On 30.08.2014 00:37, Eric Blake wrote:
On 08/29/2014 03:45 PM, Max Reitz wrote:
A refblock entry may have a different size than 16 bits, it may even be
smaller than a byte. Correct the refcount_block_entries calculation
accordingly.
Signed-off-by: Max Reitz <mre...@redhat.com>
---
docs/specs/qcow2.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index cfbc8b0..531c478 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -183,7 +183,7 @@ blocks and are exactly one cluster in size.
Given a offset into the image file, the refcount of its cluster can be
obtained
as follows:
- refcount_block_entries = (cluster_size / sizeof(uint16_t))
+ refcount_block_entries = (cluster_size / (refcount_bits / 8))
Consider refcount_order == 0 (that is, no shared blocks, ALL blocks have
at most refcount 1). Then, refcount_bits is (1 << 0) == 1. But 1/8 in
integer math truncates to 0 (oops, division by zero is undefined); when
in reality, the expression you want here is (cluster_size * 8 /
refcount_bits).
If it is integer division, that is. ;-)
I'm counting on you accepting "cluster_size * 8 / refcount_bits" and not
rejecting it because of a possible integer overflow. *g*
Max
