qcow2: Avoid feature name extension on small cluster size

As the feature name table can be quite large (over 9k if all 64 bits
of all three feature fields have names; a mere 8 features leaves only
8 bytes for a backing file name in a 512-byte cluster), it is unwise
to emit this optional header in images with small cluster sizes.

Update iotest 036 to skip running on small cluster sizes; meanwhile,
note that iotest 061 never passed on alternative cluster sizes
(however, I limited this patch to tests with output affected by adding
feature names, rather than auditing for other tests that are not
robust to alternative cluster sizes).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200324174233.1622067-4-eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

3 files changed, 17 insertions, 6 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index b74cbeb..2bb536b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2823,8 +2823,15 @@ int qcow2_update_header(BlockDriverState *bs)
         buflen -= ret;
     }
 
-    /* Feature table */
-    if (s->qcow_version >= 3) {
+    /*
+     * Feature table.  A mere 8 feature names occupies 392 bytes, and
+     * when coupled with the v3 minimum header of 104 bytes plus the
+     * 8-byte end-of-extension marker, that would leave only 8 bytes
+     * for a backing file name in an image with 512-byte clusters.
+     * Thus, we choose to omit this header for cluster sizes 4k and
+     * smaller.
+     */
+    if (s->qcow_version >= 3 && s->cluster_size > 4096) {
         static const Qcow2Feature features[] = {
             {
                 .type = QCOW2_FEAT_TYPE_INCOMPATIBLE,
diff --git a/tests/qemu-iotests/036 b/tests/qemu-iotests/036
index 5125984..cf522de 100755
--- a/tests/qemu-iotests/036
+++ b/tests/qemu-iotests/036
@@ -44,8 +44,10 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 _supported_fmt qcow2
 _supported_proto file
 # Only qcow2v3 and later supports feature bits;
-# qcow2.py does not support external data files
-_unsupported_imgopts 'compat=0.10' data_file
+# qcow2.py does not support external data files;
+# this test requires a cluster size large enough for the feature table
+_unsupported_imgopts 'compat=0.10' data_file \
+		     'cluster_size=\(512\|1024\|2048\|4096\)'
 
 echo
 echo === Image with unknown incompatible feature bit ===
diff --git a/tests/qemu-iotests/061 b/tests/qemu-iotests/061
index 36b0404..ce285d3 100755
--- a/tests/qemu-iotests/061
+++ b/tests/qemu-iotests/061
@@ -44,8 +44,10 @@ _supported_os Linux
 # Conversion between different compat versions can only really work
 # with refcount_bits=16;
 # we have explicit tests for data_file here, but the whole test does
-# not work with it
-_unsupported_imgopts 'refcount_bits=\([^1]\|.\([^6]\|$\)\)' data_file
+# not work with it;
+# we have explicit tests for various cluster sizes, the remaining tests
+# require the default 64k cluster
+_unsupported_imgopts 'refcount_bits=\([^1]\|.\([^6]\|$\)\)' data_file cluster_size
 
 echo
 echo "=== Testing version downgrade with zero expansion ==="