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SubjectRe: [f2fs-dev] [PATCH][next] f2fs: Replace one-element array with flexible-array member
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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@intel.com>
> Link: https://lore.kernel.org/lkml/603647e4.DeEFbl4eqljuwAUe%25lkp@intel.com/
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> 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 */
>

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