Re: [PATCH 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT

From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: rjw@rjwysocki.net, mingo@kernel.org, vincent.guittot@linaro.org,
	dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de,
	ebiederm@xmission.com, bigeasy@linutronix.de,
	Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org, tj@kernel.org,
	linux-pm@vger.kernel.org
Subject: Re: [PATCH 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT
Date: Wed, 13 Apr 2022 15:24:52 +0200	[thread overview]
Message-ID: <20220413132451.GA27281@redhat.com> (raw)
In-Reply-To: <20220412114853.842942162@infradead.org>

Hi Peter,

I like 1-2 but I need to read them (and other patches) again, a
couple of nits right now.

On 04/12, Peter Zijlstra wrote:
>
> +static int __ptrace_freeze_cond(struct task_struct *p)
> +{
> +	if (!task_is_traced(p))
> +		return -ESRCH;

	if (!task_is_traced(p) || p->parent != current)
		return -ESRCH;

we should not spin/sleep if it is traced by another task

> +static int __ptrace_freeze(struct task_struct *p, void *arg)
> +{
> +	int ret;
> +
> +	ret = __ptrace_freeze_cond(p);
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * Task scheduled between __ptrace_pre_freeze() and here, not our task
> +	 * anymore.
> +	 */
> +	if (*(unsigned long *)arg != p->nvcsw)
> +		return -ESRCH;
> +
> +	if (looks_like_a_spurious_pid(p))
> +		return -ESRCH;

Oh, I do not think __ptrace_freeze() should check for spurious pid...
looks_like_a_spurious_pid() should be called once in ptrace_check_attach()
before task_call_func(__ptrace_freeze).

Oleg.