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qcow2: Avoid feature name extension on small cluster size

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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>
This commit is contained in:
Eric Blake 2020-03-24 12:42:32 -05:00 committed by Max Reitz
parent bb40ebce2c
commit e7be13ad3f
3 changed files with 17 additions and 6 deletions
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11
block/qcow2.c
View File

@ -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,

6
tests/qemu-iotests/036
View File

@ -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 ===

6
tests/qemu-iotests/061
View File

@ -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 ==="