runtime/native_stack_dump.cc - platform/art - Git at Google

 /*
  * Copyright (C) 2016 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
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include "native_stack_dump.h"

 #include <memory>
 #include <ostream>

 #include <stdio.h>

 #include "art_method.h"

 // For DumpNativeStack.
 #include <backtrace/Backtrace.h>
 #include <backtrace/BacktraceMap.h>

 #if defined(__linux__)

 #include <memory>
 #include <vector>

 #include <linux/unistd.h>
 #include <poll.h>
 #include <signal.h>
 #include <stdlib.h>
 #include <sys/time.h>
 #include <sys/types.h>

 #include "android-base/stringprintf.h"
 #include "android-base/strings.h"

 #include "arch/instruction_set.h"
 #include "base/aborting.h"
 #include "base/file_utils.h"
 #include "base/memory_tool.h"
 #include "base/mutex.h"
 #include "base/os.h"
 #include "base/unix_file/fd_file.h"
 #include "base/utils.h"
 #include "oat_quick_method_header.h"
 #include "thread-current-inl.h"

 #endif

 namespace art {

 #if defined(__linux__)

 using android::base::StringPrintf;

 static constexpr bool kUseAddr2line = !kIsTargetBuild;

 ALWAYS_INLINE
 static inline void WritePrefix(std::ostream& os, const char* prefix, bool odd) {
   if (prefix != nullptr) {
     os << prefix;
   }
   os << "  ";
   if (!odd) {
     os << " ";
   }
 }

 // The state of an open pipe to addr2line. In "server" mode, addr2line takes input on stdin
 // and prints the result to stdout. This struct keeps the state of the open connection.
 struct Addr2linePipe {
   Addr2linePipe(int in_fd, int out_fd, const std::string& file_name, pid_t pid)
       : in(in_fd, false), out(out_fd, false), file(file_name), child_pid(pid), odd(true) {}

   ~Addr2linePipe() {
     kill(child_pid, SIGKILL);
   }

   File in;      // The file descriptor that is connected to the output of addr2line.
   File out;     // The file descriptor that is connected to the input of addr2line.

   const std::string file;     // The file addr2line is working on, so that we know when to close
                               // and restart.
   const pid_t child_pid;      // The pid of the child, which we should kill when we're done.
   bool odd;                   // Print state for indentation of lines.
 };

 static std::unique_ptr<Addr2linePipe> Connect(const std::string& name, const char* args[]) {
   int caller_to_addr2line[2];
   int addr2line_to_caller[2];

   if (pipe(caller_to_addr2line) == -1) {
     return nullptr;
   }
   if (pipe(addr2line_to_caller) == -1) {
     close(caller_to_addr2line[0]);
     close(caller_to_addr2line[1]);
     return nullptr;
   }

   pid_t pid = fork();
   if (pid == -1) {
     close(caller_to_addr2line[0]);
     close(caller_to_addr2line[1]);
     close(addr2line_to_caller[0]);
     close(addr2line_to_caller[1]);
     return nullptr;
   }

   if (pid == 0) {
     dup2(caller_to_addr2line[0], STDIN_FILENO);
     dup2(addr2line_to_caller[1], STDOUT_FILENO);

     close(caller_to_addr2line[0]);
     close(caller_to_addr2line[1]);
     close(addr2line_to_caller[0]);
     close(addr2line_to_caller[1]);

     execv(args[0], const_cast<char* const*>(args));
     exit(1);
   } else {
     close(caller_to_addr2line[0]);
     close(addr2line_to_caller[1]);
     return std::make_unique<Addr2linePipe>(addr2line_to_caller[0],
                                            caller_to_addr2line[1],
                                            name,
                                            pid);
   }
 }

 static void Drain(size_t expected,
                   const char* prefix,
                   std::unique_ptr<Addr2linePipe>* pipe /* inout */,
                   std::ostream& os) {
   DCHECK(pipe != nullptr);
   DCHECK(pipe->get() != nullptr);
   int in = pipe->get()->in.Fd();
   DCHECK_GE(in, 0);

   bool prefix_written = false;

   for (;;) {
     constexpr uint32_t kWaitTimeExpectedMilli = 500;
     constexpr uint32_t kWaitTimeUnexpectedMilli = 50;

