Re: [PATCH v9 06/27] rust: add C helpers

Kees Cook
2022-08-18T16:08:45Z

On Thu, Aug 18, 2022 at 06:03:04PM +0200, Miguel Ojeda wrote:
> On Thu, Aug 18, 2022 at 1:56 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > Perfect. It may be worth stating this explicitly with the helper. i.e.
> > "This is for handling any panic!() calls in core Rust, but should not
> > ever be used in the 'kernel' create; failures should be handled."
>
> I am not sure we should say "ever", because there are sometimes
> situations where we statically know a situation is impossible. Of
> course, "impossible" in practice is possible -- even if it is due to a
> single-event upset.
>
> For the "statically impossible" cases, we could simply trigger UB
> instead of panicking. However, while developing and debugging one
> would like to detect bugs as soon as possible. Moreover, in
> production, people may have use cases where killing the world is
> better as soon as anything "funny" is detected, no matter what.

Please, no UB. I will take a panic over UB any day. It'd be best to
handle things with some error path, but those are the rare exception.

> So we could make it configurable, so that "Rust statically impossible
> panics" can be defined as UB, `make_task_dead()` or a full `BUG()`.

C is riddled with UB and it's just terrible. Let's make sure we don't
continue that mistake. :)

> By the way, I should have mentioned the `unwrap()s` too, since they
> are pretty much explicit panics. We don't have any in v9 either, but
> we do have a couple dozens in the full code (in the 97% not submitted)
> in non-test or examples code. Many are of the "statically impossible"
> kind, but any that is not merits some discussion, which we can do as
> we upstream the different pieces.

The simple answer is that if an "impossible" situation can be recovered
from, it should error instead of panic. As long as that's the explicit
design goal, I think we're good. Yes there will be cases where it is
really and truly unrecoverable, but those will be rare and can be well
documented.

--
Kees Cook