Re: [PATCH v2] hex2bin: make the function hex_to_bin constant-time

[Jason A. Donenfeld]

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