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* Copyright (C) 2008 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include "fault_handler.h"
#include <setjmp.h>
#include <sys/mman.h>
#include <sys/ucontext.h>
#include "mirror/art_method.h"
#include "mirror/class.h"
#include "sigchain.h"
#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.
namespace art {
// Static fault manger object accessed by signal handler.
FaultManager fault_manager;
extern "C" {
void art_sigsegv_fault() {
// Set a breakpoint here to be informed when a SIGSEGV is unhandled by ART.
VLOG(signals)<< "Caught unknown SIGSEGV in ART fault handler - chaining to next handler.";
// Signal handler called on SIGSEGV.
static void art_fault_handler(int sig, siginfo_t* info, void* context) {
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_);
FaultManager::~FaultManager() {
static void SetUpArtAction(struct sigaction* action) {
action->sa_sigaction = art_fault_handler;
action->sa_flags = SA_SIGINFO | SA_ONSTACK;
#if !defined(__APPLE__) && !defined(__mips__)
action->sa_restorer = nullptr;
void FaultManager::EnsureArtActionInFrontOfSignalChain() {
if (initialized_) {
struct sigaction action;
EnsureFrontOfChain(SIGSEGV, &action);
} else {
LOG(WARNING) << "Can't call " << __FUNCTION__ << " due to unitialized fault manager";
void FaultManager::Init() {
struct sigaction action;
// Set our signal handler now.
int e = sigaction(SIGSEGV, &action, &oldaction_);
if (e != 0) {
VLOG(signals) << "Failed to claim SEGV: " << strerror(errno);
// Make sure our signal handler is called before any user handlers.
ClaimSignalChain(SIGSEGV, &oldaction_);
initialized_ = true;
void FaultManager::Shutdown() {
if (initialized_) {
initialized_ = false;
void FaultManager::HandleFault(int sig, siginfo_t* info, void* context) {
// 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)) {
// 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).
// We have handled a signal so it's time to return from the
// signal handler to the appropriate place.
// 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.
for (const auto& handler : other_handlers_) {
if (handler->Action(sig, info, context)) {
// Set a breakpoint in this function to catch unhandled signals.
// Pass this on to the next handler in the chain, or the default if none.
InvokeUserSignalHandler(sig, info, context);
void FaultManager::AddHandler(FaultHandler* handler, bool generated_code) {
if (generated_code) {
} else {
void FaultManager::RemoveHandler(FaultHandler* handler) {
auto it = std::find(generated_code_handlers_.begin(), generated_code_handlers_.end(), handler);
if (it != generated_code_handlers_.end()) {
auto it2 = std::find(other_handlers_.begin(), other_handlers_.end(), handler);
if (it2 != other_handlers_.end()) {
LOG(FATAL) << "Attempted to remove non existent handler " << handler;
// This function is called within the signal handler. It checks that
// the mutator_lock is held (shared). No annotalysis is done.
bool FaultManager::IsInGeneratedCode(siginfo_t* siginfo, void* context, bool check_dex_pc) {
// We can only be running Java code in the current thread if it
// is in Runnable state.
VLOG(signals) << "Checking for generated code";
Thread* thread = Thread::Current();
if (thread == nullptr) {
VLOG(signals) << "no current thread";
return false;
ThreadState state = thread->GetState();
if (state != kRunnable) {
VLOG(signals) << "not runnable";
return false;
// Current thread is runnable.
// Make sure it has the mutator lock.
if (!Locks::mutator_lock_->IsSharedHeld(thread)) {
VLOG(signals) << "no lock";
return false;
mirror::ArtMethod* method_obj = 0;
uintptr_t return_pc = 0;
uintptr_t sp = 0;
// Get the architecture specific method address and return address. These
// are in architecture specific files in arch/<arch>/fault_handler_<arch>.
GetMethodAndReturnPcAndSp(siginfo, context, &method_obj, &return_pc, &sp);
// If we don't have a potential method, we're outta here.
VLOG(signals) << "potential method: " << method_obj;
if (method_obj == 0 || !IsAligned<kObjectAlignment>(method_obj)) {
VLOG(signals) << "no method";
return false;
// Verify that the potential method is indeed a method.
// TODO: check the GC maps to make sure it's an object.
// Check that the class pointer inside the object is not null and is aligned.
