Re: [PATCH 05/16] ptrace: Remove dead code from __ptrace_detach

Oleg Nesterov
2022-05-25T14:33:03Z

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.