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

Eric W. Biederman
2022-06-06T16:06:01Z

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