Re: [PATCH] [RFC] bcache: hide variable-sized types from uapi headers

Coly Li
2021-09-27T15:22:26Z

On 9/27/21 8:43 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
>
> 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]; }
> ^
>
> We could either try to shut up the warning or remove those parts from
> the user-visible section of this header. This does the second,
> under the assumption that they are not actually used.

Hi Arnd,

Yes, the variable part is necessary for bcache-tools to understand the
on-disk format. If other program wants to understand the bcache on-disk
format, IMHO such variable part might be necessary too.

BKEY_PADDED() is a special usage of the variable size array. In this
case it indicates uuid_bucket is 8 x u64, and 6 x u64 space for ptr[].
And the bcache code make sure no more than 6 pointers are used for
uuid_bucket.

I know BKEY_PADDED() works, but I don't know how to simply remove the
-Wgnu-variable-sized-type-not-at-end warning.

Maybe Kent may offer some hint ?

Coly Li

>
> Fixes: 81ab4190ac17 ("bcache: Pull on disk data structures out into a separate header")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> Coli, Kent: can you check to see if the hidden parts are used anywhere
> from user space, and apply the patch if not?
>
> Any other ideas for addressing this warning?
> ---
> 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..e3e4889aa53e 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;
> +#ifdef __KERNEL__
> + /* gcc extension not meant for user space */
> __u64 ptr[];
> +#endif
> };
>
> +#ifdef __KERNEL__
> #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
>