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

[Mikulas Patocka]

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@xxxxxxxxxxxxxxxxxxxx> 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