| From: |
| ebiederm-AT-xmission.com (Eric W. Biederman) |
| To: |
| Ingo Molnar <mingo-AT-kernel.org> |
| 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 |
| Message-ID: |
| <878tjj8exc.fsf@xmission.com> |
| Cc: |
| Andrew Morton <akpm-AT-linux-foundation.org>, Davidlohr Bueso <dave-AT-stgolabs.net>, Elena Reshetova <elena.reshetova-AT-intel.com>, linux-kernel-AT-vger.kernel.org, peterz-AT-infradead.org, gregkh-AT-linuxfoundation.org, mingo-AT-redhat.com, adobriyan-AT-gmail.com, serge-AT-hallyn.com, arozansk-AT-redhat.com, keescook-AT-chromium.org, Hans Liljestrand <ishkamiel-AT-gmail.com>, David Windsor <dwindsor-AT-gmail.com> |
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
