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Date: Fri, 9 Mar 2018 19:11:54 -0800
From: Randy Dunlap <rdunlap@...radead.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
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>, 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 03/09/2018 04:07 PM, Andrew Morton wrote:
> 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
I'm seeing that problem with
> gcc --version
gcc (SUSE Linux) 4.8.5
in mmotm.
> 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?
--
~Randy
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