[Qemu-devel] [PATCH] qcow2: count_contiguous_clusters and compression

[Qemu-devel] [PATCH] qcow2: count_contiguous_clusters and compression

[Max Reitz]

The function is not intended to be used on compressed clusters and will
not work correctly, if used anyway, since L2E_OFFSET_MASK is not the
right mask for determining the offset of compressed clusters. Therefore,
assert that the first cluster is not compressed and always include the
compression flag in the mask of significant flags, i.e., stop the search
as soon as a compressed cluster occurs.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cluster.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 738ff73..d695a2b 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -281,12 +281,15 @@ static int count_contiguous_clusters(uint64_t nb_clusters, int cluster_size,
         uint64_t *l2_table, uint64_t start, uint64_t stop_flags)
 {
     int i;
-    uint64_t mask = stop_flags | L2E_OFFSET_MASK;
-    uint64_t offset = be64_to_cpu(l2_table[0]) & mask;
+    uint64_t mask = stop_flags | L2E_OFFSET_MASK | QCOW2_CLUSTER_COMPRESSED;
+    uint64_t first_entry = be64_to_cpu(l2_table[0]);
+    uint64_t offset = first_entry & mask;
 
     if (!offset)
         return 0;
 
+    assert(qcow2_get_cluster_type(first_entry) != QCOW2_CLUSTER_COMPRESSED);
+
     for (i = start; i < start + nb_clusters; i++) {
         uint64_t l2_entry = be64_to_cpu(l2_table[i]) & mask;
         if (offset + (uint64_t) i * cluster_size != l2_entry) {
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH] qcow2: count_contiguous_clusters and compression

[Kevin Wolf]

Am 25.09.2013 um 17:47 hat Max Reitz geschrieben:
> The function is not intended to be used on compressed clusters and will
> not work correctly, if used anyway, since L2E_OFFSET_MASK is not the
> right mask for determining the offset of compressed clusters. Therefore,
> assert that the first cluster is not compressed and always include the
> compression flag in the mask of significant flags, i.e., stop the search
> as soon as a compressed cluster occurs.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>

This is supposed to be a cleanup patch with no functional change, right?
How about removing explicit QCOW_OFLAG_COMPRESSED arguments from
callers?

Kevin