Re: [PATCHSET RFC] ptrace,signal: clean transition between STOPPED and TRACED

Roland McGrath
2011-01-18T02:12:13Z

> 1. When attaching to a STOPPED task or a traced task stops for group
> stop, the tracee now enters TRACED instead of STOPPED. This is
> visible via fs/proc but, more importantly, SIGCONT is ignored if a
> task is TRACED.

That is probably OK, but I'm still not entirely sure about it.

> This may, for example, affect the operation of strace but given how
> strace always need to issue further ptrace operations on trap to
> determine what's going on, I doubt it would actually be worse.

I'm not clear on what effect on strace you have in mind.

> 2. The transition between STOPPED and TRACED involves a short window
> of RUNNING inbetween. On attach, the transition is hidden from the
> tracer using GROUP_STOP_TRAPPING but it still is visible to other
> threads in the tracer's group. IOW, if another thread performs
> WNOHANG wait(2) on the tracee while attach is in progress, the
> wait(2) may fail even if the tracee is known to be in stopped state
> before.
>
> The same problem exists the other direction during detach.
> Currently, the code doesn't try to hide this transition even from
> the tracer. IOW, if the tracer attaches to a stopped task,
> detaches, reattaches and then performs WNOHANG wait(2), the wait(2)
> may fail. However, given the previous behavior where the tracee is
> always woken up by wake_up_process() on detach, this is highly
> unlikely to cause any problem.

This seems more problematic to me. I don't like that start/stop window
at all.

Saying "wait may fail" is not sufficiently precise to be helpful. Please
be more clear. If "fail" means ECHILD, that is unacceptable. If "fail"
means a WNOHANG wait returns 0 when userland already "knows" that the
thread is topped, that might be more acceptable.

Thanks,
Roland