Re: [PATCH v2 0/6] Rework REFCOUNT_FULL using atomic_fetch_* operations

From: Ard Biesheuvel
Date: Sat Aug 31 2019 - 16:54:41 EST

On Sat, 31 Aug 2019 at 22:02, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Sat, Aug 31, 2019 at 08:48:56PM +0300, Ard Biesheuvel wrote:
> > It's been ~2 years since I looked at this code in detail, but IIRC, it
> > looked like the inc-from-zero check was missing from the x86
> > implementation because it requires a load/compare/increment/store
> > sequence instead of a single increment instruction taking a memory
> > operand. Was there more rationale at the time for omitting this
> > particular case, and if so, was it based on a benchmark? Can we run it
> > against this implementation as well?
> It was based on providing a protection against the pre-exploitation case
> (overflow: "something bad is about to happen, let's stop it") rather
> than the post-exploitation case (inc from zero, "something bad already
> happened, eek") with absolutely the fewest possible extra cycles, as
> various subsystem maintainers had zero tolerance for any measurable
> changes in refcounting performance.

Ah, of course.

> I much prefer the full coverage, even if it's a tiny bit slower. And
> based on the worse-case timings (where literally nothing else is
> happening) it seems like these changes should be WELL under the noise.