problem: utrace-ptrace && jctl stacked stop

[Oleg Nesterov]

I seem to find the problem, and I do not see a simple solution.
Will try to think more tomorrow with a fresh head, but perhaps
you can help. looks like we need some changes in utrace/signal
layer.

Suppose that a tracee is going to report, say, PTRACE_EVENT_FORK.
The callback returns UTRACE_STOP. Of course, utrace_report_xxx()
does not do finish_resume_report() or utrace_stop(), we rely on
utrace_resume() or utrace_get_signal() which should call utrace_stop()
eventually.

Now suppose that the tracee's sub-thread initiates the group-stop.
In this case the tracee will call do_signal_stop(), without doing
utrace_resume/utrace_get_signal. And this means the tracee will
stop in TASK_STOPPED with the wrong ->exit_code.

ptrace can hook ->report_jctl(), but this can't help. We can do

        --- a/kernel/signal.c
        +++ b/kernel/signal.c
        @@ -1569,6 +1569,10 @@ static int do_signal_stop(int signr)
                                        signal_wake_up(t, 0);
                                }
                }
        +
        +       // tracehook_notify_jctl() can change ->exit_code.
        +       // If we won't stop ->exit_code will be cleared anyway
        +       current->exit_code = sig->group_exit_code;
                /*
                 * If there are no other threads in the group, or if there is
                 * a group stop in progress and we are the last to stop, report
        @@ -1584,7 +1588,6 @@ static int do_signal_stop(int signr)
                if (sig->group_stop_count) {
                        if (!--sig->group_stop_count)
                                sig->flags = SIGNAL_STOP_STOPPED;
        -               current->exit_code = sig->group_exit_code;
                        __set_current_state(TASK_STOPPED);
                }
                spin_unlock_irq(&current->sighand->siglock);

But this doesn't really help too. tracehook_notify_jctl() unlocks
->siglock, if SIGCONT comes in between we can't notice it. In this
case the stop reports will be wrong again.

What do you think?

Oleg.