     int timeout = expected > 0 ? kWaitTimeExpectedMilli : kWaitTimeUnexpectedMilli;
     struct pollfd read_fd{in, POLLIN, 0};
     int retval = TEMP_FAILURE_RETRY(poll(&read_fd, 1, timeout));
     if (retval == -1) {
       // An error occurred.
       pipe->reset();
       return;
     }

     if (retval == 0) {
       // Timeout.
       return;
     }

     if (!(read_fd.revents & POLLIN)) {
       // addr2line call exited.
       pipe->reset();
       return;
     }

     constexpr size_t kMaxBuffer = 128;  // Relatively small buffer. Should be OK as we're on an
     // alt stack, but just to be sure...
     char buffer[kMaxBuffer];
     memset(buffer, 0, kMaxBuffer);
     int bytes_read = TEMP_FAILURE_RETRY(read(in, buffer, kMaxBuffer - 1));
     if (bytes_read <= 0) {
       // This should not really happen...
       pipe->reset();
       return;
     }
     buffer[bytes_read] = '\0';

     char* tmp = buffer;
     while (*tmp != 0) {
       if (!prefix_written) {
         WritePrefix(os, prefix, (*pipe)->odd);
         prefix_written = true;
       }
       char* new_line = strchr(tmp, '\n');
       if (new_line == nullptr) {
         os << tmp;

         break;
       } else {
         char saved = *(new_line + 1);
         *(new_line + 1) = 0;
         os << tmp;
         *(new_line + 1) = saved;

         tmp = new_line + 1;
         prefix_written = false;
         (*pipe)->odd = !(*pipe)->odd;

         if (expected > 0) {
           expected--;
         }
       }
     }
   }
 }

 static void Addr2line(const std::string& map_src,
                       uintptr_t offset,
                       std::ostream& os,
                       const char* prefix,
                       std::unique_ptr<Addr2linePipe>* pipe /* inout */) {
   DCHECK(pipe != nullptr);

   if (map_src == "[vdso]" || android::base::EndsWith(map_src, ".vdex")) {
     // addr2line will not work on the vdso.
     // vdex files are special frames injected for the interpreter
     // so they don't have any line number information available.
     return;
   }

   if (*pipe == nullptr || (*pipe)->file != map_src) {
     if (*pipe != nullptr) {
       Drain(0, prefix, pipe, os);
     }
     pipe->reset();  // Close early.

     const char* args[7] = {
         "/usr/bin/addr2line",
         "--functions",
         "--inlines",
         "--demangle",
         "-e",
         map_src.c_str(),
         nullptr
     };
     *pipe = Connect(map_src, args);
   }

   Addr2linePipe* pipe_ptr = pipe->get();
   if (pipe_ptr == nullptr) {
     // Failed...
     return;
   }

   // Send the offset.
   const std::string hex_offset = StringPrintf("%zx\n", offset);

   if (!pipe_ptr->out.WriteFully(hex_offset.data(), hex_offset.length())) {
     // Error. :-(
     pipe->reset();
     return;
   }

   // Now drain (expecting two lines).
   Drain(2U, prefix, pipe, os);
 }

 static bool RunCommand(const std::string& cmd) {
   FILE* stream = popen(cmd.c_str(), "r");
   if (stream) {
     pclose(stream);
     return true;
   } else {
     return false;
   }
 }

 static bool PcIsWithinQuickCode(ArtMethod* method, uintptr_t pc) NO_THREAD_SAFETY_ANALYSIS {
   uintptr_t code = reinterpret_cast<uintptr_t>(EntryPointToCodePointer(
       method->GetEntryPointFromQuickCompiledCode()));
   if (code == 0) {
     return pc == 0;
   }
   uintptr_t code_size = reinterpret_cast<const OatQuickMethodHeader*>(code)[-1].GetCodeSize();
   return code <= pc && pc <= (code + code_size);
 }

 void DumpNativeStack(std::ostream& os,
                      pid_t tid,
                      BacktraceMap* existing_map,
                      const char* prefix,
                      ArtMethod* current_method,
                      void* ucontext_ptr,
                      bool skip_frames) {
   // Historical note: This was disabled when running under Valgrind (b/18119146).

