1 /*
2 * Copyright 2019 The WebRTC project authors. All Rights Reserved.
3 *
4 * Use of this source code is governed by a BSD-style license
5 * that can be found in the LICENSE file in the root of the source
6 * tree. An additional intellectual property rights grant can be found
7 * in the file PATENTS. All contributing project authors may
8 * be found in the AUTHORS file in the root of the source tree.
9 */
10
11 #include "sdk/android/native_api/stacktrace/stacktrace.h"
12
13 #include <dlfcn.h>
14 #include <errno.h>
15 #include <linux/futex.h>
16 #include <sys/ptrace.h>
17 #include <sys/ucontext.h>
18 #include <syscall.h>
19 #include <ucontext.h>
20 #include <unistd.h>
21 #include <unwind.h>
22 #include <atomic>
23
24 // ptrace.h is polluting the namespace. Clean up to avoid conflicts with rtc.
25 #if defined(DS)
26 #undef DS
27 #endif
28
29 #include "absl/base/attributes.h"
30 #include "rtc_base/logging.h"
31 #include "rtc_base/strings/string_builder.h"
32 #include "rtc_base/synchronization/mutex.h"
33
34 namespace webrtc {
35
36 namespace {
37
38 // Maximum stack trace depth we allow before aborting.
39 constexpr size_t kMaxStackSize = 100;
40 // Signal that will be used to interrupt threads. SIGURG ("Urgent condition on
41 // socket") is chosen because Android does not set up a specific handler for
42 // this signal.
43 constexpr int kSignal = SIGURG;
44
45 // Note: This class is only meant for use within this file, and for the
46 // simplified use case of a single Wait() and a single Signal(), followed by
47 // discarding the object (never reused).
48 // This is a replacement of rtc::Event that is async-safe and doesn't use
49 // pthread api. This is necessary since signal handlers cannot allocate memory
50 // or use pthread api. This class is ported from Chromium.
51 class AsyncSafeWaitableEvent {
52 public:
AsyncSafeWaitableEvent()53 AsyncSafeWaitableEvent() {
54 std::atomic_store_explicit(&futex_, 0, std::memory_order_release);
55 }
56
~AsyncSafeWaitableEvent()57 ~AsyncSafeWaitableEvent() {}
58
59 // Returns false in the event of an error and errno is set to indicate the
60 // cause of the error.
Wait()61 bool Wait() {
62 // futex() can wake up spuriously if this memory address was previously used
63 // for a pthread mutex. So, also check the condition.
64 while (true) {
65 int res = syscall(SYS_futex, &futex_, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0,
66 nullptr, nullptr, 0);
67 if (std::atomic_load_explicit(&futex_, std::memory_order_acquire) != 0)
68 return true;
69 if (res != 0)
70 return false;
71 }
72 }
73
Signal()74 void Signal() {
75 std::atomic_store_explicit(&futex_, 1, std::memory_order_release);
76 syscall(SYS_futex, &futex_, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1, nullptr,
77 nullptr, 0);
78 }
79
80 private:
81 std::atomic<int> futex_;
82 };
83
84 // Struct to store the arguments to the signal handler.
85 struct SignalHandlerOutputState {
86 // This function is called iteratively for each stack trace element and stores
87 // the element in the array from `unwind_output_state`.
88 static _Unwind_Reason_Code UnwindBacktrace(
89 struct _Unwind_Context* unwind_context,
90 void* unwind_output_state);
91
92 // This event is signalled when signal handler is done executing.
93 AsyncSafeWaitableEvent signal_handler_finish_event;
94 // Running counter of array index below.
95 size_t stack_size_counter = 0;
96 // Array storing the stack trace.
97 uintptr_t addresses[kMaxStackSize];
98 };
99
100 // This function is called iteratively for each stack trace element and stores
101 // the element in the array from `unwind_output_state`.
