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

From: Linus Torvalds
Date: Mon Apr 25 2022 - 13:53:46 EST

On Mon, Apr 25, 2022 at 5:07 AM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> We are subtracting values that are in the 0 ... 255 range.

Well, except that's not what the original patch did.

It was subtracting values that were in the -128 ... 255 range (where
the exact range depended on the sign of 'char').

But yeah, I think bit8 was always safe. Probably. Particularly as the
possible ranges across different architectures is bigger than the
range within one _particular_ architecture (so you'll never see "255 -
-128" even when the sign wasn't defined ;)

> > Also, I do worry that this is *exactly* the kind of trick that a
> > compiler could easily turn back into a conditional. Usually compilers
> > tend to go the other way (ie turn conditionals into arithmetic if
> > possible), but..
> Some old version that I tried used "(ch - '0' + 1) * ((unsigned)(ch - '0')
> <= 9)" - it worked with gcc, but clang was too smart and turned it into a
> cmov when compiling for i686 and to a conditional branch when compiling
> for i586.
> Another version used "-(c - '0' + 1) * (((unsigned)(c - '0') >= 10) - 1)"
> - it almost worked, except that clang still turned it into a conditional
> jump on sparc32 and powerpc32.
> So, I came up with this version that avoids comparison operators at all
> and tested it with gcc and clang on all architectures that I could try.

Yeah, the thing about those compiler heuristics is that they are often
based on almost arbitrary patterns that just happen to be what
somebody has found in some benchmark.

Hopefully nobody ever uses something like this as a benchmark.