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

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Date:   Mon, 25 Apr 2022 19:38:21 -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: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT

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

> On Mon, Apr 25, 2022 at 04:35:37PM +0200, Oleg Nesterov wrote:
>> On 04/21, Peter Zijlstra wrote:
>> >
>> > +static void clear_traced_quiesce(void)
>> > +{
>> > +	spin_lock_irq(&current->sighand->siglock);
>> > +	WARN_ON_ONCE(!(current->jobctl & JOBCTL_TRACED_QUIESCE));
>> 
>> This WARN_ON_ONCE() doesn't look right, the task can be killed right
>> after ptrace_stop() sets JOBCTL_TRACED | JOBCTL_TRACED_QUIESCE and
>> drops siglock.
>
> OK, will look at that.
>
>> > @@ -2290,14 +2303,26 @@ static int ptrace_stop(int exit_code, in
>> >  		/*
>> >  		 * Don't want to allow preemption here, because
>> >  		 * sys_ptrace() needs this task to be inactive.
>> > -		 *
>> > -		 * XXX: implement read_unlock_no_resched().
>> >  		 */
>> >  		preempt_disable();
>> >  		read_unlock(&tasklist_lock);
>> > -		cgroup_enter_frozen();
>> > +		cgroup_enter_frozen(); // XXX broken on PREEMPT_RT !!!
>> > +
>> > +		/*
>> > +		 * JOBCTL_TRACE_QUIESCE bridges the gap between
>> > +		 * set_current_state(TASK_TRACED) above and schedule() below.
>> > +		 * There must not be any blocking (specifically anything that
>> > +		 * touched ->saved_state on PREEMPT_RT) between here and
>> > +		 * schedule().
>> > +		 *
>> > +		 * ptrace_check_attach() relies on this with its
>> > +		 * wait_task_inactive() usage.
>> > +		 */
>> > +		clear_traced_quiesce();
>> 
>> Well, I think it should be called earlier under tasklist_lock,
>> before preempt_disable() above.
>> 
>> We need tasklist_lock to protect ->parent, debugger can be killed
>> and go away right after read_unlock(&tasklist_lock).
>> 
>> Still trying to convince myself everything is right with
>> JOBCTL_STOPPED/TRACED ...
>
> Can't do it earlier, since cgroup_enter_frozen() can do spinlock (eg.
> use ->saved_state).

There are some other issues in this part of ptrace_stop().


I don't see JOBCTL_TRACED_QUIESCE being cleared "if (!current->ptrace)".


Currently in ptrace_check_attach a parameter of __TASK_TRACED is passed
so that wait_task_inactive cane fail if the "!current->ptrace" branch
of ptrace_stop is take and ptrace_stop does not stop.  With the
TASK_FROZEN state it appears that "!current->ptrace" branch can continue
and freeze somewhere else and wait_task_inactive could decided it was
fine.


I have to run, but hopefully tommorrow I will post the patches that
remove the "!current->ptrace" case altogether and basically
remove the need for quiesce and wait_task_inactive detecting
which branch is taken.

The spinlock in cgroup_enter_frozen remains an issue for PREEMPT_RT.
But the rest of the issues are cleared up by using siglock instead
of tasklist_lock.  Plus the code is just easier to read and understand.

Eric