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

[Stafford Horne <shorne@gmail.com>]

On Wed, May 04, 2022 at 01:10:03PM -0700, Linus Torvalds wrote:
> On Wed, May 4, 2022 at 12:58 PM Stafford Horne <shorne@gmail.com> wrote:
> >
> > I have uploaded a diff I created here:
> >   https://gist.github.com/54334556f2907104cd12374872a0597c
> >
> > It shows the same output.
> 
> In hex_to_bin itself it seems to only be a difference due to some
> register allocation (r19 and r3 switched around).
> 
> But then it gets inlined into hex2bin and there changes there seem to
> be about instruction and basic block scheduling, so it's a lot harder
> to see what's going on.
> 
> And a lot of constant changes, which honestly look just like code code
> moved around by 16 bytes and offsets changed due to that.
> 
> So I doubt it's hex_to_bin() that is causing problems, I think it's
> purely code movement. Which explains why adding a nop or a fake printk
> fixes things.
> 
> Some alignment assumption that got broken?

This is what it looks like to me too.  I will have to do a deep dive on what is
going on with this particular build combination as I can't figure out what it is
off the top of my head.

This test is using a gcc 11 compiler, I tried with my gcc 12 toolchain and the
issue cannot be reproduced.

  - musl gcc 11 - https://musl.cc/or1k-linux-musl-cross.tgz
  - openrisc gcc 12 - https://github.com/openrisc/or1k-gcc/releases/tag/or1k-12.0.1-20220210-20220304

But again the difference between the two compiler outputs is a lot of register
allocation and offsets changes.  Its not easy to see anything that stands out.
I checked the change log for the openrisc specific changes from gcc 11 to gcc
12.  Nothing seems to stand out, mcount profiler fix for PIC, a new large binary
link flag.

-Stafford