Re: [PATCH v2 0/2] Lock and Pointer guards

Kees Cook
2023-06-06T16:08:56Z

On Tue, Jun 06, 2023 at 08:45:49AM -0700, Linus Torvalds wrote:
> On Tue, Jun 6, 2023 at 8:31 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > nit: Linus's example was "(void *)8" (instead of 1) because we've had
> > issues in the past with alignment warnings on archs that are sensitive
> > to it. (e.g. see the __is_constexpr() macro which is doing NULL/!NULL
> > comparisons.)
>
> Note that I don't think we ever saw such a warning, it was just a
> theoretical observation that depending on type, the compiler might
> warn about known mis-aligned pointer bits.
>
> So I'm not sure the 1-vs-8 actually matters. We do other things that
> assume that low bits in a pointer are retained and valid, even if in
> theory the C type system might have issues with it.
>
> But maybe I mis-remember - if you did get an actual warning, maybe we
> should document that warning just to keep the memory alive.

I've never seen a warning, but since this came up in the dissection of
the __is_constexpr() behavior, it's been burned into my mind. ;)

--
Kees Cook