Hi, when full_discard is false in discard_in_l2_slice() then the selected cluster should be deallocated and it should read back as zeroes. This is done by clearing the cluster offset field and setting 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 previous data from the backing file. This can be trivially reproduced like this: qemu-img create -f qcow2 backing.img 64k qemu-io -c 'write -P 0xff 0 64k' backing.img qemu-img create -f qcow2 -o compat=0.10 -b backing.img top.img qemu-io -c 'write -P 0x01 0 64k' top.img After this, top.img is filled with 0x01. Now we issue a discard command: qemu-io -c 'discard 0 64k' top.img top.img should now read as zeroes, but instead you get the data from the backing file (0xff). If top.img was created with compat=1.1 instead (the default) then it would read as zeroes after the discard. This seems like a bug to me, and I would simply forbid using discard in this case (see below). The other user of full_discard = false is qcow2_snapshot_create() but I think that one is safe and should be allowed? --- a/block/qcow2.c +++ b/block/qcow2.c @@ -3763,6 +3763,10 @@ static coroutine_fn int qcow2_co_pdiscard(BlockDriverState *bs, int ret; BDRVQcow2State *s = bs->opaque; + if (s->qcow_version < 3) { + return -ENOTSUP; + } + if (!QEMU_IS_ALIGNED(offset | bytes, s->cluster_size)) { assert(bytes < s->cluster_size); /* Ignore partial clusters, except for the special case of the Berto