Re: [Qemu-devel] [PATCH v2 4/4] ARRAY_SIZE: check that argument is an array

[Markus Armbruster <armbru@redhat.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> 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.

Yes, please!

> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/qemu/osdep.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 689f253..24bfda0 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -199,7 +199,13 @@ extern int daemon(int, int);
>  #endif
>  
>  #ifndef ARRAY_SIZE
> -#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
> +/*
> + * &(x)[0] is always a pointer - if it's same type as x then the argument is a
> + * pointer, not an array as expected.
> + */
> +#define ARRAY_SIZE(x) ((sizeof(x) / sizeof((x)[0])) + QEMU_BUILD_BUG_ON_ZERO( \
> +                        __builtin_types_compatible_p(typeof(x), \
> +                                                     typeof(&(x)[0]))))

Please break the line near the operator, not within one of its operands:

   #define ARRAY_SIZE(x) ((sizeof(x) / sizeof((x)[0]))			\
                          + QEMU_BUILD_BUG_ON_ZERO(			\
                               __builtin_types_compatible_p(typeof(x),	\
                                                            typeof(&(x)[0]))))
>  #endif
>  
>  int qemu_daemon(int nochdir, int noclose);

With the confusing line break tiedied up:
Reviewed-by: Markus Armbruster <armbru@redhat.com>