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[PATCH v2 3/4] qcow2: Avoid feature name extension on small cluster size
From: |
Eric Blake |
Subject: |
[PATCH v2 3/4] qcow2: Avoid feature name extension on small cluster size |
Date: |
Tue, 24 Mar 2020 12:42:32 -0500 |
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 <address@hidden>
Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
Reviewed-by: Alberto Garcia <address@hidden>
---
block/qcow2.c | 11 +++++++++--
tests/qemu-iotests/036 | 6 ++++--
tests/qemu-iotests/061 | 6 ++++--
3 files changed, 17 insertions(+), 6 deletions(-)
diff --git a/block/qcow2.c b/block/qcow2.c
index 67b0c214fba4..9475ace57163 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 512598421c20..cf522de7a1aa 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 36b040491fef..ce285d308408 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 ==="
--
2.26.0.rc2
Re: [PATCH for-5.0 v2 0/4] bug fixes extracted from larger qcow2 zero bit work, Max Reitz, 2020/03/26