netdev - Re: [RFC PATCH] lib: Introduce generic __cmpxchg

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: Windows password security audit tool. GUI, reports in PDF.

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

Message-Id: <20181102133728.GR4170@linux.ibm.com>
Date:   Fri, 2 Nov 2018 06:37:28 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Trond Myklebust <trondmy@...merspace.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ralf@...ux-mips.org" <ralf@...ux-mips.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "bfields@...ldses.org" <bfields@...ldses.org>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        "linux@...ck-us.net" <linux@...ck-us.net>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "paul.burton@...s.com" <paul.burton@...s.com>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "jhogan@...nel.org" <jhogan@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "arnd@...db.de" <arnd@...db.de>,
        "paulus@...ba.org" <paulus@...ba.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "aryabinin@...tuozzo.com" <aryabinin@...tuozzo.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed

On Fri, Nov 02, 2018 at 10:56:31AM +0000, David Laight wrote:
> From: Paul E. McKenney
> > Sent: 01 November 2018 17:02
> ...
> > And there is a push to define C++ signed arithmetic as 2s complement,
> > but there are still 1s complement systems with C compilers.  Just not
> > C++ compilers.  Legacy...
> 
> Hmmm... I've used C compilers for DSPs where signed integer arithmetic
> used the 'data registers' and would saturate, unsigned used the 'address
> registers' and wrapped.
> That was deliberate because it is much better to clip analogue values.

There are no C++ compilers for those DSPs, correct?  (Some of the
C++ standards commmittee members believe that they have fully checked,
but they might well have missed something.)

> Then there was the annoying cobol run time that didn't update the
> result variable if the result wouldn't fit.
> Took a while to notice that the sum of a list of values was even wrong!
> That would be perfectly valid for C - if unexpected.

Heh!  COBOL and FORTRAN also helped fund my first pass through university.

> > > But for us using -fno-strict-overflow which actually defines signed
> > > overflow
> 
> I wonder how much real code 'strict-overflow' gets rid of?
> IIRC gcc silently turns loops like:
> 	int i; for (i = 1; i != 0; i *= 2) ...
> into infinite ones.
> Which is never what is required.

The usual response is something like this:

	for (i = 1; i < n; i++)

where the compiler has no idea what range of values "n" might take on.
Can't say that I am convinced by that example, but at least we do have
-fno-strict-overflow.

							Thanx, Paul