   BacktraceMap* map = existing_map;
   std::unique_ptr<BacktraceMap> tmp_map;
   if (map == nullptr) {
     tmp_map.reset(BacktraceMap::Create(getpid()));
     map = tmp_map.get();
   }
   std::unique_ptr<Backtrace> backtrace(Backtrace::Create(BACKTRACE_CURRENT_PROCESS, tid, map));
   backtrace->SetSkipFrames(skip_frames);
   if (!backtrace->Unwind(0, reinterpret_cast<ucontext*>(ucontext_ptr))) {
     os << prefix << "(backtrace::Unwind failed for thread " << tid
        << ": " <<  backtrace->GetErrorString(backtrace->GetError()) << ")" << std::endl;
     return;
   } else if (backtrace->NumFrames() == 0) {
     os << prefix << "(no native stack frames for thread " << tid << ")" << std::endl;
     return;
   }

   // Check whether we have and should use addr2line.
   bool use_addr2line;
   if (kUseAddr2line) {
     // Try to run it to see whether we have it. Push an argument so that it doesn't assume a.out
     // and print to stderr.
     use_addr2line = (gAborting > 0) && RunCommand("addr2line -h");
   } else {
     use_addr2line = false;
   }

   std::unique_ptr<Addr2linePipe> addr2line_state;

   for (Backtrace::const_iterator it = backtrace->begin();
        it != backtrace->end(); ++it) {
     // We produce output like this:
     // ]    #00 pc 000075bb8  /system/lib/libc.so (unwind_backtrace_thread+536)
     // In order for parsing tools to continue to function, the stack dump
     // format must at least adhere to this format:
     //  #XX pc <RELATIVE_ADDR>  <FULL_PATH_TO_SHARED_LIBRARY> ...
     // The parsers require a single space before and after pc, and two spaces
     // after the <RELATIVE_ADDR>. There can be any prefix data before the
     // #XX. <RELATIVE_ADDR> has to be a hex number but with no 0x prefix.
     os << prefix << StringPrintf("#%02zu pc ", it->num);
     bool try_addr2line = false;
     if (!BacktraceMap::IsValid(it->map)) {
       os << StringPrintf(Is64BitInstructionSet(kRuntimeISA) ? "%016" PRIx64 "  ???"
                                                             : "%08" PRIx64 "  ???",
                          it->pc);
     } else {
       os << StringPrintf(Is64BitInstructionSet(kRuntimeISA) ? "%016" PRIx64 "  "
                                                             : "%08" PRIx64 "  ",
                          it->rel_pc);
       if (it->map.name.empty()) {
         os << StringPrintf("<anonymous:%" PRIx64 ">", it->map.start);
       } else {
         os << it->map.name;
       }
       if (it->map.offset != 0) {
         os << StringPrintf(" (offset %" PRIx64 ")", it->map.offset);
       }
       os << " (";
       if (!it->func_name.empty()) {
         os << it->func_name;
         if (it->func_offset != 0) {
           os << "+" << it->func_offset;
         }
         // Functions found using the gdb jit interface will be in an empty
         // map that cannot be found using addr2line.
         if (!it->map.name.empty()) {
           try_addr2line = true;
         }
       } else if (current_method != nullptr &&
           Locks::mutator_lock_->IsSharedHeld(Thread::Current()) &&
           PcIsWithinQuickCode(current_method, it->pc)) {
         const void* start_of_code = current_method->GetEntryPointFromQuickCompiledCode();
         os << current_method->JniLongName() << "+"
            << (it->pc - reinterpret_cast<uint64_t>(start_of_code));
       } else {
         os << "???";
       }
       os << ")";
     }
     os << std::endl;
     if (try_addr2line && use_addr2line) {
       Addr2line(it->map.name, it->rel_pc, os, prefix, &addr2line_state);
     }
   }

   if (addr2line_state != nullptr) {
     Drain(0, prefix, &addr2line_state, os);
   }
 }

 void DumpKernelStack(std::ostream& os, pid_t tid, const char* prefix, bool include_count) {
   if (tid == GetTid()) {
     // There's no point showing that we're reading our stack out of /proc!
     return;
   }

