linux-hardening - Re: [PATCH v2] creds: Convert cred.usage to refcount

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Date:   Fri, 18 Aug 2023 13:24:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>, linux-hardening@...r.kernel.org,
        Elena Reshetova <elena.reshetova@...el.com>,
        David Windsor <dwindsor@...il.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Neil Brown <neilb@...e.de>,
        Olga Kornievskaia <kolga@...app.com>,
        Dai Ngo <Dai.Ngo@...cle.com>, Tom Talpey <tom@...pey.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Alexey Gladkov <legion@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Yu Zhao <yuzhao@...gle.com>, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2] creds: Convert cred.usage to refcount_t

On Fri, Aug 18, 2023 at 04:10:49PM -0400, Jeff Layton wrote:
> [...]
> extra checks (supposedly) compile down to nothing. It should be possible
> to build alternate refcount_t handling functions that are just wrappers
> around atomic_t with no extra checks, for folks who want to really run
> "fast and loose".

No -- there's no benefit for this. We already did all this work years
ago with the fast vs full break-down. All that got tossed out since it
didn't matter. We did all the performance benchmarking and there was no
meaningful difference -- refcount _is_ atomic with an added check that
is branch-predicted away. Peter Zijlstra and Will Deacon spent a lot of
time making it run smoothly. :)

-Kees

-- 
Kees Cook