f2fs: Replace one-element array with flexible-array member

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].

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/79Build-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>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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;
 	}
 }
 

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 */