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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <d2bb7f4a8b214e1f8d50176e6d94b118@AcuMS.aculab.com>
Date:   Mon, 21 Aug 2023 10:18:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kees Cook' <keescook@...omium.org>,
        Jeff Layton <jlayton@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        "linux-hardening@...r.kernel.org" <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-kernel@...r.kernel.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH v2] creds: Convert cred.usage to refcount_t

From: Kees Cook
> Sent: Friday, August 18, 2023 9:25 PM
> 
> 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.

Hmmm IIRC recent Intel x86 cpu never do static branch prediction.
So you can't avoid mis-predicted branches in cold code.

	David

> Peter Zijlstra and Will Deacon spent a lot of
> time making it run smoothly. :)
> 
> -Kees
> 
> --
> Kees Cook

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)