   std::string kernel_stack_filename(StringPrintf("/proc/self/task/%d/stack", tid));
   std::string kernel_stack;
   if (!ReadFileToString(kernel_stack_filename, &kernel_stack)) {
     os << prefix << "(couldn't read " << kernel_stack_filename << ")\n";
     return;
   }

   std::vector<std::string> kernel_stack_frames;
   Split(kernel_stack, '\n', &kernel_stack_frames);
   if (kernel_stack_frames.empty()) {
     os << prefix << "(" << kernel_stack_filename << " is empty)\n";
     return;
   }
   // We skip the last stack frame because it's always equivalent to "[<ffffffff>] 0xffffffff",
   // which looking at the source appears to be the kernel's way of saying "that's all, folks!".
   kernel_stack_frames.pop_back();
   for (size_t i = 0; i < kernel_stack_frames.size(); ++i) {
     // Turn "[<ffffffff8109156d>] futex_wait_queue_me+0xcd/0x110"
     // into "futex_wait_queue_me+0xcd/0x110".
     const char* text = kernel_stack_frames[i].c_str();
     const char* close_bracket = strchr(text, ']');
     if (close_bracket != nullptr) {
       text = close_bracket + 2;
     }
     os << prefix;
     if (include_count) {
       os << StringPrintf("#%02zd ", i);
     }
     os << text << std::endl;
   }
 }

 #elif defined(__APPLE__)

 void DumpNativeStack(std::ostream& os ATTRIBUTE_UNUSED,
                      pid_t tid ATTRIBUTE_UNUSED,
                      BacktraceMap* existing_map ATTRIBUTE_UNUSED,
                      const char* prefix ATTRIBUTE_UNUSED,
                      ArtMethod* current_method ATTRIBUTE_UNUSED,
                      void* ucontext_ptr ATTRIBUTE_UNUSED,
                      bool skip_frames ATTRIBUTE_UNUSED) {
 }

 void DumpKernelStack(std::ostream& os ATTRIBUTE_UNUSED,
                      pid_t tid ATTRIBUTE_UNUSED,
                      const char* prefix ATTRIBUTE_UNUSED,
                      bool include_count ATTRIBUTE_UNUSED) {
 }

 #else
 #error "Unsupported architecture for native stack dumps."
 #endif

 }  // namespace art
	/*
	* Copyright (C) 2016 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
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include "native_stack_dump.h"

	#include <memory>
	#include <ostream>

	#include <stdio.h>

	#include "art_method.h"

	// For DumpNativeStack.
	#include <backtrace/Backtrace.h>
	#include <backtrace/BacktraceMap.h>

	#if defined(__linux__)

	#include <memory>
	#include <vector>

	#include <linux/unistd.h>
	#include <poll.h>
	#include <signal.h>
	#include <stdlib.h>
	#include <sys/time.h>
	#include <sys/types.h>

	#include "android-base/stringprintf.h"
	#include "android-base/strings.h"

	#include "arch/instruction_set.h"
	#include "base/aborting.h"
	#include "base/file_utils.h"
	#include "base/memory_tool.h"
	#include "base/mutex.h"
	#include "base/os.h"
	#include "base/unix_file/fd_file.h"
	#include "base/utils.h"
	#include "oat_quick_method_header.h"
	#include "thread-current-inl.h"

	#endif

	namespace art {

	#if defined(__linux__)

	using android::base::StringPrintf;

	static constexpr bool kUseAddr2line = !kIsTargetBuild;

	ALWAYS_INLINE
	static inline void WritePrefix(std::ostream& os, const char* prefix, bool odd) {
	if (prefix != nullptr) {
	os << prefix;
	}
	os << " ";
	if (!odd) {
	os << " ";
	}
	}

	// The state of an open pipe to addr2line. In "server" mode, addr2line takes input on stdin
	// and prints the result to stdout. This struct keeps the state of the open connection.
	struct Addr2linePipe {
	Addr2linePipe(int in_fd, int out_fd, const std::string& file_name, pid_t pid)
	: in(in_fd, false), out(out_fd, false), file(file_name), child_pid(pid), odd(true) {}

	~Addr2linePipe() {
	kill(child_pid, SIGKILL);
	}

	File in; // The file descriptor that is connected to the output of addr2line.
	File out; // The file descriptor that is connected to the input of addr2line.

