1 files changed, 76 insertions, 70 deletions

diff --git a/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc b/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
index d20b524..ad20e39 100644
--- a/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
+++ b/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cc
@@ -14,7 +14,7 @@
 
 
 #include "sanitizer_platform.h"
-#if SANITIZER_LINUX && defined(__x86_64__)
+#if SANITIZER_LINUX && (defined(__x86_64__) || defined(__mips__))
 
 #include "sanitizer_stoptheworld.h"
 
@@ -89,36 +89,50 @@ class ThreadSuspender {
   bool SuspendThread(SuspendedThreadID thread_id);
 };
 
-bool ThreadSuspender::SuspendThread(SuspendedThreadID thread_id) {
+bool ThreadSuspender::SuspendThread(SuspendedThreadID tid) {
   // 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))
+  if (suspended_threads_list_.Contains(tid))
     return false;
   int pterrno;
-  if (internal_iserror(internal_ptrace(PTRACE_ATTACH, thread_id, NULL, NULL),
+  if (internal_iserror(internal_ptrace(PTRACE_ATTACH, tid, 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,
-            pterrno);
+    VReport(1, "Could not attach to thread %d (errno %d).\n", tid, pterrno);
     return false;
   } else {
-    VReport(1, "Attached to thread %d.\n", thread_id);
+    VReport(1, "Attached to thread %d.\n", tid);
     // 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;
+    // wait on it. Note: if the thread receives a signal concurrently,
+    // we can get notification about the signal before notification about stop.
+    // In such case we need to forward the signal to the thread, otherwise
+    // the signal will be missed (as we do PTRACE_DETACH with arg=0) and
+    // any logic relying on signals will break. After forwarding we need to
+    // continue to wait for stopping, because the thread is not stopped yet.
+    // We do ignore delivery of SIGSTOP, because we want to make stop-the-world
+    // as invisible as possible.
+    for (;;) {
+      int status;
+      uptr waitpid_status;
+      HANDLE_EINTR(waitpid_status, internal_waitpid(tid, &status, __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",
+                tid, wperrno);
+        internal_ptrace(PTRACE_DETACH, tid, NULL, NULL);
+        return false;
+      }
+      if (WIFSTOPPED(status) && WSTOPSIG(status) != SIGSTOP) {
+        internal_ptrace(PTRACE_CONT, tid, 0, (void*)(uptr)WSTOPSIG(status));
+        continue;
+      }
+      break;
     }
-    suspended_threads_list_.Append(thread_id);
+    suspended_threads_list_.Append(tid);
     return true;
   }
 }
@@ -170,10 +184,9 @@ bool ThreadSuspender::SuspendAllThreads() {
 // 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,
-                                         SIGBUS, SIGXCPU, SIGXFSZ };
+// Synchronous signals that should not be blocked.
+static const int kSyncSignals[] = { SIGABRT, SIGILL, SIGFPE, SIGSEGV, SIGBUS,
+                                    SIGXCPU, SIGXFSZ };
 
 // Structure for passing arguments into the tracer thread.
 struct TracerThreadArgument {
@@ -188,7 +201,7 @@ struct TracerThreadArgument {
 static DieCallbackType old_die_callback;
 
 // Signal handler to wake up suspended threads when the tracer thread dies.
-void TracerThreadSignalHandler(int signum, void *siginfo, void *) {
+static void TracerThreadSignalHandler(int signum, void *siginfo, void *) {
   if (thread_suspender_instance != NULL) {
     if (signum == SIGABRT)
       thread_suspender_instance->KillAllThreads();
@@ -228,6 +241,7 @@ static int TracerThread(void* argument) {
   tracer_thread_argument->mutex.Lock();
   tracer_thread_argument->mutex.Unlock();
 
+  old_die_callback = GetDieCallback();
   SetDieCallback(TracerThreadDieCallback);
 
