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

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Date:   Wed, 13 Apr 2022 15:24:52 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        ebiederm@...ssion.com, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT

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.