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Date:   Tue, 6 Jun 2023 09:08:32 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, gregkh@linuxfoundation.org,
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Subject: Re: [PATCH v2 0/2] Lock and Pointer guards
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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