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

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Date:	Mon, 17 Jan 2011 18:11:33 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	oleg@...hat.com, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCHSET RFC] ptrace,signal: clean transition between STOPPED and TRACED

> 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
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