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

[Randy Dunlap]

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

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