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Date: Sat, 10 Mar 2018 08:30:40 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Randy Dunlap <rdunlap@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>, linux-kernel <linux-kernel@...r.kernel.org>,
Josh Poimboeuf <jpoimboe@...hat.com>, Rasmus Villemoes <linux@...musvillemoes.dk>,
"Gustavo A. R. Silva" <gustavo@...eddedor.com>, "Tobin C. Harding" <me@...in.cc>,
Steven Rostedt <rostedt@...dmis.org>, Jonathan Corbet <corbet@....net>, Chris Mason <clm@...com>,
Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
"David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
Masahiro Yamada <yamada.masahiro@...ionext.com>, Borislav Petkov <bp@...e.de>,
Ian Abbott <abbotti@....co.uk>, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
Petr Mladek <pmladek@...e.com>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Pantelis Antoniou <pantelis.antoniou@...sulko.com>, Linux Btrfs <linux-btrfs@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()
On Sat, Mar 10, 2018 at 7:33 AM, Kees Cook <keescook@...omium.org> wrote:
>
> Alright, I'm giving up on fixing max(). I'll go back to STACK_MAX() or
> some other name for the simple macro. Bleh.
Oh, and I'm starting to see the real problem.
It's not that our current "min/max()" are broiken. It's that "-Wvla" is garbage.
Lookie here:
int array[(1,2)];
results in gcc saying
warning: ISO C90 forbids variable length array ‘array’ [-Wvla]
int array[(1,2)];
^~~
and that error message - and the name of the flag - is obviously pure garbage.
What is *actually* going on is that ISO C90 requires an array size to
be not a constant value, but a constant *expression*. Those are two
different things.
A constant expression has little to do with "compile-time constant".
It's a more restricted form of it, and has actual syntax requirements.
A comma expression is not a constant expression, for example, which
was why I tested this.
So "-Wvla" is garbage, with a misleading name, and a misleading
warning string. It has nothing to do with "variable length" and
whether the compiler can figure it out at build time, and everything
to do with a _syntax_ rule.
Linus
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