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    Subject[PATCH AUTOSEL 5.5 420/542] btrfs: safely advance counter when looking up bio csums
    Date
    From: David Sterba <dsterba@suse.com>

    [ Upstream commit 4babad10198fa73fe73239d02c2e99e3333f5f5c ]

    Dan's smatch tool reports

    fs/btrfs/file-item.c:295 btrfs_lookup_bio_sums()
    warn: should this be 'count == -1'

    which points to the while (count--) loop. With count == 0 the check
    itself could decrement it to -1. There's a WARN_ON a few lines below
    that has never been seen in practice though.

    It turns out that the value of page_bytes_left matches the count (by
    sectorsize multiples). The loop never reaches the state where count
    would go to -1, because page_bytes_left == 0 is found first and this
    breaks out.

    For clarity, use only plain check on count (and only for positive
    value), decrement safely inside the loop. Any other discrepancy after
    the whole bio list processing should be reported by the exising
    WARN_ON_ONCE as well.

    Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
    Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/btrfs/file-item.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
    index b1bfdc5c1387a..6f18333e83c33 100644
    --- a/fs/btrfs/file-item.c
    +++ b/fs/btrfs/file-item.c
    @@ -274,7 +274,8 @@ static blk_status_t __btrfs_lookup_bio_sums(struct inode *inode, struct bio *bio
    csum += count * csum_size;
    nblocks -= count;
    next:
    - while (count--) {
    + while (count > 0) {
    + count--;
    disk_bytenr += fs_info->sectorsize;
    offset += fs_info->sectorsize;
    page_bytes_left -= fs_info->sectorsize;
    --
    2.20.1
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