#include "thread-inl.h"
#include "verify_object-inl.h"
-// Note on nested signal support
-// -----------------------------
-//
-// Typically a signal handler should not need to deal with signals that occur within it.
-// However, when a SIGSEGV occurs that is in generated code and is not one of the
-// handled signals (implicit checks), we call a function to try to dump the stack
-// to the log. This enhances the debugging experience but may have the side effect
-// that it may not work. If the cause of the original SIGSEGV is a corrupted stack or other
-// memory region, the stack backtrace code may run into trouble and may either crash
-// or fail with an abort (SIGABRT). In either case we don't want that (new) signal to
-// mask the original signal and thus prevent useful debug output from being presented.
-//
-// In order to handle this situation, before we call the stack tracer we do the following:
-//
-// 1. shutdown the fault manager so that we are talking to the real signal management
-// functions rather than those in sigchain.
-// 2. use pthread_sigmask to allow SIGSEGV and SIGABRT signals to be delivered to the
-// thread running the signal handler.
-// 3. set the handler for SIGSEGV and SIGABRT to a secondary signal handler.
-// 4. save the thread's state to the TLS of the current thread using 'setjmp'
-//
-// We then call the stack tracer and one of two things may happen:
-// a. it completes successfully
-// b. it crashes and a signal is raised.
-//
-// In the former case, we fall through and everything is fine. In the latter case
-// our secondary signal handler gets called in a signal context. This results in
-// a call to FaultManager::HandledNestedSignal(), an archirecture specific function
-// whose purpose is to call 'longjmp' on the jmp_buf saved in the TLS of the current
-// thread. This results in a return with a non-zero value from 'setjmp'. We detect this
-// and write something to the log to tell the user that it happened.
-//
-// Regardless of how we got there, we reach the code after the stack tracer and we
-// restore the signal states to their original values, reinstate the fault manager (thus
-// reestablishing the signal chain) and continue.
-
-// This is difficult to test with a runtime test. To invoke the nested signal code
-// on any signal, uncomment the following line and run something that throws a
-// NullPointerException.
-// #define TEST_NESTED_SIGNAL
-
namespace art {
// Static fault manger object accessed by signal handler.
FaultManager fault_manager;
fault_manager.HandleFault(sig, info, context);
}
-// Signal handler for dealing with a nested signal.
-static void art_nested_signal_handler(int sig, siginfo_t* info, void* context) {
- fault_manager.HandleNestedSignal(sig, info, context);
-}
-
FaultManager::FaultManager() : initialized_(false) {
sigaction(SIGSEGV, nullptr, &oldaction_);
}
DCHECK(self != nullptr);
DCHECK(Runtime::Current() != nullptr);
DCHECK(Runtime::Current()->IsStarted());
-
- // Now set up the nested signal handler.
-
- // TODO: add SIGSEGV back to the nested signals when we can handle running out stack gracefully.
- static const int handled_nested_signals[] = {SIGABRT};
- constexpr size_t num_handled_nested_signals = arraysize(handled_nested_signals);
-
- // Release the fault manager so that it will remove the signal chain for
- // SIGSEGV and we call the real sigaction.
- fault_manager.Release();
-
- // The action for SIGSEGV should be the default handler now.
-
- // Unblock the signals we allow so that they can be delivered in the signal handler.
- sigset_t sigset;
- sigemptyset(&sigset);
- for (int signal : handled_nested_signals) {
- sigaddset(&sigset, signal);
+ for (const auto& handler : other_handlers_) {
+ if (handler->Action(sig, info, context)) {
+ return true;
+ }
}
- pthread_sigmask(SIG_UNBLOCK, &sigset, nullptr);
+ return false;
+}
- // If we get a signal in this code we want to invoke our nested signal
- // handler.
- struct sigaction action;
- struct sigaction oldactions[num_handled_nested_signals];
- action.sa_sigaction = art_nested_signal_handler;
-
- // Explicitly mask out SIGSEGV and SIGABRT from the nested signal handler. This
- // should be the default but we definitely don't want these happening in our
- // nested signal handler.
- sigemptyset(&action.sa_mask);
- for (int signal : handled_nested_signals) {
- sigaddset(&action.sa_mask, signal);
+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) {
+ PLOG(FATAL) << "failed to unblock signals";
+ }
}
- action.sa_flags = SA_SIGINFO | SA_ONSTACK;
-#if !defined(__APPLE__) && !defined(__mips__)
- action.sa_restorer = nullptr;
-#endif
-
- // Catch handled signals to invoke our nested handler.
- bool success = true;
- for (size_t i = 0; i < num_handled_nested_signals; ++i) {
- success = sigaction(handled_nested_signals[i], &action, &oldactions[i]) == 0;
- if (!success) {
- PLOG(ERROR) << "Unable to set up nested signal handler";
- break;
+ ~ScopedSignalUnblocker() {
+ if (sigprocmask(SIG_SETMASK, &previous_mask_, nullptr) != 0) {
+ PLOG(FATAL) << "failed to unblock signals";
}
}
- if (success) {
- // Save the current state and call the handlers. If anything causes a signal
- // our nested signal handler will be invoked and this will longjmp to the saved
- // state.
