Re: [PATCH] [v2] bcache: hide variable-sized types from uapi header check

[Coly Li]

On 9/28/21 4:55 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> The headers_check helper complains about a GNU extension in
> one of the exported headers:
>
> linux/bcache.h:354:2: warning: field '' with variable sized type 'union jset::(anonymous at ./usr/include/linux/bcache.h:354:2)' not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>          BKEY_PADDED(uuid_bucket);
>          ^
> linux/bcache.h:134:2: note: expanded from macro 'BKEY_PADDED'
>          union { struct bkey key; __u64 key ## _pad[BKEY_PAD]; }
>          ^
>
> Address this by enclosing the GNU extensions in an #ifdef: Since the
> header check is done with --std=c90, this shuts up the warning and makes
> it possible to include the header file C90 user space applications, but
> allows applications built with --std=gnu89 or higher to use those
> parts.
>
> Another alternative would be to exclude this header from the check,
> but the GNU extension check seems more sensible.
>
> Fixes: 81ab4190ac17 ("bcache: Pull on disk data structures out into a separate header")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Hi Arnd,

IMHO, remove bcache related header from uapi check might be better 
solution. So far only bcache-tools uses this header with its own copy, 
no application includes the header(s) so far. It makes sense to exclude 
bcache.h from upai headers check.

Thanks.

Coly Li

> ---
> v2: use __STRICT_ANSI__ check instead of __KERNEL__.
>
> I think this version is better than the first, let me know if that
> works for you.
> ---
>   include/uapi/linux/bcache.h | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
> index cf7399f03b71..b7901e986193 100644
> --- a/include/uapi/linux/bcache.h
> +++ b/include/uapi/linux/bcache.h
> @@ -23,9 +23,13 @@ static inline void SET_##name(type *k, __u64 v)			\
>   struct bkey {
>   	__u64	high;
>   	__u64	low;
> +#ifndef __STRICT_ANSI__
> +	/* gcc extension not meant for user space */
>   	__u64	ptr[];
> +#endif
>   };
>   
> +#ifndef __STRICT_ANSI__
>   #define KEY_FIELD(name, field, offset, size)				\
>   	BITMASK(name, struct bkey, field, offset, size)
>   
> @@ -127,6 +131,8 @@ static inline struct bkey *bkey_idx(const struct bkey *k, unsigned int nr_keys)
>   
>   	return (struct bkey *) (d + nr_keys);
>   }
> +#endif
> +
>   /* Enough for a key with 6 pointers */
>   #define BKEY_PAD		8
>