linux-kernel - Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT

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Date:   Tue, 26 Apr 2022 12:19:20 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.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>,
        linux-kernel@...r.kernel.org, tj@...nel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT

Oleg Nesterov <oleg@...hat.com> writes:

> On 04/25, Eric W. Biederman wrote:
>>
>> I don't see JOBCTL_TRACED_QUIESCE being cleared "if (!current->ptrace)".
>
> As Peter explained, in this case we can rely on __ptrace_unlink() which
> should clear this flag.

I had missed that that signal_wake_up_state was clearing
JOBCTL_TRACED_QUIESCE.

Relying on __ptrace_unlink assumes the __ptrace_unlink happens after
siglock is taken before calling ptrace_stop.  Especially with the
ptrace_notify in signal_delivered that does not look guaranteed.

The __ptrace_unlink could also happen during arch_ptrace_stop.

Relying on siglock is sufficient because __ptrace_unlink holds siglock
over clearing task->ptrace.  Which means that the simple fix for this is
to just test task->ptrace before we set JOBCTL_TRACED_QUEIESCE.

Eric