	const std::string file; // The file addr2line is working on, so that we know when to close
	// and restart.
	const pid_t child_pid; // The pid of the child, which we should kill when we're done.
	bool odd; // Print state for indentation of lines.
	};

	static std::unique_ptr<Addr2linePipe> Connect(const std::string& name, const char* args[]) {
	int caller_to_addr2line[2];
	int addr2line_to_caller[2];

	if (pipe(caller_to_addr2line) == -1) {
	return nullptr;
	}
	if (pipe(addr2line_to_caller) == -1) {
	close(caller_to_addr2line[0]);
	close(caller_to_addr2line[1]);
	return nullptr;
	}

	pid_t pid = fork();
	if (pid == -1) {
	close(caller_to_addr2line[0]);
	close(caller_to_addr2line[1]);
	close(addr2line_to_caller[0]);
	close(addr2line_to_caller[1]);
	return nullptr;
	}

	if (pid == 0) {
	dup2(caller_to_addr2line[0], STDIN_FILENO);
	dup2(addr2line_to_caller[1], STDOUT_FILENO);

	close(caller_to_addr2line[0]);
	close(caller_to_addr2line[1]);
	close(addr2line_to_caller[0]);
	close(addr2line_to_caller[1]);

	execv(args[0], const_cast<char* const*>(args));
	exit(1);
	} else {
	close(caller_to_addr2line[0]);
	close(addr2line_to_caller[1]);
	return std::make_unique<Addr2linePipe>(addr2line_to_caller[0],
	caller_to_addr2line[1],
	name,
	pid);
	}
	}

	static void Drain(size_t expected,
	const char* prefix,
	std::unique_ptr<Addr2linePipe>* pipe /* inout */,
	std::ostream& os) {
	DCHECK(pipe != nullptr);
	DCHECK(pipe->get() != nullptr);
	int in = pipe->get()->in.Fd();
	DCHECK_GE(in, 0);

	bool prefix_written = false;

	for (;;) {
	constexpr uint32_t kWaitTimeExpectedMilli = 500;
	constexpr uint32_t kWaitTimeUnexpectedMilli = 50;

	int timeout = expected > 0 ? kWaitTimeExpectedMilli : kWaitTimeUnexpectedMilli;
	struct pollfd read_fd{in, POLLIN, 0};
	int retval = TEMP_FAILURE_RETRY(poll(&read_fd, 1, timeout));
	if (retval == -1) {
	// An error occurred.
	pipe->reset();
	return;
	}

	if (retval == 0) {
	// Timeout.
	return;
	}

	if (!(read_fd.revents & POLLIN)) {
	// addr2line call exited.
	pipe->reset();
	return;
	}

	constexpr size_t kMaxBuffer = 128; // Relatively small buffer. Should be OK as we're on an
	// alt stack, but just to be sure...
	char buffer[kMaxBuffer];
	memset(buffer, 0, kMaxBuffer);
	int bytes_read = TEMP_FAILURE_RETRY(read(in, buffer, kMaxBuffer - 1));
	if (bytes_read <= 0) {
	// This should not really happen...
	pipe->reset();
	return;
	}
	buffer[bytes_read] = '\0';

	char* tmp = buffer;
	while (*tmp != 0) {
	if (!prefix_written) {
	WritePrefix(os, prefix, (*pipe)->odd);
	prefix_written = true;
	}
	char* new_line = strchr(tmp, '\n');
	if (new_line == nullptr) {
	os << tmp;

	break;
	} else {
	char saved = *(new_line + 1);
	*(new_line + 1) = 0;
	os << tmp;
	*(new_line + 1) = saved;

	tmp = new_line + 1;
	prefix_written = false;
	(pipe)->odd = !(pipe)->odd;

	if (expected > 0) {
	expected--;
	}
	}
	}
	}
	}

	static void Addr2line(const std::string& map_src,
	uintptr_t offset,
	std::ostream& os,
	const char* prefix,
	std::unique_ptr<Addr2linePipe>* pipe /* inout */) {
	DCHECK(pipe != nullptr);

	if (map_src == "[vdso]" \|\| android::base::EndsWith(map_src, ".vdex")) {
	// addr2line will not work on the vdso.
	// vdex files are special frames injected for the interpreter
	// so they don't have any line number information available.
	return;
	}

	if (pipe == nullptr \|\| (pipe)->file != map_src) {
	if (*pipe != nullptr) {
	Drain(0, prefix, pipe, os);
	}
	pipe->reset(); // Close early.