UnwindBacktrace(struct _Unwind_Context * unwind_context,void * unwind_output_state)102 _Unwind_Reason_Code SignalHandlerOutputState::UnwindBacktrace(
103 struct _Unwind_Context* unwind_context,
104 void* unwind_output_state) {
105 SignalHandlerOutputState* const output_state =
106 static_cast<SignalHandlerOutputState*>(unwind_output_state);
107
108 // Abort if output state is corrupt.
109 if (output_state == nullptr)
110 return _URC_END_OF_STACK;
111
112 // Avoid overflowing the stack trace array.
113 if (output_state->stack_size_counter >= kMaxStackSize)
114 return _URC_END_OF_STACK;
115
116 // Store the instruction pointer in the array. Subtract 2 since we want to get
117 // the call instruction pointer, not the return address which is the
118 // instruction after.
119 output_state->addresses[output_state->stack_size_counter] =
120 _Unwind_GetIP(unwind_context) - 2;
121 ++output_state->stack_size_counter;
122
123 return _URC_NO_REASON;
124 }
125
126 class GlobalStackUnwinder {
127 public:
Get()128 static GlobalStackUnwinder& Get() {
129 static GlobalStackUnwinder* const instance = new GlobalStackUnwinder();
130 return *instance;
131 }
132 const char* CaptureRawStacktrace(int pid,
133 int tid,
134 SignalHandlerOutputState* params);
135
136 private:
GlobalStackUnwinder()137 GlobalStackUnwinder() { current_output_state_.store(nullptr); }
138
139 // Temporarily installed signal handler.
140 static void SignalHandler(int signum, siginfo_t* info, void* ptr);
141
142 Mutex mutex_;
143
144 // Accessed by signal handler.
145 static std::atomic<SignalHandlerOutputState*> current_output_state_;
146 // A signal handler mustn't use locks.
147 static_assert(std::atomic<SignalHandlerOutputState*>::is_always_lock_free);
148 };
149
150 std::atomic<SignalHandlerOutputState*>
151 GlobalStackUnwinder::current_output_state_;
152
153 // This signal handler is exectued on the interrupted thread.
SignalHandler(int signum,siginfo_t * info,void * ptr)154 void GlobalStackUnwinder::SignalHandler(int signum,
155 siginfo_t* info,
156 void* ptr) {
157 // This should have been set by the thread requesting the stack trace.
158 SignalHandlerOutputState* signal_handler_output_state =
159 current_output_state_.load();
160 if (signal_handler_output_state != nullptr) {
161 _Unwind_Backtrace(&SignalHandlerOutputState::UnwindBacktrace,
162 signal_handler_output_state);
163 signal_handler_output_state->signal_handler_finish_event.Signal();
164 }
165 }
166
167 // Temporarily change the signal handler to a function that records a raw stack
168 // trace and interrupt the given tid. This function will block until the output
169 // thread stack trace has been stored in `params`. The return value is an error
170 // string on failure and null on success.
CaptureRawStacktrace(int pid,int tid,SignalHandlerOutputState * params)171 const char* GlobalStackUnwinder::CaptureRawStacktrace(
172 int pid,
173 int tid,
174 SignalHandlerOutputState* params) {
175 // This function is under a global lock since we are changing the signal
176 // handler and using global state for the output. The lock is to ensure only
177 // one thread at a time gets captured. The lock also means we need to be very
178 // careful with what statements we put in this function, and we should even
179 // avoid logging here.
180 struct sigaction act;
181 struct sigaction old_act;
182 memset(&act, 0, sizeof(act));
183 act.sa_sigaction = &SignalHandler;
184 act.sa_flags = SA_RESTART | SA_SIGINFO;
185 sigemptyset(&act.sa_mask);
186
187 MutexLock loch(&mutex_);
188 current_output_state_.store(params);
189
190 if (sigaction(kSignal, &act, &old_act) != 0)
191 return "Failed to change signal action";
192
193 // Interrupt the thread which will execute SignalHandler() on the given
194 // thread.
