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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 6 Jun 2023 08:45:49 -0700
Message-ID: <CAHk-=wh=vs298o0gewFH725pQAYQ57CAQzj05FOx_VZubZ3jiA@mail.gmail.com>
Subject: Re: [PATCH v2 0/2] Lock and Pointer guards
To:     Kees Cook <keescook@chromium.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, gregkh@linuxfoundation.org,
        pbonzini@redhat.com, linux-kernel@vger.kernel.org,
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On Tue, Jun 6, 2023 at 8:31=E2=80=AFAM Kees Cook <keescook@chromium.org> wr=
ote:
>
> 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.

            Linus