static bool initialized = false;
  if (initialized) {
    // Don't initialize twice.
    return;
  }

  void* linked_sigaction_sym = dlsym(RTLD_NEXT, "sigaction");
  if (linked_sigaction_sym == nullptr) {
    linked_sigaction_sym = dlsym(RTLD_DEFAULT, "sigaction");
    if (linked_sigaction_sym == nullptr ||
        linked_sigaction_sym == reinterpret_cast<void*>(sigaction)) {
      fatal("Unable to find next sigaction in signal chain");
    }
  }

  void* linked_sigprocmask_sym = dlsym(RTLD_NEXT, "sigprocmask");
  if (linked_sigprocmask_sym == nullptr) {
    linked_sigprocmask_sym = dlsym(RTLD_DEFAULT, "sigprocmask");
    if (linked_sigprocmask_sym == nullptr ||
        linked_sigprocmask_sym == reinterpret_cast<void*>(sigprocmask)) {
      fatal("Unable to find next sigprocmask in signal chain");
    }
  }

  linked_sigaction = reinterpret_cast<decltype(linked_sigaction)>(linked_sigaction_sym);
  linked_sigprocmask = reinterpret_cast<decltype(linked_sigprocmask)>(linked_sigprocmask_sym);
  initialized = true;
}

extern "C" void AddSpecialSignalHandlerFn(int signal, SpecialSignalHandlerFn fn) {
  if (signal <= 0 || signal >= _NSIG) {
    fatal("Invalid signal %d", signal);
  }

  // Set the managed_handler.
  chains[signal].AddSpecialHandler(fn);
  chains[signal].Claim(signal);
}

extern "C" void RemoveSpecialSignalHandlerFn(int signal, SpecialSignalHandlerFn fn) {
  if (signal <= 0 || signal >= _NSIG) {
    fatal("Invalid signal %d", signal);
  }

  chains[signal].RemoveSpecialHandler(fn);
}

extern "C" void EnsureFrontOfChain(int signal) {
  if (signal <= 0 || signal >= _NSIG) {
    fatal("Invalid signal %d", signal);
  }

  // Read the current action without looking at the chain, it should be the expected action.
  struct sigaction current_action;
  InitializeSignalChain();
  linked_sigaction(signal, nullptr, &current_action);
  // If the sigactions don't match then we put the current action on the chain and make ourself as
  // the main action.
  if (current_action.sa_sigaction != SignalChain::Handler) {
    log("Warning: Unexpected sigaction action found %p\n", current_action.sa_sigaction);
    chains[signal].Register(signal);
  }
}

}   // namespace art