From: ebiederm@xmission.com (Eric W. Biederman)
To: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Davidlohr Bueso <dave@stgolabs.net>,
Elena Reshetova <elena.reshetova@intel.com>,
linux-kernel@vger.kernel.org, peterz@infradead.org,
gregkh@linuxfoundation.org, mingo@redhat.com,
adobriyan@gmail.com, serge@hallyn.com, arozansk@redhat.com,
keescook@chromium.org, Hans Liljestrand <ishkamiel@gmail.com>,
David Windsor <dwindsor@gmail.com>
Subject: Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t
Date: Thu, 20 Jul 2017 07:34:55 -0500 [thread overview]
Message-ID: <878tjj8exc.fsf@xmission.com> (raw)
In-Reply-To: <20170720093402.55alnsgsodgs4mfk@gmail.com> (Ingo Molnar's message of "Thu, 20 Jul 2017 11:34:02 +0200")
Ingo Molnar <mingo@kernel.org> writes:
> * Andrew Morton <akpm@linux-foundation.org> wrote:
>
>> On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
>>
>> > On Wed, 19 Jul 2017, Andrew Morton wrote:
>> >
>> > >I do rather dislike these conversions from the point of view of
>> > >performance overhead and general code bloat. But I seem to have lost
>> > >that struggle and I don't think any of these are fastpath(?).
>> >
>> > Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
>> > implementation), performance is supposed to be ok.
>>
>> Sure, things are OK for people who disable the feature.
>
> So with the WIP fast-refcount series from Kees:
>
> [PATCH v6 0/2] x86: Implement fast refcount overflow protection
>
> I believe the robustness difference between optimized-refcount_t and
> full-refcount_t will be marginal.
>
> I.e. we'll be able to have both higher API safety _and_ performance.
>
>> But for people who want to enable the feature we really should minimize the cost
>> by avoiding blindly converting sites which simply don't need it: simple, safe,
>> old, well-tested code. Why go and slow down such code? Need to apply some
>> common sense here...
>
> It's old, well-tested code _for existing, sane parameters_, until someone finds a
> decade old bug in one of these with an insane parameters no-one stumbled upon so
> far, and builds an exploit on top of it.
>
> Only by touching all these places do we have a chance to improve things measurably
> in terms of reducing the probability of bugs.
The more I hear people pushing the upsides of refcount_t without
considering the downsides the more I dislike it.
- refcount_t is really the wrong thing because it uses saturation
semantics. So by definition it includes a bug.
- refcount_t will only really prevent something if there is an extra
increment. That is not the kind of bug people are likely to make.
- refcount_t won't help if you have an extra decrement. The bad
use-after-free will still happen.
- refcount_t won't help if there is a memory stomp. As with an extra
decrement the bad use-after-free will still happen.
So all I see is a huge amount of code churn to implement a buggy (by
definition) refcounting API, that risks adding new bugs and only truly
helps with bugs that are unlikely in the first place.
I really don't think this is an obvious slam dunk.
Eric
next prev parent reply other threads:[~2017-07-20 12:43 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-07 8:59 [PATCH 0/3] v2 ipc subsystem refcount coversions Elena Reshetova
2017-07-07 8:59 ` [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t Elena Reshetova
2017-07-09 21:59 ` Eric W. Biederman
2017-07-10 6:48 ` Reshetova, Elena
2017-07-10 8:37 ` Eric W. Biederman
2017-07-10 9:34 ` Alexey Dobriyan
2017-07-10 11:19 ` Eric W. Biederman
2017-07-10 9:56 ` Reshetova, Elena
2017-07-10 11:26 ` Eric W. Biederman
2017-07-10 12:11 ` Reshetova, Elena
2017-07-10 20:32 ` Eric W. Biederman
2017-07-12 9:21 ` Reshetova, Elena
2017-07-19 22:35 ` Andrew Morton
2017-07-19 22:54 ` Davidlohr Bueso
2017-07-19 22:58 ` Andrew Morton
2017-07-19 23:11 ` Davidlohr Bueso
2017-07-19 23:20 ` Kees Cook
2017-07-20 0:32 ` Kees Cook
2017-07-20 9:34 ` Ingo Molnar
2017-07-20 12:34 ` Eric W. Biederman [this message]
2017-07-20 15:12 ` Kees Cook
2017-07-07 8:59 ` [PATCH 2/3] ipc: convert sem_undo_list.refcnt " Elena Reshetova
2017-07-07 8:59 ` [PATCH 3/3] ipc: convert kern_ipc_perm.refcount " Elena Reshetova
-- strict thread matches above, loose matches on Subject: below --
2017-02-20 11:29 [PATCH 0/3] ipc subsystem refcounter conversions Elena Reshetova
2017-02-20 11:29 ` [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t Elena Reshetova
2017-05-27 19:41 ` Kees Cook
2017-05-28 12:10 ` Manfred Spraul
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