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

[Miguel Ojeda]

On Sat, Mar 10, 2018 at 4:11 AM, Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
> On 03/09/2018 04:07 PM, Andrew Morton wrote:
>> On Fri, 9 Mar 2018 12:05:36 -0800 Kees Cook <keescook@xxxxxxxxxxxx> 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 âidsâ [-Wvla]
>>> -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ânamebufâ [-Wvla]
>>> -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array âsymâ [-Wvla]
>>> -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array âbuffâ [-Wvla]
>>> -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array âbuffâ [-Wvla]
>>> -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array âbuff64â [-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