	const char* args[7] = {
	"/usr/bin/addr2line",
	"--functions",
	"--inlines",
	"--demangle",
	"-e",
	map_src.c_str(),
	nullptr
	};
	*pipe = Connect(map_src, args);
	}

	Addr2linePipe* pipe_ptr = pipe->get();
	if (pipe_ptr == nullptr) {
	// Failed...
	return;
	}

	// Send the offset.
	const std::string hex_offset = StringPrintf("%zx\n", offset);

	if (!pipe_ptr->out.WriteFully(hex_offset.data(), hex_offset.length())) {
	// Error. :-(
	pipe->reset();
	return;
	}

	// Now drain (expecting two lines).
	Drain(2U, prefix, pipe, os);
	}

	static bool RunCommand(const std::string& cmd) {
	FILE* stream = popen(cmd.c_str(), "r");
	if (stream) {
	pclose(stream);
	return true;
	} else {
	return false;
	}
	}

	static bool PcIsWithinQuickCode(ArtMethod* method, uintptr_t pc) NO_THREAD_SAFETY_ANALYSIS {
	uintptr_t code = reinterpret_cast<uintptr_t>(EntryPointToCodePointer(
	method->GetEntryPointFromQuickCompiledCode()));
	if (code == 0) {
	return pc == 0;
	}
	uintptr_t code_size = reinterpret_cast<const OatQuickMethodHeader*>(code)[-1].GetCodeSize();
	return code <= pc && pc <= (code + code_size);
	}

	void DumpNativeStack(std::ostream& os,
	pid_t tid,
	BacktraceMap* existing_map,
	const char* prefix,
	ArtMethod* current_method,
	void* ucontext_ptr,
	bool skip_frames) {
	// Historical note: This was disabled when running under Valgrind (b/18119146).

	BacktraceMap* map = existing_map;
	std::unique_ptr<BacktraceMap> tmp_map;
	if (map == nullptr) {
	tmp_map.reset(BacktraceMap::Create(getpid()));
	map = tmp_map.get();
	}
	std::unique_ptr<Backtrace> backtrace(Backtrace::Create(BACKTRACE_CURRENT_PROCESS, tid, map));
	backtrace->SetSkipFrames(skip_frames);
	if (!backtrace->Unwind(0, reinterpret_cast<ucontext*>(ucontext_ptr))) {
	os << prefix << "(backtrace::Unwind failed for thread " << tid
	<< ": " << backtrace->GetErrorString(backtrace->GetError()) << ")" << std::endl;
	return;
	} else if (backtrace->NumFrames() == 0) {
	os << prefix << "(no native stack frames for thread " << tid << ")" << std::endl;
	return;
	}

	// Check whether we have and should use addr2line.
	bool use_addr2line;
	if (kUseAddr2line) {
	// Try to run it to see whether we have it. Push an argument so that it doesn't assume a.out
	// and print to stderr.
	use_addr2line = (gAborting > 0) && RunCommand("addr2line -h");
	} else {
	use_addr2line = false;
	}

	std::unique_ptr<Addr2linePipe> addr2line_state;

