linux-kernel - Re: [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped.

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Message-ID: <8760ir192p.fsf@xmission.com>
Date:   Thu, 30 Mar 2017 03:07:42 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <asarai@...e.com>,
        Andy Lutomirski <luto@...capital.net>,
        Attila Fazekas <afazekas@...hat.com>,
        Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...nel.org>,
        Ulrich Obergfell <uobergfe@...hat.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped.

Oleg Nesterov <oleg@...hat.com> writes:

> On 03/03, Eric W. Biederman wrote:
>> @@ -1065,11 +1065,8 @@ static int de_thread(struct task_struct *tsk)
>>  	}
>>
>>  	sig->group_exit_task = tsk;
>> -	sig->notify_count = zap_other_threads(tsk);
>> -	if (!thread_group_leader(tsk))
>> -		sig->notify_count--;
>> -
>> -	while (sig->notify_count) {
>> +	zap_other_threads(tsk);
>> +	while (atomic_read(&sig->live) > 1) {
>>  		__set_current_state(TASK_KILLABLE);
>>  		spin_unlock_irq(lock);
>>  		schedule();
>
> Very nice. So de_thread() returns as soon as all other threads decrement
> signal->live in do_exit(). Before they do, say, exit_mm(). This is already
> wrong, for example this breaks OOM. Plus a lot more problems afaics,  but
> lets ignore this.

Which means that we need to keep sig->notify_count.

> Note that de_thread() also unshares ->sighand before return. So in the
> case of mt exec it will likely see oldsighand->count != 1 and alloc the
> new sighand_struct and this breaks the locking.
>
> Because the execing thread will use newsighand->siglock to protect its
> signal_struct while the zombie threads will use oldsighand->siglock to
> protect the same signal struct. Yes, tasklist_lock + the fact irq_disable
> implies rcu_lock mostly save us but not entirely, say, a foreign process
> doing __send_signal() can take the right or the wrong lock depending on
> /dev/random.

Which leads to the question how can we get back tot he 2.4 behavior
of freeing sighand_struct in do_exit?

At which point as soon as we free sighand_struct if we are the last
to dying thread notify de_thread and everything works.

There are only two uses of sighand->siglock that I can see in
release_task:  __ptrace_unlink and __exit_signal.

For what __ptrace_unlink is doing we should just be able to skip
acquiring of siglock if PF_EXITING is set.

__exit_signal is a little more interesting but half of what it is
doing looks like it was pulled out of do_exit and just needs to
be put back.

Which probably adds up to 4 or 5 small carefully written patches to sort
out that part of the exit path, but I think it leads to a very simple
and straight forward result.  With the only side effect that we
can exec a process and still have a debugger reaping zombie threads.

Oleg does that sound reasonable to you?

Eric