Am 24.03.2020 um 13:16 hat Alberto Garcia geschrieben: > A discard request deallocates the selected clusters so they read back > as zeroes. This is done by clearing the cluster offset field and > setting QCOW_OFLAG_ZERO in the L2 entry. > > This flag is however only supported when qcow_version >= 3. In older > images the cluster is simply deallocated, exposing any possible stale > data from the backing file. > > Since discard is an advisory operation it's safer to simply forbid it > in this scenario. > > Note that we are adding this check to qcow2_co_pdiscard() and not to > qcow2_cluster_discard() or discard_in_l2_slice() because the last > two are also used by qcow2_snapshot_create() to discard the clusters > used by the VM state. In this case there's no risk of exposing stale > data to the guest and we really want that the clusters are always > discarded. > > Signed-off-by: Alberto Garcia <be...@igalia.com>
The test number 289 is already taken, so this needs a rebase. > +echo > +echo "### Test 'qemu-io -c discard' on a QCOW2 image without a backing file" > +echo > +for qcow2_compat in 0.10 1.1; do > + echo "# Create an image with compat=$qcow2_compat without a backing file" > + _make_test_img -o "compat=$qcow2_compat" 128k > + > + echo "# Fill all clusters with data and then discard them" > + $QEMU_IO -c 'write -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io > + $QEMU_IO -c 'discard 0 128k' "$TEST_IMG" | _filter_qemu_io > + > + echo "# Read the data from the discarded clusters" > + $QEMU_IO -c 'read -P 0x00 0 128k' "$TEST_IMG" | _filter_qemu_io > +done How about adding a 'qemu-img map' call just to have some more direct information about what happened to the allocation status? > + > +echo > +echo "### Test 'qemu-io -c discard' on a QCOW2 image with a backing file" > +echo > + > +echo "# Create a backing image and fill it with data" > +BACKING_IMG="$TEST_IMG.base" > +TEST_IMG="$BACKING_IMG" _make_test_img 128k > +$QEMU_IO -c 'write -P 0xff 0 128k' "$BACKING_IMG" | _filter_qemu_io > + > +for qcow2_compat in 0.10 1.1; do > + echo "# Create an image with compat=$qcow2_compat and a backing file" > + _make_test_img -o "compat=$qcow2_compat" -b "$BACKING_IMG" I would write some non-zero data to the backing file so that you can later distinguish zero clusters from unallocated clusters. > + echo "# Fill all clusters with data and then discard them" > + $QEMU_IO -c 'write -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io > + $QEMU_IO -c 'discard 0 128k' "$TEST_IMG" | _filter_qemu_io > + > + echo "# Read the data from the discarded clusters" > + if [ "$qcow2_compat" = "1.1" ]; then > + # In qcow2 v3 clusters are zeroed (with QCOW_OFLAG_ZERO) > + $QEMU_IO -c 'read -P 0x00 0 128k' "$TEST_IMG" | _filter_qemu_io > + else > + # In qcow2 v2 if there's a backing image we cannot zero the clusters > + # without exposing the backing file data so discard does nothing > + $QEMU_IO -c 'read -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io > + fi > +done Kevin