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

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Date:   Mon, 1 Mar 2021 11:17:07 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Chao Yu <chao@...nel.org>, <linux-hardening@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH][next] f2fs: Replace one-element array with
 flexible-array member

On 2021/2/28 13:00, Jaegeuk Kim wrote:
> On 02/25, Chao Yu wrote:
>> 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.
> 
> Merged, thanks.
> This looks better reason than code readability. :)

Agreed.

Reviewed-by: Chao Yu <yuchao0@...wei.com>

Thanks,

> 
>>
>>>
>>> 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@...el.com>
>>> Link: https://lore.kernel.org/lkml/603647e4.DeEFbl4eqljuwAUe%25lkp@intel.com/
>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.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 */
>>>
> .
>