f2fs: fix potential overflow

In build_sit_entries(), if valid_blocks in SIT block is smaller than
valid_blocks in journal, for below calculation:

sbi->discard_blks += old_valid_blocks - se->valid_blocks;

There will be two times potential overflow:
- old_valid_blocks - se->valid_blocks will overflow, and be a very
large number.
- sbi->discard_blks += result will overflow again, comes out a correct
result accidently.

Anyway, it should be fixed.

Fixes: d600af236da5 ("f2fs: avoid unneeded loop in build_sit_entries")
Fixes: 1f43e2ad7bff ("f2fs: introduce CP_TRIMMED_FLAG to avoid unneeded discard")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 1576b9d9a172c1f4ed725daacb89fb633ccec4ef..fec698d17d6a5a21e912ef2b05c3b7e9d9d0d254 100644 (file)
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -3714,14 +3714,17 @@ static int build_sit_entries(struct f2fs_sb_info *sbi)
                        } else {
                                memcpy(se->discard_map, se->cur_valid_map,
                                                        SIT_VBLOCK_MAP_SIZE);
-                               sbi->discard_blks += old_valid_blocks -
-                                                       se->valid_blocks;
+                               sbi->discard_blks += old_valid_blocks;
+                               sbi->discard_blks -= se->valid_blocks;
                        }
                }
 
-               if (sbi->segs_per_sec > 1)
+               if (sbi->segs_per_sec > 1) {
                        get_sec_entry(sbi, start)->valid_blocks +=
-                               se->valid_blocks - old_valid_blocks;
+                                                       se->valid_blocks;
+                       get_sec_entry(sbi, start)->valid_blocks -=
+                                                       old_valid_blocks;
+               }
        }
        up_read(&curseg->journal_rwsem);