[PATCH] btrfs: safely advance counter when looking up bio csums

[PATCH] btrfs: safely advance counter when looking up bio csums

[David Sterba]

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>
Signed-off-by: David Sterba <dsterba@suse.com>
---
 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 bb374042d297..c2f365662d55 100644
--- 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;
-- 
2.24.0

Re: [PATCH] btrfs: safely advance counter when looking up bio csums

[Josef Bacik]

On 1/8/20 9:40 AM, David Sterba wrote:
> 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>
> Signed-off-by: David Sterba <dsterba@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef