On Fri, 9 Mar 2018 12:05:36 -0800 Kees Cook <keesc...@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
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?
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