blob: 372038e3728b9b1a304f31c7bc385b7ded67c9f2 [file] [log] [blame]
// Copyright 2019 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/profiler/stack_copier_signal.h"
#include <linux/futex.h>
#include <signal.h>
#include <sys/ucontext.h>
#include <syscall.h>
#include <atomic>
#include "base/profiler/metadata_recorder.h"
#include "base/profiler/register_context.h"
#include "base/profiler/sample_metadata.h"
#include "base/profiler/stack_buffer.h"
#include "base/profiler/suspendable_thread_delegate.h"
#include "base/trace_event/trace_event.h"
#include "build/build_config.h"
namespace base {
namespace {
// Waitable event implementation with futex and without DCHECK(s), since signal
// handlers cannot allocate memory or use pthread api.
class AsyncSafeWaitableEvent {
AsyncSafeWaitableEvent() {, std::memory_order_release); }
~AsyncSafeWaitableEvent() {}
bool Wait() {
// futex() can wake up spuriously if this memory address was previously used
// for a pthread mutex. So, also check the condition.
while (true) {
int res =
syscall(SYS_futex, futex_int_ptr(), FUTEX_WAIT | FUTEX_PRIVATE_FLAG,
0, nullptr, nullptr, 0);
if (futex_.load(std::memory_order_acquire) != 0)
return true;
if (res != 0)
return false;
void Signal() {, std::memory_order_release);
syscall(SYS_futex, futex_int_ptr(), FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1,
nullptr, nullptr, 0);
// Provides a pointer to the atomic's storage. std::atomic_int has standard
// layout so its address can be used for the pointer as long as it only
// contains the int.
int* futex_int_ptr() {
static_assert(sizeof(futex_) == sizeof(int),
"Expected std::atomic_int to be the same size as int");
return reinterpret_cast<int*>(&futex_);
std::atomic_int futex_{0};
// Scoped signal event that calls Signal on the AsyncSafeWaitableEvent at
// destructor.
class ScopedEventSignaller {
ScopedEventSignaller(AsyncSafeWaitableEvent* event) : event_(event) {}
~ScopedEventSignaller() { event_->Signal(); }
AsyncSafeWaitableEvent* event_;
// Struct to store the arguments to the signal handler.
struct HandlerParams {
uintptr_t stack_base_address;
// The event is signalled when signal handler is done executing.
AsyncSafeWaitableEvent* event;
// Return values:
// Successfully copied the stack segment.
bool* success;
// The thread context of the leaf function.
mcontext_t* context;
// Buffer to copy the stack segment.
StackBuffer* stack_buffer;
const uint8_t** stack_copy_bottom;
// The timestamp when the stack was copied.
TimeTicks* timestamp;
// Pointer to the parameters to be "passed" to the CopyStackSignalHandler() from
// the sampling thread to the sampled (stopped) thread. This value is set just
// before sending the signal to the thread and reset when the handler is done.
std::atomic<HandlerParams*> g_handler_params;
// CopyStackSignalHandler is invoked on the stopped thread and records the
// thread's stack and register context at the time the signal was received. This
// function may only call reentrant code.
void CopyStackSignalHandler(int n, siginfo_t* siginfo, void* sigcontext) {
HandlerParams* params = g_handler_params.load(std::memory_order_acquire);
// TimeTicks::Now() is implemented in terms of clock_gettime on Linux, which
// is signal safe per the signal-safety(7) man page.
*params->timestamp = TimeTicks::Now();
ScopedEventSignaller e(params->event);
*params->success = false;
const ucontext_t* ucontext = static_cast<ucontext_t*>(sigcontext);
memcpy(params->context, &ucontext->uc_mcontext, sizeof(mcontext_t));
const uintptr_t bottom = RegisterContextStackPointer(params->context);
const uintptr_t top = params->stack_base_address;
if ((top - bottom) > params->stack_buffer->size()) {
// The stack exceeds the size of the allocated buffer. The buffer is sized
// such that this shouldn't happen under typical execution so we can safely
// punt in this situation.
*params->stack_copy_bottom =
reinterpret_cast<uint8_t*>(bottom), reinterpret_cast<uintptr_t*>(top),
StackBuffer::kPlatformStackAlignment, params->stack_buffer->buffer());
// TODO( Record metadata while the thread is
// suspended.
*params->success = true;
// Sets the global handler params for the signal handler function.
class ScopedSetSignalHandlerParams {
ScopedSetSignalHandlerParams(HandlerParams* params) {, std::memory_order_release);
~ScopedSetSignalHandlerParams() {, std::memory_order_release);
class ScopedSigaction {
ScopedSigaction(int signal,
struct sigaction* action,
struct sigaction* original_action)
: signal_(signal),
succeeded_(sigaction(signal, action, original_action) == 0) {}
bool succeeded() const { return succeeded_; }
~ScopedSigaction() {
if (!succeeded_)
bool reset_succeeded = sigaction(signal_, original_action_, action_) == 0;
const int signal_;
struct sigaction* const action_;
struct sigaction* const original_action_;
const bool succeeded_;
} // namespace
std::unique_ptr<ThreadDelegate> thread_delegate)
: thread_delegate_(std::move(thread_delegate)) {}
StackCopierSignal::~StackCopierSignal() = default;
bool StackCopierSignal::CopyStack(StackBuffer* stack_buffer,
uintptr_t* stack_top,
ProfileBuilder* profile_builder,
TimeTicks* timestamp,
RegisterContext* thread_context) {
AsyncSafeWaitableEvent wait_event;
bool copied = false;
const uint8_t* stack_copy_bottom = nullptr;
const uintptr_t stack_base_address = thread_delegate_->GetStackBaseAddress();
HandlerParams params = {stack_base_address, &wait_event, &copied,
thread_context, stack_buffer, &stack_copy_bottom,
ScopedSetSignalHandlerParams scoped_handler_params(&params);
// Set the signal handler for the thread to the stack copy function.
struct sigaction action;
struct sigaction original_action;
memset(&action, 0, sizeof(action));
action.sa_sigaction = CopyStackSignalHandler;
action.sa_flags = SA_RESTART | SA_SIGINFO;
"StackCopierSignal copy stack");
// SIGURG is chosen here because we observe no crashes with this signal and
// neither Chrome or the AOSP sets up a special handler for this signal.
ScopedSigaction scoped_sigaction(SIGURG, &action, &original_action);
if (!scoped_sigaction.succeeded())
return false;
if (syscall(SYS_tgkill, getpid(), thread_delegate_->GetThreadId(),
SIGURG) != 0) {
return false;
bool finished_waiting = wait_event.Wait();
"StackCopierSignal copy stack");
if (!finished_waiting) {
return false;
const uintptr_t bottom = RegisterContextStackPointer(params.context);
for (uintptr_t* reg :
thread_delegate_->GetRegistersToRewrite(thread_context)) {
*reg = StackCopierSignal::RewritePointerIfInOriginalStack(
reinterpret_cast<uintptr_t*>(stack_base_address), stack_copy_bottom,
*stack_top = reinterpret_cast<uintptr_t>(stack_copy_bottom) +
(stack_base_address - bottom);
return copied;
} // namespace base