linux-kernel - Re: [RFC][PATCH] ptrace: Don't change _

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Date:   Thu, 21 Apr 2022 09:45:54 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, 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: [RFC][PATCH] ptrace: Don't change __state

Peter Zijlstra <peterz@...radead.org> writes:

> On Wed, Apr 20, 2022 at 03:54:15PM -0500, Eric W. Biederman wrote:
>> 
>> I was thinking about this and I have an approach from a different
>> direction.  In particular it removes the need for ptrace_freeze_attach
>> and ptrace_unfreeze_attach to change __state.  Instead a jobctl
>> bit is used to suppress waking up a process with TASK_WAKEKILL.
>> 
>> I think this would be a good technique to completely decouple
>> PREEMPT_RT from the work that ptrace_freeze_attach does.
>> 
>> Comments?
>
> On first read-through, I like it! A few comments down below..
>
>> @@ -216,13 +217,11 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
>>  	 * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
>>  	 * Recheck state under the lock to close this race.
>>  	 */
>> -	spin_lock_irq(&task->sighand->siglock);
>> -	if (READ_ONCE(task->__state) == __TASK_TRACED) {
>> -		if (__fatal_signal_pending(task))
>> -			wake_up_state(task, __TASK_TRACED);
>> -		else
>> -			WRITE_ONCE(task->__state, TASK_TRACED);
>> -	}
>> +	spin_unlock_irq(&task->sighand->siglock);
>
>   ^^^^ this should be spin_lock_irq(...)

Doh!

Thank you for spotting that.  That solves my nasty splat in
__send_signal.


>
>> +	WARN_ON(!(task->jobctl & JOBCTL_DELAY_WAKEKILL));
>> +	task->jobctl &= ~JOBCTL_DELAY_WAKEKILL;
>> +	if (fatal_signal_pending(task))
>> +		wake_up_state(task, TASK_WAKEKILL);
>>  	spin_unlock_irq(&task->sighand->siglock);
>>  }
>>  
>> @@ -256,7 +255,7 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
>>  	 */
>>  	read_lock(&tasklist_lock);
>>  	if (child->ptrace && child->parent == current) {
>> -		WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED);
>> +		WARN_ON(child->jobctl & JOBCTL_DELAY_WAKEKILL);
>>  		/*
>>  		 * child->sighand can't be NULL, release_task()
>>  		 * does ptrace_unlink() before __exit_signal().
>> @@ -267,13 +266,13 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
>>  	read_unlock(&tasklist_lock);
>>  
>>  	if (!ret && !ignore_state) {
>> -		if (!wait_task_inactive(child, __TASK_TRACED)) {
>> +		if (!wait_task_inactive(child, TASK_TRACED)) {
>
> This is still very dubious, there are spinlocks between
> set_current_state(TASK_TRACED) and schedule(), so wait_task_inactive()
> can fail where we don't want it to due to TASK_TRACED being temporarily
> held in ->saved_state.

When it comes to PREEMPT_RT yes.

I think we might be able to remove the wait_task_inactive, I am
not certain what it gets us.

All this change gets us is the removal of playing with __state.

Eric