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Subject: Re: [PATCH] [RFC] bcache: hide variable-sized types from uapi headers
To:     Arnd Bergmann <arnd@kernel.org>,
        Kent Overstreet <kmo@daterainc.com>
Cc:     Arnd Bergmann <arnd@arndb.de>, Jens Axboe <axboe@kernel.dk>,
        linux-kernel@vger.kernel.org
References: <20210927124331.1487876-1-arnd@kernel.org>
From:   Coly Li <colyli@suse.de>
Message-ID: <7d90109f-7e25-ae2d-0dc9-80618593a3e4@suse.de>
Date:   Mon, 27 Sep 2021 23:22:16 +0800
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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
>