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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Oleg Nesterov <oleg@redhat.com>
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References: <87a6bv6dl6.fsf_-_@email.froward.int.ebiederm.org>
        <20220505182645.497868-10-ebiederm@xmission.com>
        <20220510142202.GB23277@redhat.com>
Date:   Tue, 10 May 2022 10:17:32 -0500
In-Reply-To: <20220510142202.GB23277@redhat.com> (Oleg Nesterov's message of
        "Tue, 10 May 2022 16:23:18 +0200")
Message-ID: <87ee11wh6b.fsf@email.froward.int.ebiederm.org>
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Subject: Re: [PATCH v4 10/12] ptrace: Don't change __state
Precedence: bulk

Oleg Nesterov <oleg@redhat.com> writes:

> On 05/05, Eric W. Biederman wrote:
>>
>>  static void ptrace_unfreeze_traced(struct task_struct *task)
>>  {
>> -	if (READ_ONCE(task->__state) != __TASK_TRACED)
>> -		return;
>> -
>> -	WARN_ON(!task->ptrace || task->parent != current);
>> +	unsigned long flags;
>>  
>>  	/*
>> -	 * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
>> -	 * Recheck state under the lock to close this race.
>> +	 * The child may be awake and may have cleared
>> +	 * JOBCTL_PTRACE_FROZEN (see ptrace_resume).  The child will
>> +	 * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew.
>>  	 */
>> -	spin_lock_irq(&task->sighand->siglock);
>> -	if (READ_ONCE(task->__state) == __TASK_TRACED) {
>> +	if (lock_task_sighand(task, &flags)) {
>
> But I still think that a lockless
>
> 	if (!(task->jobctl & JOBCTL_PTRACE_FROZEN))
> 		return;
>
> check at the start of ptrace_unfreeze_traced() makes sense to avoid
> lock_task_sighand() if possible.
>
> And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with
> JOBCTL_TRACED to make this optimization work better. The same for
> ptrace_signal_wake_up().

What do you have that suggests that taking siglock there is a problem?

What you propose will definitely work as an incremental change, and
in an incremental change we can explain why doing the stupid simple
thing is not good enough.

I am not really opposed on any grounds except that simplicity is good,
and hard to get wrong.

Eric