linux-kernel - Re: [PATCH v2 07/12] ptrace: Don't change _

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Date:   Wed, 4 May 2022 16:02:38 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, 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>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH v2 07/12] ptrace: Don't change __state

On 05/03, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > But why is it bad if the tracee doesn't sleep in schedule ? If it races
> > with SIGKILL. I still can't understand this.
> >
> > Yes, wait_task_inactive() can fail, so you need to remove WARN_ON_ONCE()
> > in 11/12.
>
> >
> > Why is removing TASK_WAKEKILL from TASK_TRACED and complicating
> > *signal_wake_up() better?
>
> Not changing __state is better because it removes special cases
> from the scheduler that only apply to ptrace.

Hmm. But I didn't argue with that? I like the idea of JOBCTL_TASK_FROZEN.

I meant, I do not think that removing KILLABLE from TASK_TRACED (not
from __state) and complicating *signal_wake_up() (I mean, compared
to your previous version) is a good idea.

And. At least in context of this series it is fine if the JOBCTL_TASK_FROZEN
tracee do not block in schedule(), just you need to remove WARN_ON_ONCE()
around wait_task_inactive().

> > And even if we need to ensure the tracee will always block after
> > ptrace_freeze_traced(), we can change signal_pending_state() to
> > return false if JOBCTL_PTRACE_FROZEN. Much simpler, imo. But still
> > looks unnecessary to me.
>
> We still need to change signal_wake_up in that case.  Possibly
> signal_wake_up_state.

Of course. See above.

> >> if we depend on wait_task_inactive failing if the process is in the
> >> wrong state.
> >
> > OK, I guess this is what I do not understand. Could you spell please?
> >
> > And speaking of RT, wait_task_inactive() still can fail because
> > cgroup_enter_frozen() takes css_set_lock? And it is called under
> > preempt_disable() ? I don't understand the plan :/
>
> Let me describe his freezer change as that is much easier to get to the
> final result.  RT has more problems as it turns all spin locks into
> sleeping locks.  When a task is frozen

[...snip...]

Oh, thanks Eric, but I understand this part. But I still can't understand
why is it that critical to block in schedule... OK, I need to think about
it. Lets assume this is really necessary.

Anyway. I'd suggest to not change TASK_TRACED in this series and not
complicate signal_wake_up() more than you did in your previous version:

	static inline void signal_wake_up(struct task_struct *t, bool resume)
	{
		bool wakekill = resume && !(t->jobctl & JOBCTL_DELAY_WAKEKILL);
		signal_wake_up_state(t, wakekill ? TASK_WAKEKILL : 0);
	}

JOBCTL_PTRACE_FROZEN is fine.

ptrace_check_attach() can do

	if (!ret && !ignore_state &&
	    /*
	     * This can only fail if the frozen tracee races with
	     * SIGKILL and enters schedule() with fatal_signal_pending
	     */
	    !wait_task_inactive(child, __TASK_TRACED))
		ret = -ESRCH;

	return ret;


Now. If/when we really need to ensure that the frozen tracee always
blocks and wait_task_inactive() never fails, we can just do

	- add the fatal_signal_pending() check into ptrace_stop()
	  (like this patch does)

	- say, change signal_pending_state:

	static inline int signal_pending_state(unsigned int state, struct task_struct *p)
	{
		if (!(state & (TASK_INTERRUPTIBLE | TASK_WAKEKILL)))
			return 0;
		if (!signal_pending(p))
			return 0;
		if (p->jobctl & JOBCTL_TASK_FROZEN)
			return 0;
		return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
	}

in a separate patch which should carefully document the need for this
change.

> > I didn't look at JOBCTL_PTRACE_SIGNR yet. But this looks minor to me,
> > I mean, I am not sure it worth the trouble.
>
> The immediate problem the JOBCTL_PTRACE_SIGNR patch solves is:
> - stopping in ptrace_report_syscall.
> - Not having PT_TRACESYSGOOD set.
> - The tracee being killed with a fatal signal
        ^^^^^^
        tracer ?
> - The tracee sending SIGTRAP to itself.

Oh, but this is clear. But do we really care? If the tracer exits
unexpectedly, the tracee can have a lot more problems, I don't think
that this particular one is that important.

Oleg.