Re: [f2fs-dev] [PATCH][next] f2fs: Replace one-element array with flexible-array member

[Chao Yu]

Hello, Gustavo,

On 2021/2/25 3:03, Gustavo A. R. Silva wrote:
> There is a regular need in the kernel to provide a way to declare having
> a dynamically sized set of trailing elements in a structure. Kernel code
> should always use “flexible array members”[1] for these cases. The older
> style of one-element or zero-length arrays should no longer be used[2].

I proposal to do the similar cleanup, and I've no objection on doing this.

https://lore.kernel.org/patchwork/patch/869440/

Let's ask for Jaegeuk' opinion.

> Refactor the code according to the use of a flexible-array member in
> struct f2fs_checkpoint, instead of a one-element arrays.
>
> Notice that a temporary pointer to void '*tmp_ptr' was used in order to
> fix the following errors when using a flexible array instead of a one
> element array in struct f2fs_checkpoint:
>
>    CC [M]  fs/f2fs/dir.o
> In file included from fs/f2fs/dir.c:13:
> fs/f2fs/f2fs.h: In function ‘__bitmap_ptr’:
> fs/f2fs/f2fs.h:2227:40: error: invalid use of flexible array member
>   2227 |   return &ckpt->sit_nat_version_bitmap + offset + sizeof(__le32);
>        |                                        ^
> fs/f2fs/f2fs.h:2227:49: error: invalid use of flexible array member
>   2227 |   return &ckpt->sit_nat_version_bitmap + offset + sizeof(__le32);
>        |                                                 ^
> fs/f2fs/f2fs.h:2238:40: error: invalid use of flexible array member
>   2238 |   return &ckpt->sit_nat_version_bitmap + offset;
>        |                                        ^
> make[2]: *** [scripts/Makefile.build:287: fs/f2fs/dir.o] Error 1
> make[1]: *** [scripts/Makefile.build:530: fs/f2fs] Error 2
> make: *** [Makefile:1819: fs] Error 2
>
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays
>
> Link: https://github.com/KSPP/linux/issues/79
> Build-tested-by: kernel test robot <lkp@xxxxxxxxx>
> Link: https://lore.kernel.org/lkml/603647e4.DeEFbl4eqljuwAUe%25lkp@xxxxxxxxx/
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> ---
>   fs/f2fs/f2fs.h          | 5 +++--
>   include/linux/f2fs_fs.h | 2 +-
>   2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index e2d302ae3a46..3f5cb097c30f 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -2215,6 +2215,7 @@ static inline block_t __cp_payload(struct f2fs_sb_info *sbi)
>   static inline void *__bitmap_ptr(struct f2fs_sb_info *sbi, int flag)
>   {
>   	struct f2fs_checkpoint *ckpt = F2FS_CKPT(sbi);
> +	void *tmp_ptr = &ckpt->sit_nat_version_bitmap;
>   	int offset;
>   
>   	if (is_set_ckpt_flags(sbi, CP_LARGE_NAT_BITMAP_FLAG)) {
> @@ -2224,7 +2225,7 @@ static inline void *__bitmap_ptr(struct f2fs_sb_info *sbi, int flag)
>   		 * if large_nat_bitmap feature is enabled, leave checksum
>   		 * protection for all nat/sit bitmaps.
>   		 */
> -		return &ckpt->sit_nat_version_bitmap + offset + sizeof(__le32);
> +		return tmp_ptr + offset + sizeof(__le32);
>   	}
>   
>   	if (__cp_payload(sbi) > 0) {
> @@ -2235,7 +2236,7 @@ static inline void *__bitmap_ptr(struct f2fs_sb_info *sbi, int flag)
>   	} else {
>   		offset = (flag == NAT_BITMAP) ?
>   			le32_to_cpu(ckpt->sit_ver_bitmap_bytesize) : 0;
> -		return &ckpt->sit_nat_version_bitmap + offset;
> +		return tmp_ptr + offset;
>   	}
>   }
>   
> diff --git a/include/linux/f2fs_fs.h b/include/linux/f2fs_fs.h
> index c6cc0a566ef5..5487a80617a3 100644
> --- a/include/linux/f2fs_fs.h
> +++ b/include/linux/f2fs_fs.h
> @@ -168,7 +168,7 @@ struct f2fs_checkpoint {
>   	unsigned char alloc_type[MAX_ACTIVE_LOGS];
>   
>   	/* SIT and NAT version bitmap */
> -	unsigned char sit_nat_version_bitmap[1];
> +	unsigned char sit_nat_version_bitmap[];
>   } __packed;
>   
>   #define CP_CHKSUM_OFFSET	4092	/* default chksum offset in checkpoint */