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

[David Laight]

From: Linus Torvalds
> Sent: 24 April 2022 22:42
> 
> 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).

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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