Record robust futex changes before we send SIGKILL in case the SIGKILL destroys the threadgroup without sending a proper PTRACE_EVENT_EXIT

Author: rocallahan

src/RecordSession.cc

@@ -73,6 +73,11 @@ static void record_robust_futex_change(
  */
 template <typename Arch>
 static void record_robust_futex_changes_arch(RecordTask* t) {
+  if (t->did_record_robust_futex_changes) {
+    return;
+  }
+  t->did_record_robust_futex_changes = true;
+
   auto head_ptr = t->robust_list().cast<typename Arch::robust_list_head>();
   if (head_ptr.is_null()) {
     return;
@@ -488,6 +493,13 @@ bool RecordSession::handle_ptrace_event(RecordTask** t_ptr,
           case SECCOMP_RET_KILL:
             LOG(debug) << "  seccomp kill for syscall: "
                        << syscall_name(syscallno, t->arch());
+            for (Task* tt : t->thread_group()->task_set()) {
+              // Record robust futex changes now in case the taskgroup dies
+              // synchronously without a regular PTRACE_EVENT_EXIT (as seems
+              // to happen on Ubuntu 4.2.0-42-generic)
+              RecordTask* rt = static_cast<RecordTask*>(tt);
+              record_robust_futex_changes(rt);
+            }
             t->tgkill(SIGKILL);
             step_state->continue_type = RecordSession::CONTINUE;
             break;

src/RecordTask.cc

@@ -190,7 +190,8 @@ RecordTask::RecordTask(RecordSession& session, pid_t _tid, uint32_t serial,
       break_at_syscallbuf_traced_syscalls(false),
       break_at_syscallbuf_untraced_syscalls(false),
       break_at_syscallbuf_final_instruction(false),
-      next_pmc_interrupt_is_for_user(false) {
+      next_pmc_interrupt_is_for_user(false),
+      did_record_robust_futex_changes(false) {
   push_event(Event::sentinel());
   if (session.tasks().empty()) {
     // Initial tracee. It inherited its state from this process, so set it up.

src/RecordTask.h

@@ -669,6 +669,8 @@ class RecordTask : public Task {
   // The pmc is programmed to interrupt at a value requested by the tracee, not
   // by rr.
   bool next_pmc_interrupt_is_for_user;
+
+  bool did_record_robust_futex_changes;
 };
 
 } // namespace rr