linux-kernel - Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

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Message-ID: <CANiq72k_vsTPMXvcfTNo+sjRoVMkihvRz-fzeXdcXvNEEeY-Pw@mail.gmail.com>
Date:   Sat, 10 Mar 2018 07:10:47 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.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>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

On Sat, Mar 10, 2018 at 4:11 AM, Randy Dunlap <rdunlap@...radead.org> wrote:
> 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

Same here, 4.8.5 fails. gcc 5.4.1 seems to work. I compiled a minimal
5.1.0 and it seems to work as well.

Cheers,
Miguel