linux-kernel - Re: [PATCH v2 0/6] Rework REFCOUNT_FULL using atomic_fetch

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Message-ID: <CAKv+Gu899ZsEG43aSQ0nn9suHp-ekKeSMbPRb2OYwsAu=_Q93Q@mail.gmail.com>
Date:   Sat, 31 Aug 2019 23:54:11 +0300
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Jan Glauber <jglauber@...vell.com>
Subject: Re: [PATCH v2 0/6] Rework REFCOUNT_FULL using atomic_fetch_* operations

On Sat, 31 Aug 2019 at 22:02, Kees Cook <keescook@...omium.org> wrote:
>
> On Sat, Aug 31, 2019 at 08:48:56PM +0300, Ard Biesheuvel wrote:
> > It's been ~2 years since I looked at this code in detail, but IIRC, it
> > looked like the inc-from-zero check was missing from the x86
> > implementation because it requires a load/compare/increment/store
> > sequence instead of a single increment instruction taking a memory
> > operand. Was there more rationale at the time for omitting this
> > particular case, and if so, was it based on a benchmark? Can we run it
> > against this implementation as well?
>
> It was based on providing a protection against the pre-exploitation case
> (overflow: "something bad is about to happen, let's stop it") rather
> than the post-exploitation case (inc from zero, "something bad already
> happened, eek") with absolutely the fewest possible extra cycles, as
> various subsystem maintainers had zero tolerance for any measurable
> changes in refcounting performance.
>

Ah, of course.

> I much prefer the full coverage, even if it's a tiny bit slower. And
> based on the worse-case timings (where literally nothing else is
> happening) it seems like these changes should be WELL under the noise.
>

 Agreed.