// TODO: Method might be not a heap address, and GetClass could fault.
mirror::Class* cls = method_obj->GetClass<kVerifyNone>();
if (cls == nullptr) {
VLOG(signals) << "not a class";
return false;
if (!IsAligned<kObjectAlignment>(cls)) {
VLOG(signals) << "not aligned";
return false;
if (!VerifyClassClass(cls)) {
VLOG(signals) << "not a class class";
return false;
// Now make sure the class is a mirror::ArtMethod.
if (!cls->IsArtMethodClass()) {
VLOG(signals) << "not a method";
return false;
// We can be certain that this is a method now. Check if we have a GC map
// at the return PC address.
if (true || kIsDebugBuild) {
VLOG(signals) << "looking for dex pc for return pc " << std::hex << return_pc;
const void* code = Runtime::Current()->GetInstrumentation()->GetQuickCodeFor(method_obj,
uint32_t sought_offset = return_pc - reinterpret_cast<uintptr_t>(code);
VLOG(signals) << "pc offset: " << std::hex << sought_offset;
uint32_t dexpc = method_obj->ToDexPc(return_pc, false);
VLOG(signals) << "dexpc: " << dexpc;
return !check_dex_pc || dexpc != DexFile::kDexNoIndex;
FaultHandler::FaultHandler(FaultManager* manager) : manager_(manager) {
// Null pointer fault handler
NullPointerHandler::NullPointerHandler(FaultManager* manager) : FaultHandler(manager) {
manager_->AddHandler(this, true);
// Suspension fault handler
SuspensionHandler::SuspensionHandler(FaultManager* manager) : FaultHandler(manager) {
manager_->AddHandler(this, true);
// Stack overflow fault handler
StackOverflowHandler::StackOverflowHandler(FaultManager* manager) : FaultHandler(manager) {
manager_->AddHandler(this, true);
// Stack trace handler, used to help get a stack trace from SIGSEGV inside of compiled code.
JavaStackTraceHandler::JavaStackTraceHandler(FaultManager* manager) : FaultHandler(manager) {
manager_->AddHandler(this, false);
bool JavaStackTraceHandler::Action(int sig, siginfo_t* siginfo, void* context) {
// Make sure that we are in the generated code, but we may not have a dex pc.
bool in_generated_code = true;
bool in_generated_code = manager_->IsInGeneratedCode(siginfo, context, false);
if (in_generated_code) {
LOG(ERROR) << "Dumping java stack trace for crash in generated code";
mirror::ArtMethod* method = nullptr;
uintptr_t return_pc = 0;
uintptr_t sp = 0;
Thread* self = Thread::Current();
// Shutdown the fault manager so that it will remove the signal chain for
// SIGSEGV and we call the real sigaction.
// 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;
sigaddset(&sigset, SIGSEGV);
sigaddset(&sigset, SIGABRT);
pthread_sigmask(SIG_UNBLOCK, &sigset, nullptr);
// If we get a signal in this code we want to invoke our nested signal
// handler.
struct sigaction action, oldsegvaction, oldabortaction;
action.sa_sigaction = art_nested_signal_handler;
// Explictly 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.
sigaddset(&action.sa_mask, SIGSEGV);
sigaddset(&action.sa_mask, SIGABRT);
action.sa_flags = SA_SIGINFO | SA_ONSTACK;
#if !defined(__APPLE__) && !defined(__mips__)
action.sa_restorer = nullptr;
// Catch SIGSEGV and SIGABRT to invoke our nested handler
int e1 = sigaction(SIGSEGV, &action, &oldsegvaction);
int e2 = sigaction(SIGABRT, &action, &oldabortaction);
if (e1 != 0 || e2 != 0) {
LOG(ERROR) << "Unable to register nested signal handler - no stack trace possible";
// If sigaction failed we have a serious problem. We cannot catch
// any failures in the stack tracer and it's likely to occur since
// the program state is bad. Therefore we don't even try to give
// a stack trace.
} else {
// Save the current state and try to dump the stack. If this causes a signal
// our nested signal handler will be invoked and this will longjmp to the saved
// state.
if (setjmp(*self->GetNestedSignalState()) == 0) {
manager_->GetMethodAndReturnPcAndSp(siginfo, context, &method, &return_pc, &sp);
// Inside of generated code, sp[0] is the method, so sp is the frame.
StackReference<mirror::ArtMethod>* frame =
self->SetTopOfStack(frame, 0); // Since we don't necessarily have a dex pc, pass in 0.
// 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.
} else {
LOG(ERROR) << "Stack trace aborted due to nested signal - original signal being reported";
// Restore the signal handlers.
sigaction(SIGSEGV, &oldsegvaction, nullptr);
sigaction(SIGABRT, &oldabortaction, nullptr);
// Now put the fault manager back in place.
// And we're done.
return false; // Return false since we want to propagate the fault to the main signal handler.
} // namespace art