btrfs: safely advance counter when looking up bio csums

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>

diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index bb374042d297860fc7853c58ef83909d64645a49..c2f365662d55045cc530fd81f99a2018c6f87f84 100644 (file)
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -283,7 +283,8 @@ 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;