blob: 48894374acd9728cf9f3337bf7127e15432af058 [file] [log] [blame]
* Copyright 2019 The WebRTC project authors. All Rights Reserved.
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
#include "sdk/android/native_api/stacktrace/stacktrace.h"
#include <dlfcn.h>
#include <errno.h>
#include <linux/futex.h>
#include <sys/ptrace.h>
#include <sys/ucontext.h>
#include <syscall.h>
#include <ucontext.h>
#include <unistd.h>
#include <unwind.h>
#include <atomic>
// ptrace.h is polluting the namespace. Clean up to avoid conflicts with rtc.
#if defined(DS)
#undef DS
#include "absl/base/attributes.h"
#include "rtc_base/logging.h"
#include "rtc_base/strings/string_builder.h"
#include "rtc_base/synchronization/mutex.h"
namespace webrtc {
namespace {
// Maximum stack trace depth we allow before aborting.
constexpr size_t kMaxStackSize = 100;
// Signal that will be used to interrupt threads. SIGURG ("Urgent condition on
// socket") is chosen because Android does not set up a specific handler for
// this signal.
constexpr int kSignal = SIGURG;
// Note: This class is only meant for use within this file, and for the
// simplified use case of a single Wait() and a single Signal(), followed by
// discarding the object (never reused).
// This is a replacement of rtc::Event that is async-safe and doesn't use
// pthread api. This is necessary since signal handlers cannot allocate memory
// or use pthread api. This class is ported from Chromium.
class AsyncSafeWaitableEvent {
AsyncSafeWaitableEvent() {
std::atomic_store_explicit(&futex_, 0, std::memory_order_release);
~AsyncSafeWaitableEvent() {}
// Returns false in the event of an error and errno is set to indicate the
// cause of the error.
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_, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0,
nullptr, nullptr, 0);
if (std::atomic_load_explicit(&futex_, std::memory_order_acquire) != 0)
return true;
if (res != 0)
return false;
void Signal() {
std::atomic_store_explicit(&futex_, 1, std::memory_order_release);
syscall(SYS_futex, &futex_, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1, nullptr,
nullptr, 0);
std::atomic<int> futex_;
// Struct to store the arguments to the signal handler.
struct SignalHandlerOutputState {
// This event is signalled when signal handler is done executing.
AsyncSafeWaitableEvent signal_handler_finish_event;
// Running counter of array index below.
size_t stack_size_counter = 0;
// Array storing the stack trace.
uintptr_t addresses[kMaxStackSize];
// Global lock to ensure only one thread gets interrupted at a time.
ABSL_CONST_INIT GlobalMutex g_signal_handler_lock(absl::kConstInit);
// Argument passed to the ThreadSignalHandler() from the sampling thread to the
// sampled (stopped) thread. This value is set just before sending signal to the
// thread and reset when handler is done.
SignalHandlerOutputState* volatile g_signal_handler_output_state;
// This function is called iteratively for each stack trace element and stores
// the element in the array from `unwind_output_state`.
_Unwind_Reason_Code UnwindBacktrace(struct _Unwind_Context* unwind_context,
void* unwind_output_state) {
SignalHandlerOutputState* const output_state =
// Abort if output state is corrupt.
if (output_state == nullptr)
// Avoid overflowing the stack trace array.
if (output_state->stack_size_counter >= kMaxStackSize)
// Store the instruction pointer in the array. Subtract 2 since we want to get
// the call instruction pointer, not the return address which is the
// instruction after.
output_state->addresses[output_state->stack_size_counter] =
_Unwind_GetIP(unwind_context) - 2;
return _URC_NO_REASON;
// This signal handler is exectued on the interrupted thread.
void SignalHandler(int signum, siginfo_t* info, void* ptr) {
// This should have been set by the thread requesting the stack trace.
