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>
References: <1499417992-3238-1-git-send-email-elena.reshetova@intel.com>
        <1499417992-3238-2-git-send-email-elena.reshetova@intel.com>
        <87bmottgo4.fsf@xmission.com>
        <20170719153546.37567fbf77861653172fa263@linux-foundation.org>
        <20170719225427.GD14395@linux-80c1.suse>
        <20170719155833.641a283467bf6b89a7d2e56b@linux-foundation.org>
        <20170720093402.55alnsgsodgs4mfk@gmail.com>
Date: Thu, 20 Jul 2017 07:34:55 -0500
In-Reply-To: <20170720093402.55alnsgsodgs4mfk@gmail.com> (Ingo Molnar's
        message of "Thu, 20 Jul 2017 11:34:02 +0200")
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Subject: Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t
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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