[PATCH] f2fs: relocate chksum_offset for large_nat_bitmap feature

[PATCH] f2fs: relocate chksum_offset for large_nat_bitmap feature

[Chao Yu]

For large_nat_bitmap feature, there is a design flaw:

Previous:

struct f2fs_checkpoint layout:
+--------------------------+  0x0000
| checkpoint_ver           |
| ......                   |
| checksum_offset          |------+
| ......                   |      |
| sit_nat_version_bitmap[] |<-----|-------+
| ......                   |      |       |
| checksum_value           |<-----+       |
+--------------------------+  0x1000      |
|                          |      nat_bitmap + sit_bitmap
| payload blocks           |              |
|                          |              |
+--------------------------|<-------------+

Obviously, if nat_bitmap size + sit_bitmap size is larger than
MAX_BITMAP_SIZE_IN_CKPT, nat_bitmap or sit_bitmap may overlap
checkpoint checksum's position, once checkpoint() is triggered
from kernel, nat or sit bitmap will be damaged by checksum field.

In order to fix this, let's relocate checksum_value's position
to the head of sit_nat_version_bitmap as below, then nat/sit
bitmap and chksum value update will become safe.

After:

struct f2fs_checkpoint layout:
+--------------------------+  0x0000
| checkpoint_ver           |
| ......                   |
| checksum_offset          |------+
| ......                   |      |
| sit_nat_version_bitmap[] |<-----+
| ......                   |<-------------+
|                          |              |
+--------------------------+  0x1000      |
|                          |      nat_bitmap + sit_bitmap
| payload blocks           |              |
|                          |              |
+--------------------------|<-------------+

Related report and discussion:

https://sourceforge.net/p/linux-f2fs/mailman/message/36642346/

Reported-by: Park Ju Hyung <qkrwngud825@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
---
 fs/f2fs/f2fs.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 31531711e148..f5ffc09705eb 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -1911,7 +1911,11 @@ static inline void *__bitmap_ptr(struct f2fs_sb_info *sbi, int flag)
 	if (is_set_ckpt_flags(sbi, CP_LARGE_NAT_BITMAP_FLAG)) {
 		offset = (flag == SIT_BITMAP) ?
 			le32_to_cpu(ckpt->nat_ver_bitmap_bytesize) : 0;
-		return &ckpt->sit_nat_version_bitmap + offset;
+		/*
+		 * if large_nat_bitmap feature is enabled, leave checksum
+		 * protection for all nat/sit bitmaps.
+		 */
+		return &ckpt->sit_nat_version_bitmap + offset + sizeof(__le32);
 	}
 
 	if (__cp_payload(sbi) > 0) {
-- 
2.18.0.rc1