195 if (tgkill(pid, tid, kSignal) != 0)
196 return "Failed to interrupt thread";
197
198 // Wait until the thread is done recording its stack trace.
199 if (!params->signal_handler_finish_event.Wait())
200 return "Failed to wait for thread to finish stack trace";
201
202 // Restore previous signal handler.
203 sigaction(kSignal, &old_act, /* old_act= */ nullptr);
204
205 return nullptr;
206 }
207
208 // Translate addresses into symbolic information using dladdr().
FormatStackTrace(const SignalHandlerOutputState & params)209 std::vector<StackTraceElement> FormatStackTrace(
210 const SignalHandlerOutputState& params) {
211 std::vector<StackTraceElement> stack_trace;
212 for (size_t i = 0; i < params.stack_size_counter; ++i) {
213 const uintptr_t address = params.addresses[i];
214
215 Dl_info dl_info = {};
216 if (!dladdr(reinterpret_cast<void*>(address), &dl_info)) {
217 RTC_LOG(LS_WARNING)
218 << "Could not translate address to symbolic information for address "
219 << address << " at stack depth " << i;
220 continue;
221 }
222
223 StackTraceElement stack_trace_element;
224 stack_trace_element.shared_object_path = dl_info.dli_fname;
225 stack_trace_element.relative_address = static_cast<uint32_t>(
226 address - reinterpret_cast<uintptr_t>(dl_info.dli_fbase));
227 stack_trace_element.symbol_name = dl_info.dli_sname;
228
229 stack_trace.push_back(stack_trace_element);
230 }
231
232 return stack_trace;
233 }
234
235 } // namespace
236
GetStackTrace(int tid)237 std::vector<StackTraceElement> GetStackTrace(int tid) {
238 // Only a thread itself can unwind its stack, so we will interrupt the given
239 // tid with a custom signal handler in order to unwind its stack. The stack
240 // will be recorded to `params` through the use of the global pointer
241 // `g_signal_handler_param`.
242 SignalHandlerOutputState params;
243
244 const char* error_string =
245 GlobalStackUnwinder::Get().CaptureRawStacktrace(getpid(), tid, ¶ms);
246 if (error_string != nullptr) {
247 RTC_LOG(LS_ERROR) << error_string << ". tid: " << tid
248 << ". errno: " << errno;
249 return {};
250 }
251 if (params.stack_size_counter >= kMaxStackSize) {
252 RTC_LOG(LS_WARNING) << "Stack trace for thread " << tid << " was truncated";
253 }
254 return FormatStackTrace(params);
255 }
256
GetStackTrace()257 std::vector<StackTraceElement> GetStackTrace() {
258 SignalHandlerOutputState params;
259 _Unwind_Backtrace(&SignalHandlerOutputState::UnwindBacktrace, ¶ms);
260 if (params.stack_size_counter >= kMaxStackSize) {
261 RTC_LOG(LS_WARNING) << "Stack trace was truncated";
262 }
263 return FormatStackTrace(params);
264 }
265
StackTraceToString(const std::vector<StackTraceElement> & stack_trace)266 std::string StackTraceToString(
267 const std::vector<StackTraceElement>& stack_trace) {
268 rtc::StringBuilder string_builder;
269
270 for (size_t i = 0; i < stack_trace.size(); ++i) {
271 const StackTraceElement& stack_trace_element = stack_trace[i];
272 string_builder.AppendFormat(
273 "#%02zu pc %08x %s", i,
274 static_cast<uint32_t>(stack_trace_element.relative_address),
275 stack_trace_element.shared_object_path);
276 // The symbol name is only available for unstripped .so files.
277 if (stack_trace_element.symbol_name != nullptr)
278 string_builder.AppendFormat(" %s", stack_trace_element.symbol_name);
279
280 string_builder.AppendFormat("\n");
281 }
282
283 return string_builder.Release();
284 }
285
286 } // namespace webrtc
287