[Qemu-devel] [PATCH] qcow2: Fix alloc_clusters_noref() overflow detection

[Max Reitz <mreitz@redhat.com>]

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 <mreitz@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