   ThreadSuspender thread_suspender(internal_getppid());
@@ -242,17 +256,14 @@ static int TracerThread(void* argument) {
   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;
-    internal_sigfillset(&new_sigaction.sa_mask);
-    internal_sigaction_norestorer(kUnblockedSignals[signal_index],
-                                  &new_sigaction, NULL);
+  // Install our handler for synchronous signals. Other signals should be
+  // blocked by the mask we inherited from the parent thread.
+  for (uptr i = 0; i < ARRAY_SIZE(kSyncSignals); i++) {
+    __sanitizer_sigaction act;
+    internal_memset(&act, 0, sizeof(act));
+    act.sigaction = TracerThreadSignalHandler;
+    act.sa_flags = SA_ONSTACK | SA_SIGINFO;
+    internal_sigaction_norestorer(kSyncSignals[i], &act, 0);
   }
 
   int exit_code = 0;
@@ -265,9 +276,11 @@ static int TracerThread(void* argument) {
     thread_suspender.ResumeAllThreads();
     exit_code = 0;
   }
+  // Note, this is a bad race. If TracerThreadDieCallback is already started
+  // in another thread and observed that thread_suspender_instance != 0,
+  // it can call KillAllThreads on the destroyed variable.
+  SetDieCallback(old_die_callback);
   thread_suspender_instance = NULL;
-  handler_stack.ss_flags = SS_DISABLE;
-  internal_sigaltstack(&handler_stack, NULL);
   return exit_code;
 }
 
@@ -299,53 +312,21 @@ class ScopedStackSpaceWithGuard {
 // into globals.
 static __sanitizer_sigset_t blocked_sigset;
 static __sanitizer_sigset_t old_sigset;
-static __sanitizer_sigaction old_sigactions
-    [ARRAY_SIZE(kUnblockedSignals)];
 
 class StopTheWorldScope {
  public:
   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).
-    internal_sigfillset(&blocked_sigset);
-    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;
-      internal_sigfillset(&new_sigaction.sa_mask);
-      internal_sigaction_norestorer(kUnblockedSignals[signal_index],
-          &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() {
-    SetDieCallback(old_die_callback);
     // 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++) {
-      internal_sigaction_norestorer(kUnblockedSignals[signal_index],
-                                    &old_sigactions[signal_index], NULL);
-    }
-    internal_sigprocmask(SIG_SETMASK, &old_sigset, &old_sigset);
   }
 
  private:
@@ -378,11 +359,36 @@ void StopTheWorld(StopTheWorldCallback callback, void *argument) {
   // Block the execution of TracerThread until after we have set ptrace
   // permissions.
   tracer_thread_argument.mutex.Lock();
+  // Signal handling story.
+  // We don't want async signals to be delivered to the tracer thread,
+  // so we block all async signals before creating the thread. An async signal
+  // handler can temporary modify errno, which is shared with this thread.
+  // We ought to use pthread_sigmask here, because sigprocmask has undefined
+  // behavior in multithreaded programs. However, on linux sigprocmask is
+  // equivalent to pthread_sigmask with the exception that pthread_sigmask
+  // does not allow to block some signals used internally in pthread
+  // implementation. We are fine with blocking them here, we are really not
+  // going to pthread_cancel the thread.
+  // The tracer thread should not raise any synchronous signals. But in case it
+  // does, we setup a special handler for sync signals that properly kills the
+  // parent as well. Note: we don't pass CLONE_SIGHAND to clone, so handlers
+  // in the tracer thread won't interfere with user program. Double note: if a
+  // user does something along the lines of 'kill -11 pid', that can kill the
+  // process even if user setup own handler for SEGV.
+  // Thing to watch out for: this code should not change behavior of user code
+  // in any observable way. In particular it should not override user signal
+  // handlers.
+  internal_sigfillset(&blocked_sigset);
+  for (uptr i = 0; i < ARRAY_SIZE(kSyncSignals); i++)
+    internal_sigdelset(&blocked_sigset, kSyncSignals[i]);
+  int rv = internal_sigprocmask(SIG_BLOCK, &blocked_sigset, &old_sigset);
+  CHECK_EQ(rv, 0);
   uptr tracer_pid = internal_clone(
       TracerThread, tracer_stack.Bottom(),
       CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_UNTRACED,
       &tracer_thread_argument, 0 /* parent_tidptr */, 0 /* newtls */, 0
       /* child_tidptr */);
+  internal_sigprocmask(SIG_SETMASK, &old_sigset, 0);
   int local_errno = 0;
   if (internal_iserror(tracer_pid, &local_errno)) {
     VReport(1, "Failed spawning a tracer thread (errno %d).\n", local_errno);
@@ -459,4 +465,4 @@ uptr SuspendedThreadsList::RegisterCount() {
 }
 }  // namespace __sanitizer
 
-#endif  // SANITIZER_LINUX && defined(__x86_64__)
+#endif  // SANITIZER_LINUX && (defined(__x86_64__) || defined(__mips__))