linux-kernel - Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

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Date:   Fri, 16 Mar 2018 20:12:15 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        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 Fri, Mar 16, 2018 at 12:27:23PM -0700, Linus Torvalds wrote:

> But it sure isn't "variable" either as far as the standard is
> concerned, because the standard doesn't even have that concept (it
> uses "variable" for argument numbers and for variables).

Huh?  6.7.5.2p4:

If the size is not present, the array type is an incomplete type.
If the size is * instead of being an expression, the array type is
a variable length array type of unspecified size, which can only be
used in declarations with function prototype scope [footnote]; such
arrays are nonetheless complete types.  If the size is an integer
constant expression and the element type has a known constant size,
the array type is not a variable length array type; otherwise, the
array type is a variable length array type.

footnote: Thus, * can be used only in function declarations that are
not definitions (see 6.7.5.3).

That's C99, straight from N1256.pdf (C99-TC3)...