	for (Backtrace::const_iterator it = backtrace->begin();
	it != backtrace->end(); ++it) {
	// We produce output like this:
	// ] #00 pc 000075bb8 /system/lib/libc.so (unwind_backtrace_thread+536)
	// In order for parsing tools to continue to function, the stack dump
	// format must at least adhere to this format:
	// #XX pc <RELATIVE_ADDR> <FULL_PATH_TO_SHARED_LIBRARY> ...
	// The parsers require a single space before and after pc, and two spaces
	// after the <RELATIVE_ADDR>. There can be any prefix data before the
	// #XX. <RELATIVE_ADDR> has to be a hex number but with no 0x prefix.
	os << prefix << StringPrintf("#%02zu pc ", it->num);
	bool try_addr2line = false;
	if (!BacktraceMap::IsValid(it->map)) {
	os << StringPrintf(Is64BitInstructionSet(kRuntimeISA) ? "%016" PRIx64 " ???"
	: "%08" PRIx64 " ???",
	it->pc);
	} else {
	os << StringPrintf(Is64BitInstructionSet(kRuntimeISA) ? "%016" PRIx64 " "
	: "%08" PRIx64 " ",
	it->rel_pc);
	if (it->map.name.empty()) {
	os << StringPrintf("<anonymous:%" PRIx64 ">", it->map.start);
	} else {
	os << it->map.name;
	}
	if (it->map.offset != 0) {
	os << StringPrintf(" (offset %" PRIx64 ")", it->map.offset);
	}
	os << " (";
	if (!it->func_name.empty()) {
	os << it->func_name;
	if (it->func_offset != 0) {
	os << "+" << it->func_offset;
	}
	// Functions found using the gdb jit interface will be in an empty
	// map that cannot be found using addr2line.
	if (!it->map.name.empty()) {
	try_addr2line = true;
	}
	} else if (current_method != nullptr &&
	Locks::mutator_lock_->IsSharedHeld(Thread::Current()) &&
	PcIsWithinQuickCode(current_method, it->pc)) {
	const void* start_of_code = current_method->GetEntryPointFromQuickCompiledCode();
	os << current_method->JniLongName() << "+"
	<< (it->pc - reinterpret_cast<uint64_t>(start_of_code));
	} else {
	os << "???";
	}
	os << ")";
	}
	os << std::endl;
	if (try_addr2line && use_addr2line) {
	Addr2line(it->map.name, it->rel_pc, os, prefix, &addr2line_state);
	}
	}

	if (addr2line_state != nullptr) {
	Drain(0, prefix, &addr2line_state, os);
	}
	}

	void DumpKernelStack(std::ostream& os, pid_t tid, const char* prefix, bool include_count) {
	if (tid == GetTid()) {
	// There's no point showing that we're reading our stack out of /proc!
	return;
	}

	std::string kernel_stack_filename(StringPrintf("/proc/self/task/%d/stack", tid));
	std::string kernel_stack;
	if (!ReadFileToString(kernel_stack_filename, &kernel_stack)) {
	os << prefix << "(couldn't read " << kernel_stack_filename << ")\n";
	return;
	}

	std::vector<std::string> kernel_stack_frames;
	Split(kernel_stack, '\n', &kernel_stack_frames);
	if (kernel_stack_frames.empty()) {
	os << prefix << "(" << kernel_stack_filename << " is empty)\n";
	return;
	}
	// We skip the last stack frame because it's always equivalent to "[<ffffffff>] 0xffffffff",
	// which looking at the source appears to be the kernel's way of saying "that's all, folks!".
	kernel_stack_frames.pop_back();
	for (size_t i = 0; i < kernel_stack_frames.size(); ++i) {
	// Turn "[<ffffffff8109156d>] futex_wait_queue_me+0xcd/0x110"
	// into "futex_wait_queue_me+0xcd/0x110".
	const char* text = kernel_stack_frames[i].c_str();
	const char* close_bracket = strchr(text, ']');
	if (close_bracket != nullptr) {
	text = close_bracket + 2;
	}
	os << prefix;
	if (include_count) {
	os << StringPrintf("#%02zd ", i);
	}
	os << text << std::endl;
	}
	}

	#elif defined(__APPLE__)

	void DumpNativeStack(std::ostream& os ATTRIBUTE_UNUSED,
	pid_t tid ATTRIBUTE_UNUSED,
	BacktraceMap* existing_map ATTRIBUTE_UNUSED,
	const char* prefix ATTRIBUTE_UNUSED,
	ArtMethod* current_method ATTRIBUTE_UNUSED,
	void* ucontext_ptr ATTRIBUTE_UNUSED,
	bool skip_frames ATTRIBUTE_UNUSED) {
	}

	void DumpKernelStack(std::ostream& os ATTRIBUTE_UNUSED,
	pid_t tid ATTRIBUTE_UNUSED,
	const char* prefix ATTRIBUTE_UNUSED,
	bool include_count ATTRIBUTE_UNUSED) {
	}

	#else
	#error "Unsupported architecture for native stack dumps."
	#endif

	} // namespace art