SignalHandlerOutputState* signal_handler_output_state =
if (signal_handler_output_state != nullptr) {
_Unwind_Backtrace(&UnwindBacktrace, signal_handler_output_state);
// Temporarily change the signal handler to a function that records a raw stack
// trace and interrupt the given tid. This function will block until the output
// thread stack trace has been stored in `params`. The return value is an error
// string on failure and null on success.
const char* CaptureRawStacktrace(int pid,
int tid,
SignalHandlerOutputState* params) {
// This function is under a global lock since we are changing the signal
// handler and using global state for the output. The lock is to ensure only
// one thread at a time gets captured. The lock also means we need to be very
// careful with what statements we put in this function, and we should even
// avoid logging here.
struct sigaction act;
struct sigaction old_act;
memset(&act, 0, sizeof(act));
act.sa_sigaction = &SignalHandler;
act.sa_flags = SA_RESTART | SA_SIGINFO;
GlobalMutexLock ls(&g_signal_handler_lock);
g_signal_handler_output_state = params;
if (sigaction(kSignal, &act, &old_act) != 0)
return "Failed to change signal action";
// Interrupt the thread which will execute SignalHandler() on the given
// thread.
if (tgkill(pid, tid, kSignal) != 0)
return "Failed to interrupt thread";
// Wait until the thread is done recording its stack trace.
if (!params->signal_handler_finish_event.Wait())
return "Failed to wait for thread to finish stack trace";
// Restore previous signal handler.
sigaction(kSignal, &old_act, /* old_act= */ nullptr);
return nullptr;
// Translate addresses into symbolic information using dladdr().
std::vector<StackTraceElement> FormatStackTrace(
const SignalHandlerOutputState& params) {
std::vector<StackTraceElement> stack_trace;
for (size_t i = 0; i < params.stack_size_counter; ++i) {
const uintptr_t address = params.addresses[i];
Dl_info dl_info = {};
if (!dladdr(reinterpret_cast<void*>(address), &dl_info)) {
<< "Could not translate address to symbolic information for address "
<< address << " at stack depth " << i;
StackTraceElement stack_trace_element;
stack_trace_element.shared_object_path = dl_info.dli_fname;
stack_trace_element.relative_address = static_cast<uint32_t>(
address - reinterpret_cast<uintptr_t>(dl_info.dli_fbase));
stack_trace_element.symbol_name = dl_info.dli_sname;
return stack_trace;
} // namespace
std::vector<StackTraceElement> GetStackTrace(int tid) {
// Only a thread itself can unwind its stack, so we will interrupt the given
// tid with a custom signal handler in order to unwind its stack. The stack
// will be recorded to `params` through the use of the global pointer
// `g_signal_handler_param`.
SignalHandlerOutputState params;
const char* error_string = CaptureRawStacktrace(getpid(), tid, &params);
if (error_string != nullptr) {
RTC_LOG(LS_ERROR) << error_string << ". tid: " << tid
<< ". errno: " << errno;
return {};
if (params.stack_size_counter >= kMaxStackSize) {
RTC_LOG(LS_WARNING) << "Stack trace for thread " << tid << " was truncated";
return FormatStackTrace(params);
std::vector<StackTraceElement> GetStackTrace() {
SignalHandlerOutputState params;
_Unwind_Backtrace(&UnwindBacktrace, &params);
if (params.stack_size_counter >= kMaxStackSize) {
RTC_LOG(LS_WARNING) << "Stack trace was truncated";
return FormatStackTrace(params);
std::string StackTraceToString(
const std::vector<StackTraceElement>& stack_trace) {
rtc::StringBuilder string_builder;
for (size_t i = 0; i < stack_trace.size(); ++i) {
const StackTraceElement& stack_trace_element = stack_trace[i];
"#%02zu pc %08x %s", i,
// The symbol name is only available for unstripped .so files.
if (stack_trace_element.symbol_name != nullptr)
string_builder.AppendFormat(" %s", stack_trace_element.symbol_name);
return string_builder.Release();
} // namespace webrtc