- if (setjmp(*self->GetNestedSignalState()) == 0) {
- for (const auto& handler : other_handlers_) {
- if (handler->Action(sig, info, context)) {
- // Restore the signal handlers, reinit the fault manager and return. Signal was
- // handled.
- for (size_t i = 0; i < num_handled_nested_signals; ++i) {
- success = sigaction(handled_nested_signals[i], &oldactions[i], nullptr) == 0;
- if (!success) {
- PLOG(ERROR) << "Unable to restore signal handler";
- }
- }
- fault_manager.Init();
- return true;
- }
- }
- } else {
- LOG(ERROR) << "Nested signal detected - original signal being reported";
- }
+ private:
+ sigset_t previous_mask_;
+};
- // Restore the signal handlers.
- for (size_t i = 0; i < num_handled_nested_signals; ++i) {
- success = sigaction(handled_nested_signals[i], &oldactions[i], nullptr) == 0;
- if (!success) {
- PLOG(ERROR) << "Unable to restore signal handler";
- }
- }
+class ScopedHandlingSignalSetter {
+ public:
+ explicit ScopedHandlingSignalSetter(Thread* thread) : thread_(thread) {
+ CHECK(!thread->HandlingSignal());
+ thread_->SetHandlingSignal(true);
}
- // Now put the fault manager back in place.
- fault_manager.Init();
- return false;
-}
+ ~ScopedHandlingSignalSetter() {
+ CHECK(thread_->HandlingSignal());
+ thread_->SetHandlingSignal(false);
+ }
+
+ private:
+ Thread* thread_;
+};
void FaultManager::HandleFault(int sig, siginfo_t* info, void* context) {
// BE CAREFUL ALLOCATING HERE INCLUDING USING LOG(...)
//
// If malloc calls abort, it will be holding its lock.
// If the handler tries to call malloc, it will deadlock.
- VLOG(signals) << "Handling fault";
- if (IsInGeneratedCode(info, context, true)) {
- VLOG(signals) << "in generated code, looking for handler";
- for (const auto& handler : generated_code_handlers_) {
- VLOG(signals) << "invoking Action on handler " << handler;
- if (handler->Action(sig, info, context)) {
+
+ // Use a thread local field to track whether we're recursing, and fall back.
+ // (e.g.. if one of our handlers crashed)
+ Thread* thread = Thread::Current();
+
+ if (thread != nullptr && !thread->HandlingSignal()) {
+ // Unblock some signals and set thread->handling_signal_ to true,
+ // so that we can catch crashes in our signal handler.
+ ScopedHandlingSignalSetter setter(thread);
+ ScopedSignalUnblocker unblocker { SIGABRT, SIGBUS, SIGSEGV }; // NOLINT
+
+ VLOG(signals) << "Handling fault";
+
#ifdef TEST_NESTED_SIGNAL
- // In test mode we want to fall through to stack trace handler
- // on every signal (in reality this will cause a crash on the first
- // signal).
- break;
-#else
- // We have handled a signal so it's time to return from the
- // signal handler to the appropriate place.
- return;
+ // Simulate a crash in a handler.
+ raise(SIGSEGV);
#endif
+
+ if (IsInGeneratedCode(info, context, true)) {
+ VLOG(signals) << "in generated code, looking for handler";
+ for (const auto& handler : generated_code_handlers_) {
+ VLOG(signals) << "invoking Action on handler " << handler;
+ if (handler->Action(sig, info, context)) {
+ // We have handled a signal so it's time to return from the
+ // signal handler to the appropriate place.
+ return;
+ }
}
- }
- // We hit a signal we didn't handle. This might be something for which
- // we can give more information about so call all registered handlers to see
- // if it is.
- if (HandleFaultByOtherHandlers(sig, info, context)) {
+ // We hit a signal we didn't handle. This might be something for which
+ // we can give more information about so call all registered handlers to
+ // see if it is.
+ if (HandleFaultByOtherHandlers(sig, info, context)) {
return;
+ }
}
}
bool JavaStackTraceHandler::Action(int sig ATTRIBUTE_UNUSED, siginfo_t* siginfo, void* context) {
// Make sure that we are in the generated code, but we may not have a dex pc.
-#ifdef TEST_NESTED_SIGNAL
- bool in_generated_code = true;
-#else
bool in_generated_code = manager_->IsInGeneratedCode(siginfo, context, false);
-#endif
if (in_generated_code) {
LOG(ERROR) << "Dumping java stack trace for crash in generated code";
ArtMethod* method = nullptr;
manager_->GetMethodAndReturnPcAndSp(siginfo, context, &method, &return_pc, &sp);
// Inside of generated code, sp[0] is the method, so sp is the frame.
self->SetTopOfStack(reinterpret_cast<ArtMethod**>(sp));
-#ifdef TEST_NESTED_SIGNAL
- // To test the nested signal handler we raise a signal here. This will cause the
- // nested signal handler to be called and perform a longjmp back to the setjmp
- // above.
- abort();
-#endif
self->DumpJavaStack(LOG_STREAM(ERROR));
}
OSDN Git Service
