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btrfs: safely advance counter when looking up bio csums

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[ Upstream commit 4babad1019 ]

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>
This commit is contained in:
David Sterba 2020-01-08 15:29:53 +01:00 committed by Greg Kroah-Hartman
parent 957ec72d9e
commit 3c8b2ec559
1 changed files with 2 additions and 1 deletions
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3
fs/btrfs/file-item.c
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@ -275,7 +275,8 @@ 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;