linux-kernel - Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE

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Message-ID: <20110221152855.GA20769@redhat.com>
Date:	Mon, 21 Feb 2011 16:28:55 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Roland McGrath <roland@...hat.com>, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after
	PTRACE_ATTACH

On 02/21, Tejun Heo wrote:
>
> 1. The distinction between the first SIGSTOP trapping and the second
>    can only be reliably done by GETSIGINFO which in turn will put the
>    tracee into TASK_TRACED making the tracee ignore the future SIGCONT

Yes, but please see below.

> 2. Due to reparenting, wait(2) notifications (including the SIGCLDs)
>    don't get to the real parent at all.
>
> #2 just needs fixing.

Yes.

> That preciesly is what is being discussed.  IIUC, Oleg and Roland are
> saying that the tracee should enter group stop but not ptrace trap at
> that point and then transition into ptrace trap on the first PTRACE
> call.

Actually I am not saying this (at least now, probably I did).

Once again. We have the bug with arch_ptrace_stop_needed(), but lets
ignore it to simplify the discussion.

Suppose that the tracee calls do_signal_stop() and participates in the
group stop. To me, it doesn't really matter (in the context of this
discussion) if it stops in TASK_STOPPED or TASK_TRACED (and where it
stops).

However, I am starting to agree that TASK_TRACED looks more clean.

What is important, I think ptrace should respect SIGNAL_STOP_STOPPED.
IOW, when the tracee is group-stopped (TASK_STOPPED or TASK_TRACED,
doesn't matter), ptrace_resume() should not wake it up, but merely
do set_task_state(TASK_STATE) and make it resumeable by SIGCONT.

Oleg.

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