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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 24 Apr 2022 14:42:28 -0700
Message-ID: <CAHk-=wibmkFz6dybsdpW_3kUnV20FhJazerWDcbm7yCp_Xv+CA@mail.gmail.com>
Subject: Re: [PATCH] hex2bin: make the function hex_to_bin constant-time
To:     Mikulas Patocka <mpatocka@redhat.com>
Cc:     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>
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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).

           Linus