Re: [PATCH v8 3/8] Use atomic_t for ucounts reference counting

From: Kees Cook
Date: Tue Mar 16 2021 - 15:34:01 EST

On Tue, Mar 16, 2021 at 12:26:05PM -0700, Linus Torvalds wrote:
> Note that the above very intentionally does allow the "we can go over
> the limit" case for another reason: we still have that regular
> *unconditional* get_page(), that has a "I absolutely need a temporary
> ref to this page, but I know it's not some long-term thing that a user
> can force". That's not only our traditional model, but it's something
> that some kernel code simply does need, so it's a good feature in
> itself. That might be less of an issue for ucounts, but for pages, we
> somethines do have "I need to take a ref to this page just for my own
> use while I then drop the page lock and do something else".

Right, get_page() has a whole other set of requirements. :) I just
couldn't find the "we _must_ to get a reference to ucounts" code path,
so I was scratching my head.

> And it's possible that "refcount_t" could use that exact same model,
> and actually then offer that option that ucounts wants, of a "try to
> get a refcount, but if we have too many refcounts, then never mind, I
> can just return an error to user space instead".

Yeah, if there starts to be more of these cases, I think it'd be a
nice addition. And with the recent performance work Will Deacon did on
refcount_t, I think any general performance concerns are met now. But
I'd love to just leave refcount_t alone until we can really show a need
for an API change. :P

Kees Cook