linux-kernel - Re: [PATCH 7/9] ptrace: Simplify the wait_task_inactive call in ptrace_check

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Message-ID: <20220428111911.GA3804@redhat.com>
Date:   Thu, 28 Apr 2022 13:19:11 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        inux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 7/9] ptrace: Simplify the wait_task_inactive call in
 ptrace_check_attach

On 04/28, Peter Zijlstra wrote:
>
> On Wed, Apr 27, 2022 at 05:14:57PM +0200, Oleg Nesterov wrote:
> > On 04/26, Eric W. Biederman wrote:
> > >
> > > Asking wait_task_inactive to verify that tsk->__state == __TASK_TRACED
> > > was needed to detect the when ptrace_stop would decide not to stop
> > > after calling "set_special_state(TASK_TRACED)".  With the recent
> > > cleanups ptrace_stop will always stop after calling set_special_state.
> > >
> > > Take advatnage of this by no longer asking wait_task_inactive to
> > > verify the state.  If a bug is hit and wait_task_inactive does not
> > > succeed warn and return -ESRCH.
> >
> > ACK, but I think that the changelog is wrong.
> >
> > We could do this right after may_ptrace_stop() has gone. This doesn't
> > depend on the previous changes in this series.
>
> It very much does rely on there not being any blocking between
> set_special_state() and schedule() tho. So all those PREEMPT_RT
> spinlock->rt_mutex things need to be gone.

Yes sure. But this patch doesn't add the new problems, imo.

Yes we can hit the WARN_ON_ONCE(!wait_task_inactive()), but this is
correct in that it should not fail, and this is what we need to fix.

> That is also the reason I couldn't do wait_task_inactive(task, 0)

Ah, I din't notice this patch uses wait_task_inactive(child, 0),
I think it should do wait_task_inactive(child, __TASK_TRACED).

Oleg.