If the very first allocation has a length of 0, the free_cluster_index is still 0 after the for loop, which means that subtracting one from it will underflow and signal an invalid range of clusters by returning -EFBIG. However, there is no such range, as its length is 0.
Fix this by preventing underflows on free_cluster_index during the check. Signed-off-by: Max Reitz <mre...@redhat.com> --- block/qcow2-refcount.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c index e79895d..9507aef 100644 --- a/block/qcow2-refcount.c +++ b/block/qcow2-refcount.c @@ -656,7 +656,9 @@ retry: /* Make sure that all offsets in the "allocated" range are representable * in an int64_t */ - if (s->free_cluster_index - 1 > (INT64_MAX >> s->cluster_bits)) { + if (s->free_cluster_index > 0 && + s->free_cluster_index - 1 > (INT64_MAX >> s->cluster_bits)) + { return -EFBIG; } -- 1.9.2