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SubjectRe: [PATCH 17/17] ptrace: implement PTRACE_LISTEN
On 05/30, Tejun Heo wrote:
>
> This allows ptracer to monitor group stop state without running tracee
> - use INTERRUPT to put tracee into STOP trap, issue LISTEN and then
> wait(2) to wait for the next group stop event. When it happens,
> PTRACE_GETSIGINFO provides information to determine the current state.

Great. Just a couple of questions,

> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1416,7 +1416,7 @@ static int wait_task_stopped(struct wait_opts *wo,
> spin_lock_irq(&p->sighand->siglock);
>
> p_code = task_stopped_code(p, ptrace);
> - if (unlikely(!p_code))
> + if (unlikely(!p_code) || p->jobctl & JOBCTL_LISTENING)
> goto unlock_sig;

Up to you, but perhaps this JOBCTL_LISTENING check should go into
task_stopped_code() ? Or do you think we can't check it without
siglock?

> + case PTRACE_LISTEN:
> + /*
> + * Listen for events. Tracee must be in STOP. It's not
> + * resumed per-se but is not considered to be in TRACED by
> + * wait(2) or ptrace(2). If an async event (e.g. group
> + * stop state change) happens, tracee will enter STOP trap
> + * again. Alternatively, ptracer can issue INTERRUPT to
> + * finish listening and re-trap tracee into STOP.
> + */
> + if (unlikely(!seized || !lock_task_sighand(child, &flags)))
> + break;
> +
> + si = child->last_siginfo;
> + if (unlikely(!si || si->si_code != PTRACE_STOP_SI_CODE))
> + break;
> +
> + child->jobctl |= JOBCTL_LISTENING;
> +
> + /*
> + * If NOTIFY is set, it means event happened between start
> + * of this trap and now. Trigger re-trap immediately.
> + */
> + if (child->jobctl & JOBCTL_TRAP_NOTIFY)
> + signal_wake_up(child, true);

Again, I won't insist if you prefer signal_wake_up(), but afaics
wake_up_state(__TASK_TRACED) should be enough.

> @@ -838,7 +840,7 @@ static void ptrace_trap_notify(struct task_struct *t)
> assert_spin_locked(&t->sighand->siglock);
>
> task_set_jobctl_pending(t, JOBCTL_TRAP_NOTIFY);
> - signal_wake_up(t, 0);
> + signal_wake_up(t, t->jobctl & JOBCTL_LISTENING);
> }

OK. The only thing I can't understand is why prepare_signal(SIGCONT)
calls ptrace_trap_notify() unconditionally. How about

if (likely(!(t->ptrace & PT_SEIZED)))
wake_up_state(t, __TASK_STOPPED);
- else
+ else if (why)
ptrace_trap_notify(t);
?

Oleg.



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