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//===-- ---------------------------===//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// See sanitizer_stoptheworld.h for details.
// This implementation was inspired by Markus Gutschke's
#include "sanitizer_platform.h"
#if SANITIZER_LINUX && defined(__x86_64__)
#include "sanitizer_stoptheworld.h"
#include "sanitizer_platform_limits_posix.h"
#include <errno.h>
#include <sched.h> // for CLONE_* definitions
#include <stddef.h>
#include <sys/prctl.h> // for PR_* definitions
#include <sys/ptrace.h> // for PTRACE_* definitions
#include <sys/types.h> // for pid_t
#if SANITIZER_ANDROID && defined(__arm__)
# include <linux/user.h> // for pt_regs
# include <sys/user.h> // for user_regs_struct
#include <sys/wait.h> // for signal-related stuff
#ifdef sa_handler
# undef sa_handler
#ifdef sa_sigaction
# undef sa_sigaction
#include "sanitizer_common.h"
#include "sanitizer_flags.h"
#include "sanitizer_libc.h"
#include "sanitizer_linux.h"
#include "sanitizer_mutex.h"
#include "sanitizer_placement_new.h"
// This module works by spawning a Linux task which then attaches to every
// thread in the caller process with ptrace. This suspends the threads, and
// PTRACE_GETREGS can then be used to obtain their register state. The callback
// supplied to StopTheWorld() is run in the tracer task while the threads are
// suspended.
// The tracer task must be placed in a different thread group for ptrace to
// work, so it cannot be spawned as a pthread. Instead, we use the low-level
// clone() interface (we want to share the address space with the caller
// process, so we prefer clone() over fork()).
// We don't use any libc functions, relying instead on direct syscalls. There
// are two reasons for this:
// 1. calling a library function while threads are suspended could cause a
// deadlock, if one of the treads happens to be holding a libc lock;
// 2. it's generally not safe to call libc functions from the tracer task,
// because clone() does not set up a thread-local storage for it. Any
// thread-local variables used by libc will be shared between the tracer task
// and the thread which spawned it.
COMPILER_CHECK(sizeof(SuspendedThreadID) == sizeof(pid_t));
namespace __sanitizer {
// This class handles thread suspending/unsuspending in the tracer thread.
class ThreadSuspender {
explicit ThreadSuspender(pid_t pid)
: pid_(pid) {
CHECK_GE(pid, 0);
bool SuspendAllThreads();
void ResumeAllThreads();
void KillAllThreads();
SuspendedThreadsList &suspended_threads_list() {
return suspended_threads_list_;
SuspendedThreadsList suspended_threads_list_;
pid_t pid_;
bool SuspendThread(SuspendedThreadID thread_id);
bool ThreadSuspender::SuspendThread(SuspendedThreadID thread_id) {
// Are we already attached to this thread?
// Currently this check takes linear time, however the number of threads is
// usually small.
if (suspended_threads_list_.Contains(thread_id))
return false;
int pterrno;
if (internal_iserror(internal_ptrace(PTRACE_ATTACH, thread_id, NULL, NULL),
&pterrno)) {
// Either the thread is dead, or something prevented us from attaching.
// Log this event and move on.
VReport(1, "Could not attach to thread %d (errno %d).\n", thread_id,
return false;
} else {
VReport(1, "Attached to thread %d.\n", thread_id);
// The thread is not guaranteed to stop before ptrace returns, so we must
// wait on it.
uptr waitpid_status;
HANDLE_EINTR(waitpid_status, internal_waitpid(thread_id, NULL, __WALL));
int wperrno;
if (internal_iserror(waitpid_status, &wperrno)) {
// Got a ECHILD error. I don't think this situation is possible, but it
// doesn't hurt to report it.
VReport(1, "Waiting on thread %d failed, detaching (errno %d).\n",
thread_id, wperrno);
internal_ptrace(PTRACE_DETACH, thread_id, NULL, NULL);
return false;
return true;
void ThreadSuspender::ResumeAllThreads() {
for (uptr i = 0; i < suspended_threads_list_.thread_count(); i++) {
pid_t tid = suspended_threads_list_.GetThreadID(i);
int pterrno;
if (!internal_iserror(internal_ptrace(PTRACE_DETACH, tid, NULL, NULL),
&pterrno)) {
VReport(1, "Detached from thread %d.\n", tid);
} else {
// Either the thread is dead, or we are already detached.
// The latter case is possible, for instance, if this function was called
// from a signal handler.
