| From: |
| Randy Dunlap <rdunlap-AT-infradead.org> |
| To: |
| Andrew Morton <akpm-AT-linux-foundation.org>, Kees Cook <keescook-AT-chromium.org> |
| Subject: |
| Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max() |
| Date: |
| Fri, 9 Mar 2018 19:11:54 -0800 |
| Message-ID: |
| <e8874944-f0b7-7a1f-26f3-446d7d7fcc38@infradead.org> |
| Cc: |
| linux-kernel-AT-vger.kernel.org, Josh Poimboeuf <jpoimboe-AT-redhat.com>, Rasmus Villemoes <linux-AT-rasmusvillemoes.dk>, "Gustavo A. R. Silva" <gustavo-AT-embeddedor.com>, "Tobin C. Harding" <me-AT-tobin.cc>, Steven Rostedt <rostedt-AT-goodmis.org>, Jonathan Corbet <corbet-AT-lwn.net>, Chris Mason <clm-AT-fb.com>, Josef Bacik <jbacik-AT-fb.com>, David Sterba <dsterba-AT-suse.com>, "David S. Miller" <davem-AT-davemloft.net>, Alexey Kuznetsov <kuznet-AT-ms2.inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji-AT-linux-ipv6.org>, Ingo Molnar <mingo-AT-kernel.org>, Peter Zijlstra <peterz-AT-infradead.org>, Thomas Gleixner <tglx-AT-linutronix.de>, Masahiro Yamada <yamada.masahiro-AT-socionext.com>, Borislav Petkov <bp-AT-suse.de>, Ian Abbott <abbotti-AT-mev.co.uk>, Sergey Senozhatsky <sergey.senozhatsky.work-AT-gmail.com>, Petr Mladek <pmladek-AT-suse.com>, Andy Shevchenko <andriy.shevchenko-AT-linux.intel.com>, Pantelis Antoniou <pantelis.antoniou-AT-konsulko.com>, Linux Btrfs <linux-btrfs-AT-vger.kernel.org>, Network Development <netdev-AT-vger.kernel.org>, Kernel Hardening <kernel-hardening-AT-lists.openwall.com>, Linus Torvalds <torvalds-AT-linux-foundation.org> |
On 03/09/2018 04:07 PM, Andrew Morton wrote:
> On Fri, 9 Mar 2018 12:05:36 -0800 Kees Cook <keescook@chromium.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
