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

[Linus Torvalds]

On Sun, Apr 24, 2022 at 2:37 PM Linus Torvalds
<torvalds at 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