Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 25 Apr 2022 07:04:05 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     David Laight <David.Laight@ACULAB.COM>
cc:     "'Linus Torvalds'" <torvalds@linux-foundation.org>,
        Andy Shevchenko <andy@kernel.org>,
        device-mapper development <dm-devel@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Mike Snitzer <msnitzer@redhat.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Milan Broz <gmazyland@gmail.com>
Subject: RE: [PATCH] hex2bin: make the function hex_to_bin constant-time
In-Reply-To: <789f0463ce974e90a93f4dbf8c471156@AcuMS.aculab.com>
Message-ID: <alpine.LRH.2.02.2204250701410.10912@file01.intranet.prod.int.rdu2.redhat.com>
References: <alpine.LRH.2.02.2204241648270.17244@file01.intranet.prod.int.rdu2.redhat.com> <CAHk-=wh+Z+OKH3jRttWGHbWSQq2wVMtdnA=ntDiadZu=VxAC7w@mail.gmail.com> <CAHk-=wibmkFz6dybsdpW_3kUnV20FhJazerWDcbm7yCp_Xv+CA@mail.gmail.com>
 <789f0463ce974e90a93f4dbf8c471156@AcuMS.aculab.com>
User-Agent: Alpine 2.02 (LRH 1266 2009-07-14)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Precedence: bulk


On Mon, 25 Apr 2022, David Laight wrote:

> From: Linus Torvalds
> > Sent: 24 April 2022 22:42
> > 
> > On Sun, Apr 24, 2022 at 2:37 PM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > Finally, for the same reason - please don't use ">> 8".  Because I do
> > > not believe that bit 8 is well-defined in your arithmetic. The *sign*
> > > bit will be, but I'm not convinced bit 8 is.
> > 
> > Hmm.. I think it's ok. It can indeed overflow in 'char' and change the
> > sign in bit #7, but I suspect bit #8 is always fine.
> > 
> > Still, If you want to just extend the sign bit, ">> 31" _is_ the
> > obvious thing to use (yeah, yeah, properly "sizeof(int)*8-1" or
> > whatever, you get my drift).
> 
> Except that right shifts of signed values are UB.
> In particular it has always been valid to do an unsigned
> shift right on a 2's compliment negative number.
> 
> 	David

Yes. All the standard versions (C89, C99, C11, C2X) say that right shift 
of a negative value is implementation-defined.

So, we should cast it to "unsigned" before shifting it.

Mikulas