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Message-ID: <CA+55aFwO8KZD_tZwqwL05FbzpKW4Ucd88C0tcR7LJ1utuy3WGg@mail.gmail.com>
Date: Tue, 20 Mar 2018 16:26:52 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: Al Viro <viro@...iv.linux.org.uk>, Florian Weimer <fweimer@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>, Randy Dunlap <rdunlap@...radead.org>,
Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Ingo Molnar <mingo@...nel.org>,
David Laight <David.Laight@...lab.com>, Ian Abbott <abbotti@....co.uk>,
linux-input <linux-input@...r.kernel.org>, linux-btrfs <linux-btrfs@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()
On Tue, Mar 20, 2018 at 4:23 PM, Linus Torvalds
<torvalds@...ux-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@....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
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