linux-kernel - Re: [PATCH 0/3] ipc subsystem refcounter conversions

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Date:   Fri, 3 Mar 2017 16:23:52 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        gregkh@...uxfoundation.org, ebiederm@...ssion.com,
        mingo@...hat.com, adobriyan@...il.com, serge@...lyn.com,
        arozansk@...hat.com, dave@...olabs.net
Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions

On Mon, 20 Feb 2017 13:29:46 +0200 Elena Reshetova <elena.reshetova@...el.com> wrote:

> Now when new refcount_t type and API are finally merged
> (see include/linux/refcount.h), the following
> patches convert various refcounters in the ipc susystem from atomic_t
> to refcount_t. By doing this we prevent intentional or accidental
> underflows or overflows that can led to use-after-free vulnerabilities.
> 
> The below patches are fully independent and can be cherry-picked separately.
> Since we convert all kernel subsystems in the same fashion, resulting
> in about 300 patches, we have to group them for sending at least in some
> fashion to be manageable. Please excuse the long cc list.

Again, the refcount_t operations are much more expensive than the bare
atomic_t operations.  I'm reluctant to merge any of these conversions
without either

a) a convincing demonstration that the performance impact is
   sufficiently small (ie: unmeasurable) or

b) a compile-time option to disable the refcount_t operations (make
   them generate the same code as the bare atomic_t ops).  Along with
   some suitably reliable means of preventing people from accidentally
   enabling the debug code in production builds.