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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Oleg Nesterov <oleg@redhat.com>
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Date:   Mon, 06 Jun 2022 11:06:01 -0500
In-Reply-To: <20220525143303.GB2687@redhat.com> (Oleg Nesterov's message of
        "Wed, 25 May 2022 16:33:03 +0200")
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Subject: Re: [PATCH 05/16] ptrace: Remove dead code from __ptrace_detach
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Oleg Nesterov <oleg@redhat.com> writes:

> On 05/24, Oleg Nesterov wrote:
>>
>> Sorry for delay.
>>
>> On 05/18, Eric W. Biederman wrote:
>> >
>> > Ever since commit 28d838cc4dfe ("Fix ptrace self-attach rule") it has
>> > been impossible to attach another thread in the same thread group.
>> >
>> > Remove the code from __ptrace_detach that was trying to support
>> > detaching from a thread in the same thread group.
>>
>> may be I am totally confused, but I think you misunderstood this code
>> and thus this patch is very wrong.
>>
>> The same_thread_group() check does NOT try to check if debugger and
>> tracee is in the same thread group, this is indeed impossible.
>>
>> We need this check to know if the tracee was ptrace_reparented() before
>> __ptrace_unlink() or not.
>>
>>
>> > -static int ignoring_children(struct sighand_struct *sigh)
>> > -{
>> > -	int ret;
>> > -	spin_lock(&sigh->siglock);
>> > -	ret = (sigh->action[SIGCHLD-1].sa.sa_handler == SIG_IGN) ||
>> > -	      (sigh->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT);
>> > -	spin_unlock(&sigh->siglock);
>> > -	return ret;
>> > -}
>>
>> ...
>>
>> > @@ -565,14 +552,9 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
>> >
>> >  	dead = !thread_group_leader(p);
>> >
>> > -	if (!dead && thread_group_empty(p)) {
>> > -		if (!same_thread_group(p->real_parent, tracer))
>> > -			dead = do_notify_parent(p, p->exit_signal);
>> > -		else if (ignoring_children(tracer->sighand)) {
>> > -			__wake_up_parent(p, tracer);
>> > -			dead = true;
>> > -		}
>> > -	}
>>
>> So the code above does:
>>
>> 	- if !same_thread_group(p->real_parent, tracer), then the tracee was
>> 	  ptrace_reparented(), and now we need to notify its natural parent
>> 	  to let it know it has a zombie child.
>>
>> 	- otherwise, the tracee is our natural child, and it is actually dead.
>> 	  however, since we are going to reap this task, we need to wake up our
>> 	  sub-threads possibly sleeping on ->wait_chldexit wait_queue_head_t.
>>
>> See?
>>
>> > +	if (!dead && thread_group_empty(p))
>> > +		dead = do_notify_parent(p, p->exit_signal);
>>
>> No, this looks wrong. Or I missed something?
>
> Yes, but...
>
> That said, it seems that we do not need __wake_up_parent() if it was our
> natural child?

Agreed on both counts.

Hmm.  I see where the logic comes from.  The ignoring_children test and
the __wake_up_parent are what do_notify_parent does when the parent
ignores children.  Hmm.  I even see all of this document in the comment
above __ptrace_detach.

So I am just going to drop this change.

> I'll recheck. Eric, I'll continue to read this series tomorrow, can't
> concentrate on ptrace today.

No worries.  This was entirely too close to the merge window so I
dropped it all until today.

Eric