VReport(1, "Could not detach from thread %d (errno %d).\n", tid, pterrno);
void ThreadSuspender::KillAllThreads() {
for (uptr i = 0; i < suspended_threads_list_.thread_count(); i++)
internal_ptrace(PTRACE_KILL, suspended_threads_list_.GetThreadID(i),
bool ThreadSuspender::SuspendAllThreads() {
ThreadLister thread_lister(pid_);
bool added_threads;
do {
// Run through the directory entries once.
added_threads = false;
pid_t tid = thread_lister.GetNextTID();
while (tid >= 0) {
if (SuspendThread(tid))
added_threads = true;
tid = thread_lister.GetNextTID();
if (thread_lister.error()) {
// Detach threads and fail.
return false;
} while (added_threads);
return true;
// Pointer to the ThreadSuspender instance for use in signal handler.
static ThreadSuspender *thread_suspender_instance = NULL;
// Signals that should not be blocked (this is used in the parent thread as well
// as the tracer thread).
static const int kUnblockedSignals[] = { SIGABRT, SIGILL, SIGFPE, SIGSEGV,
// Structure for passing arguments into the tracer thread.
struct TracerThreadArgument {
StopTheWorldCallback callback;
void *callback_argument;
// The tracer thread waits on this mutex while the parent finishes its
// preparations.
BlockingMutex mutex;
uptr parent_pid;
static DieCallbackType old_die_callback;
// Signal handler to wake up suspended threads when the tracer thread dies.
void TracerThreadSignalHandler(int signum, void *siginfo, void *) {
if (thread_suspender_instance != NULL) {
if (signum == SIGABRT)
internal__exit((signum == SIGABRT) ? 1 : 2);
static void TracerThreadDieCallback() {
// Generally a call to Die() in the tracer thread should be fatal to the
// parent process as well, because they share the address space.
// This really only works correctly if all the threads are suspended at this
// point. So we correctly handle calls to Die() from within the callback, but
// not those that happen before or after the callback. Hopefully there aren't
// a lot of opportunities for that to happen...
if (thread_suspender_instance)
if (old_die_callback)
// Size of alternative stack for signal handlers in the tracer thread.
static const int kHandlerStackSize = 4096;
// This function will be run as a cloned task.
static int TracerThread(void* argument) {
TracerThreadArgument *tracer_thread_argument =
(TracerThreadArgument *)argument;
internal_prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
// Check if parent is already dead.
if (internal_getppid() != tracer_thread_argument->parent_pid)
// Wait for the parent thread to finish preparations.
ThreadSuspender thread_suspender(internal_getppid());
// Global pointer for the signal handler.
thread_suspender_instance = &thread_suspender;
// Alternate stack for signal handling.
InternalScopedBuffer<char> handler_stack_memory(kHandlerStackSize);
struct sigaltstack handler_stack;
internal_memset(&handler_stack, 0, sizeof(handler_stack));
handler_stack.ss_sp =;
handler_stack.ss_size = kHandlerStackSize;
internal_sigaltstack(&handler_stack, NULL);
// Install our handler for fatal signals. Other signals should be blocked by
// the mask we inherited from the caller thread.
for (uptr signal_index = 0; signal_index < ARRAY_SIZE(kUnblockedSignals);
signal_index++) {
__sanitizer_sigaction new_sigaction;
internal_memset(&new_sigaction, 0, sizeof(new_sigaction));
new_sigaction.sigaction = TracerThreadSignalHandler;
new_sigaction.sa_flags = SA_ONSTACK | SA_SIGINFO;
&new_sigaction, NULL);
int exit_code = 0;
if (!thread_suspender.SuspendAllThreads()) {
VReport(1, "Failed suspending threads.\n");
exit_code = 3;
} else {
exit_code = 0;
thread_suspender_instance = NULL;
handler_stack.ss_flags = SS_DISABLE;
internal_sigaltstack(&handler_stack, NULL);
return exit_code;
class ScopedStackSpaceWithGuard {
explicit ScopedStackSpaceWithGuard(uptr stack_size) {
stack_size_ = stack_size;
guard_size_ = GetPageSizeCached();
// FIXME: Omitting MAP_STACK here works in current kernels but might break
// in the future.
guard_start_ = (uptr)MmapOrDie(stack_size_ + guard_size_,
CHECK_EQ(guard_start_, (uptr)Mprotect((uptr)guard_start_, guard_size_));
~ScopedStackSpaceWithGuard() {
UnmapOrDie((void *)guard_start_, stack_size_ + guard_size_);
void *Bottom() const {
return (void *)(guard_start_ + stack_size_ + guard_size_);
uptr stack_size_;
uptr guard_size_;
uptr guard_start_;
// We have a limitation on the stack frame size, so some stuff had to be moved
// into globals.
static __sanitizer_sigset_t blocked_sigset;
static __sanitizer_sigset_t old_sigset;
static __sanitizer_sigaction old_sigactions
class StopTheWorldScope {
StopTheWorldScope() {
// Block all signals that can be blocked safely, and install
// default handlers for the remaining signals.
// We cannot allow user-defined handlers to run while the ThreadSuspender
// thread is active, because they could conceivably call some libc functions
// which modify errno (which is shared between the two threads).
for (uptr signal_index = 0; signal_index < ARRAY_SIZE(kUnblockedSignals);
signal_index++) {
// Remove the signal from the set of blocked signals.
internal_sigdelset(&blocked_sigset, kUnblockedSignals[signal_index]);
// Install the default handler.
