linux-kernel - RE: [PATCH] hex2bin: make the function hex_to

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Date:   Mon, 25 Apr 2022 09:37:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Mikulas Patocka <mpatocka@...hat.com>
CC:     Andy Shevchenko <andy@...nel.org>,
        device-mapper development <dm-devel@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Mike Snitzer <msnitzer@...hat.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Milan Broz <gmazyland@...il.com>
Subject: RE: [PATCH] hex2bin: make the function hex_to_bin constant-time

From: Linus Torvalds
> Sent: 24 April 2022 22:42
> 
> On Sun, Apr 24, 2022 at 2:37 PM Linus Torvalds
> <torvalds@...ux-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

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