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

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Date:   Sat, 27 Feb 2021 21:00:51 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
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 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. :)

> 
> > 
> > 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 */
> >