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

[Andrew Morton]

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

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?