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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 20 Mar 2018 16:26:52 -0700
Message-ID: <CA+55aFwO8KZD_tZwqwL05FbzpKW4Ucd88C0tcR7LJ1utuy3WGg@mail.gmail.com>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()
To:     Kees Cook <keescook@chromium.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Florian Weimer <fweimer@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
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        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Ian Abbott <abbotti@mev.co.uk>,
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On Tue, Mar 20, 2018 at 4:23 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Hmm. So thanks to the diseased mind of Martin Uecker, there's a better
> test for "__is_constant()":
>
>   /* Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de> */
>   #define __is_constant(a) \
>         (sizeof(int) == sizeof(*(1 ? ((void*)((a) * 0l)) : (int*)1)))
>
> that is actually *specified* by the C standard to work, and doesn't
> even depend on any gcc extensions.

Well, it does depend on 'sizeof(*(void *)X)' being 1 and the compiler
not complaining about it, and that sizeof(int) is not 1.

But since we depend on those things in the kernel anyway, that's fine.

                Linus