__sanitizer_sigaction new_sigaction;
internal_memset(&new_sigaction, 0, sizeof(new_sigaction));
new_sigaction.handler = SIG_DFL;
&new_sigaction, &old_sigactions[signal_index]);
int sigprocmask_status =
internal_sigprocmask(SIG_BLOCK, &blocked_sigset, &old_sigset);
CHECK_EQ(sigprocmask_status, 0); // sigprocmask should never fail
// Make this process dumpable. Processes that are not dumpable cannot be
// attached to.
process_was_dumpable_ = internal_prctl(PR_GET_DUMPABLE, 0, 0, 0, 0);
if (!process_was_dumpable_)
internal_prctl(PR_SET_DUMPABLE, 1, 0, 0, 0);
old_die_callback = GetDieCallback();
~StopTheWorldScope() {
// Restore the dumpable flag.
if (!process_was_dumpable_)
internal_prctl(PR_SET_DUMPABLE, 0, 0, 0, 0);
// Restore the signal handlers.
for (uptr signal_index = 0; signal_index < ARRAY_SIZE(kUnblockedSignals);
signal_index++) {
&old_sigactions[signal_index], NULL);
internal_sigprocmask(SIG_SETMASK, &old_sigset, &old_sigset);
int process_was_dumpable_;
// When sanitizer output is being redirected to file (i.e. by using log_path),
// the tracer should write to the parent's log instead of trying to open a new
// file. Alert the logging code to the fact that we have a tracer.
struct ScopedSetTracerPID {
explicit ScopedSetTracerPID(uptr tracer_pid) {
stoptheworld_tracer_pid = tracer_pid;
stoptheworld_tracer_ppid = internal_getpid();
~ScopedSetTracerPID() {
stoptheworld_tracer_pid = 0;
stoptheworld_tracer_ppid = 0;
void StopTheWorld(StopTheWorldCallback callback, void *argument) {
StopTheWorldScope in_stoptheworld;
// Prepare the arguments for TracerThread.
struct TracerThreadArgument tracer_thread_argument;
tracer_thread_argument.callback = callback;
tracer_thread_argument.callback_argument = argument;
tracer_thread_argument.parent_pid = internal_getpid();
const uptr kTracerStackSize = 2 * 1024 * 1024;
ScopedStackSpaceWithGuard tracer_stack(kTracerStackSize);
// Block the execution of TracerThread until after we have set ptrace
// permissions.
uptr tracer_pid = internal_clone(
TracerThread, tracer_stack.Bottom(),
&tracer_thread_argument, 0 /* parent_tidptr */, 0 /* newtls */, 0
/* child_tidptr */);
int local_errno = 0;
if (internal_iserror(tracer_pid, &local_errno)) {
VReport(1, "Failed spawning a tracer thread (errno %d).\n", local_errno);
} else {
ScopedSetTracerPID scoped_set_tracer_pid(tracer_pid);
// On some systems we have to explicitly declare that we want to be traced
// by the tracer thread.
internal_prctl(PR_SET_PTRACER, tracer_pid, 0, 0, 0);
// Allow the tracer thread to start.
// Since errno is shared between this thread and the tracer thread, we
// must avoid using errno while the tracer thread is running.
// At this point, any signal will either be blocked or kill us, so waitpid
// should never return (and set errno) while the tracer thread is alive.
uptr waitpid_status = internal_waitpid(tracer_pid, NULL, __WALL);
if (internal_iserror(waitpid_status, &local_errno))
VReport(1, "Waiting on the tracer thread failed (errno %d).\n",
// Platform-specific methods from SuspendedThreadsList.
#if SANITIZER_ANDROID && defined(__arm__)
typedef pt_regs regs_struct;
#define REG_SP ARM_sp
#elif SANITIZER_LINUX && defined(__arm__)
typedef user_regs regs_struct;
#define REG_SP uregs[13]
#elif defined(__i386__) || defined(__x86_64__)
typedef user_regs_struct regs_struct;
#if defined(__i386__)
#define REG_SP esp
#define REG_SP rsp
#elif defined(__powerpc__) || defined(__powerpc64__)
typedef pt_regs regs_struct;
#define REG_SP gpr[PT_R1]
#elif defined(__mips__)
typedef struct user regs_struct;
#define REG_SP regs[EF_REG29]
#error "Unsupported architecture"
#endif // SANITIZER_ANDROID && defined(__arm__)
int SuspendedThreadsList::GetRegistersAndSP(uptr index,
uptr *buffer,
uptr *sp) const {
pid_t tid = GetThreadID(index);
regs_struct regs;
int pterrno;
if (internal_iserror(internal_ptrace(PTRACE_GETREGS, tid, NULL, &regs),
&pterrno)) {
VReport(1, "Could not get registers from thread %d (errno %d).\n", tid,
return -1;
*sp = regs.REG_SP;
internal_memcpy(buffer, &regs, sizeof(regs));
return 0;
uptr SuspendedThreadsList::RegisterCount() {
return sizeof(regs_struct) / sizeof(uptr);
} // namespace __sanitizer
#endif // SANITIZER_LINUX && defined(__x86_64__)