linux-kernel - Re: [PATCH -mm 2/2] ptrace_untrace: fix the SIGNAL_STOP

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Message-ID: <20090220191321.GA7564@redhat.com>
Date:	Fri, 20 Feb 2009 20:13:21 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 2/2] ptrace_untrace: fix the SIGNAL_STOP_STOPPED
	check

On 02/19, Roland McGrath wrote:
>
> > This bug is ancient too. ptrace_untrace() must not resume the task
> > if the group stop in progress, we should set TASK_STOPPED instead.
>
> Right, the change looks correct.
>
> > Unfortunately, we still have problems here:
> >
> > 	- if the process/thread was traced, SIGNAL_STOP_STOPPED
> > 	  does not necessary means this thread group is stopped.
> >
> > 	- ptrace breaks the bookkeeping of ->group_stop_count.
>
> Neither of these should be true today as I understand the code.

Suppose we have

	int main(void)
	{
		kill(getpid(), SIGSTOP);

		for (;;)
			;
	}

If you strace this task nobody clears SIGNAL_STOP_STOPPED.

As for bookkeeping of ->group_stop_count, I think ptrace_stop() is wrong
in many ways. For example, it decrements ->group_stop_count but doesn't
set SIGNAL_STOP_STOPPED if it becomes zero.

Oleg.

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