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

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Date:   Mon, 29 May 2017 01:39:04 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions

On Sat, May 27, 2017 at 12:58:14PM -0700, Kees Cook wrote:
> FAST_REFCOUNT=n: use function-based refcount_t with cmpxvhg and
> full-verification
> FAST_REFCOUNT=y without arch-specific implementation: use atomic_t
> with no verification (i.e. no functional change from now)
> FAST_REFCOUNT=y with arch-specific implementation: use atomic_t with
> overflow protection
> 
> which means FAST_REFCOUNT would need to be default-on so that mm,
> block, net users will remain happy.
> 
> Does that sound reasonable?

I'd rather turn the options around so that the atomic_t or fast
arch implementations are the defaul.  But either way it needs to
be configurable.  Once that is done we can spread refcount_t everywhere
and everyone will be better off, if only for the documentation value
of the type when they use the atomic_t based implementation.