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    Subject[PATCH 4.14 015/217] f2fs: fix potential overflow
    Date
    From: Chao Yu <yuchao0@huawei.com>

    commit a9af3fdcc4258af406879eca63d82e9d6baa892e upstream.

    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/f2fs/segment.c | 11 +++++++----
    1 file changed, 7 insertions(+), 4 deletions(-)

    --- a/fs/f2fs/segment.c
    +++ b/fs/f2fs/segment.c
    @@ -3337,14 +3337,17 @@ static int build_sit_entries(struct f2fs
    } 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);


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