Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with siglock held

From: Eric W. Biederman
Date: Wed Apr 27 2022 - 10:21:10 EST

Oleg Nesterov <oleg@xxxxxxxxxx> 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.