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Date: Wed, 15 Aug 2012 15:01:59 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: Fengguang Wu <fengguang.wu@intel.com>, LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: yama_ptrace_access_check(): possible recursive locking detected
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On 08/14, Kees Cook wrote:
>
> Okay, I've now managed to reproduce this locally. I added a bunch of
> debugging, and I think I understand what's going on. This warning is,
> actually, a false positive.

Sure. I mean that yes, this warning doesn't mean we already hit deadlock.

> get used recursively (the task_struct->alloc_lock), but they are
> separate instantiations ("task" is never "current").

Yes. But suppose that we have 2 tasks T1 and T2,

	- T1 does ptrace(PTRACE_ATTACH, T2);

	- T2 does ptrace(PTRACE_ATTACH, T1);

at the same time. This can lead to the "real" deadlock, no?

> So Oleg's suggestion of removing the locking around the reading of
> ->comm is wrong since it really does need the lock.

Nothing bad can happen without the lock. Yes, printk() can print
some string "in between" if we race with set_task_comm() but this
is all.

BTW, set_task_comm()->wmb() and memset() should die. There are
not needed afaics, and the comment is misleading.

Oleg.