From: David Sterba Date: Wed, 8 Jan 2020 14:29:53 +0000 (+0100) Subject: btrfs: safely advance counter when looking up bio csums X-Git-Tag: v5.5.6~93 X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=44b63fb0a00876281279a55283595e93590b21ee;p=users%2Fdwmw2%2Flinux.git btrfs: safely advance counter when looking up bio csums [ 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 Reviewed-by: Josef Bacik Signed-off-by: David Sterba Signed-off-by: Sasha Levin --- 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 @@ found: 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;