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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Oleg Nesterov <oleg@redhat.com>
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References: <878rrrh32q.fsf_-_@email.froward.int.ebiederm.org>
        <20220426225211.308418-6-ebiederm@xmission.com>
        <20220427141018.GA17421@redhat.com>
Date:   Wed, 27 Apr 2022 09:20:51 -0500
In-Reply-To: <20220427141018.GA17421@redhat.com> (Oleg Nesterov's message of
        "Wed, 27 Apr 2022 16:10:25 +0200")
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Subject: Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with
 siglock held
Precedence: bulk

Oleg Nesterov <oleg@redhat.com> writes:

> On 04/26, Eric W. Biederman wrote:
>>
>> @@ -2164,7 +2166,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
>>   	}
>>
>>  	sighand = parent->sighand;
>> -	spin_lock_irqsave(&sighand->siglock, flags);
>> +	lock = tsk->sighand != sighand;
>> +	if (lock)
>> +		spin_lock_nested(&sighand->siglock, SINGLE_DEPTH_NESTING);
>
> But why is it safe?
>
> Suppose we have two tasks, they both trace each other, both call
> ptrace_stop() at the same time. Of course this is ugly, they both
> will block.
>
> But with this patch in this case we have the trivial ABBA deadlock,
> no?

I was thinking in terms of the process tree (which is fine).

The ptrace parental relationship definitely has the potential to be a
graph with cycles.  Which as you point out is not fine.


The result is very nice and I don't want to give it up.  I suspect
something ptrace cycles are always a problem and can simply be
forbidden.  That is going to take some analsysis and some additional
code in ptrace_attach.

I will go look at that.

Eric