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Date: Fri, 9 Mar 2018 16:07:19 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: 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>, Randy Dunlap
 <rdunlap@...radead.org>, 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>, Linus Torvalds
 <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in
 min()/max()

On Fri, 9 Mar 2018 12:05:36 -0800 Kees Cook <keescook@...omium.org> wrote:

> When max() is used in stack array size calculations from literal values
> (e.g. "char foo[max(sizeof(struct1), sizeof(struct2))]", the compiler
> thinks this is a dynamic calculation due to the single-eval logic, which
> is not needed in the literal case. This change removes several accidental
> stack VLAs from an x86 allmodconfig build:
> 
> $ diff -u before.txt after.txt | grep ^-
> -drivers/input/touchscreen/cyttsp4_core.c:871:2: warning: ISO C90 forbids variable length array ‘ids’ [-Wvla]
> -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ‘namebuf’ [-Wvla]
> -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array ‘sym’ [-Wvla]
> -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla]
> -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla]
> -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array ‘buff64’ [-Wvla]
> 
> Based on an earlier patch from Josh Poimboeuf.

v1, v2 and v3 of this patch all fail with gcc-4.4.4:

./include/linux/jiffies.h: In function 'jiffies_delta_to_clock_t':
./include/linux/jiffies.h:444: error: first argument to '__builtin_choose_expr' not a constant

That's with

#define __max(t1, t2, x, y)						\
	__builtin_choose_expr(__builtin_constant_p(x) &&		\
			      __builtin_constant_p(y) &&		\
			      __builtin_types_compatible_p(t1, t2),	\
			      (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y),	\
			      __single_eval_max(t1, t2,			\
						__UNIQUE_ID(max1_),	\
						__UNIQUE_ID(max2_),	\
						x, y))
/**
 * max - return maximum of two values of the same or compatible types
 * @x: first value
 * @y: second value
 */
#define max(x, y)	__max(typeof(x), typeof(y), x, y)


A brief poke failed to reveal a workaround - gcc-4.4.4 doesn't appear
to know that __builtin_constant_p(x) is a constant.  Or something.

Sigh.  Wasn't there some talk about modernizing our toolchain
requirements?

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