linux-kernel - Re: [RFC] Proposal for ptrace improvements

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Message-ID: <20110303202246.GB32152@redhat.com>
Date:	Thu, 3 Mar 2011 21:22:46 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Roland McGrath <roland@...hat.com>, jan.kratochvil@...hat.com,
	Denys Vlasenko <vda.linux@...glemail.com>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [RFC] Proposal for ptrace improvements

On 03/03, Oleg Nesterov wrote:
>
> I'll ask the questions later.

Right now I do not see any holes (but I'll try more ;)

One question, to ensure I really understand you. To simplify,
consider this particular example.

Tracee:

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

		printf("I am running\n");

		for (;;)
			;
	}

To simplify again, suppose that the debugger attaches when it is
already stopped, then it does PTRACE_CONT(0).

In this case the tracee remains SIGNAL_STOP_STOPPED but prints
"I am running" and enters the endless loop.

(the new debugger can do PTRACE_SEIZE after that and "return"
 it to the stopped state without affecting jctl state).

Now, if SIGCONT comes (from anywhere) it clears SIGNAL_STOP_STOPPED,
the tracee traps and reports this event to debugger.

Correct?

And, once again. In the mt case, I assume that SIGCONT makes
every traced thread to report this event individually, right?

(I am talking about the case when the group-stop was finished,
 iow "every" probably means the threads which participated and
 reported CLD_STOPPED to the debugger).

In both cases, later then this SIGCONT will be reported again
as any "normal" signal when some thread dequeues it.

Is my understanding correct?

Thanks,

Oleg.

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