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Date:   Wed, 25 May 2022 16:33:03 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
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        Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 05/16] ptrace: Remove dead code from __ptrace_detach
Message-ID: <20220525143303.GB2687@redhat.com>
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 <20220518225355.784371-5-ebiederm@xmission.com>
 <20220524114250.GB14347@redhat.com>
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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?

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

Oleg.