Re: [Qemu-devel] [PATCH v3 4/4] ARRAY_SIZE: check that argument is an array

[Eric Blake <eblake@redhat.com>]

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On 01/19/2017 03:07 PM, Michael S. Tsirkin wrote:
> It's a familiar pattern: some code uses ARRAY_SIZE, then refactoring
> changes the argument from an array to a pointer to a dynamically
> allocated buffer.  Code keeps compiling but any ARRAY_SIZE calls now
> return the size of the pointer divided by element size.
> 
> Let's add build time checks to ARRAY_SIZE before we allow more
> of these in the code-base.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
> ---
>  include/qemu/osdep.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 689f253..56c9e22 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -198,8 +198,15 @@ extern int daemon(int, int);
>  #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
>  #endif
>  
> +/*
> + * &(x)[0] is always a pointer - if it's same type as x then the argument is a
> + * pointer, not an array.
> + */
> +#define QEMU_IS_ARRAY(x) (!__builtin_types_compatible_p(typeof(x), \
> +                                                        typeof(&(x)[0])))
>  #ifndef ARRAY_SIZE
> -#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
> +#define ARRAY_SIZE(x) ((sizeof(x) / sizeof((x)[0])) + \
> +                       QEMU_BUILD_BUG_ON_ZERO(!QEMU_IS_ARRAY(x)))

We've got some double-negation going on here ("cause a build bug if the
negation of QEMU_IS_ARRAY() is not 0") which takes some mental
gymnastics, but it is the correct result.  [I kind of like that gnulib
uses positive logic in its 'verify(x)' meaning "verify that x is true,
or cause a build error"; compared to the negative logic in the kernal
'BUILD_BUG_ON[_ZERO](x)' meaning "cause a build bug if x is non-zero" -
but that's personal preference and not something for qemu to change]

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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