linux-kernel - Re: [PATCH v2] hex2bin: make the function hex_to

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Date:   Wed, 4 May 2022 12:15:31 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Andy Shevchenko <andriy.shevchenko@...el.com>
Cc:     Stafford Horne <shorne@...il.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 v2] hex2bin: make the function hex_to_bin constant-time

On Wed, May 04, 2022 at 01:07:26PM +0300, Andy Shevchenko wrote:
> On Wed, May 04, 2022 at 11:57:29AM +0200, Jason A. Donenfeld wrote:
> > On Wed, May 04, 2022 at 11:42:27AM +0200, Jason A. Donenfeld wrote:
> > > So more likely is that this patch just helps unmask a real issue
> > > elsewhere -- linker, compiler, or register restoration after preemption.
> > > I don't think there's anything to do with regards to the patch of this
> > > thread, as it's clearly fine. 
> > 
> > The problem even goes away if I just add a nop...
> 
> Alignment? Compiler bug? HW issue?

Probably one of those, yea. Removing the instruction addresses, the only
difference between the two compiles is: https://xn--4db.cc/Rrn8usaX/diff#line-440

So either there's some alignment going on here, a compiler thing I
haven't spotted yet, or some very fragile interrupt/preemption behavior
that's interacting with this, either on the kernel side or the QEMU
side.

(I've never touched real HW for this; I just got nerd sniped when
wondering why my wireguard CI was failing...)
 
Jason