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Message-ID: <874k2ea9q4.fsf@email.froward.int.ebiederm.org>
Date: Wed, 27 Apr 2022 09:20:51 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
vincent.guittot@...aro.org, dietmar.eggemann@....com,
rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
Will Deacon <will@...nel.org>, tj@...nel.org,
linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
Richard Weinberger <richard@....at>,
Anton Ivanov <anton.ivanov@...bridgegreys.com>,
Johannes Berg <johannes@...solutions.net>,
linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
Max Filippov <jcmvbkbc@...il.com>,
inux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with
siglock held
Oleg Nesterov <oleg@...hat.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
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