linux-kernel - Re: Runtime trouble with commit dbd952127d (seccomp: introduce writer locking)

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Date:	Mon, 11 Aug 2014 12:51:22 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: Runtime trouble with commit dbd952127d (seccomp: introduce writer locking)

On Mon, Aug 11, 2014 at 7:11 AM, Guenter Roeck <linux@...ck-us.net> wrote:
> On 08/11/2014 04:48 AM, Oleg Nesterov wrote:
>>
>> On 08/10, Linus Torvalds wrote:
>>>
>>>
>>> So that should just be converted to assert_spin_is_locked().
>>
>>
>> I still think that lockdep_assert_held() is better. Unlike
>> assert_spin_locked() it checks that this lock is held by us, and this
>> is what we want in this case.
>>
>
> assert_spin_locked maps to "BUG_ON(!raw_spin_is_locked(x))"
> which it seems is exactly what the current code is doing.
> I submitted a patch to make that change to use assert_spin_locked.
> Presumably the author had a reason for using BUG_ON and not
> lockdep_assert_held(), ie to perform the checks all the time
> and not just while debugging. For me this was the safe change
> to make. Anything else should, in my opinion, come from the
> original author who introduced the code.

Thanks for the patch! Yeah, that's a weird case; I think we need some
documentation in the header file about the UP vs SMP logic when using
spin_is_locked(). I note that all other stuff gets hidden behind the
_up and _smp headers.

I don't prefer lockdep_assert_held(), though, since I want lock
failures to hit BUG. I'll apply the patch and ask James to pull it.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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