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discard and v2 qcow2 images
From: |
Alberto Garcia |
Subject: |
discard and v2 qcow2 images |
Date: |
Fri, 20 Mar 2020 19:58:48 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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
- discard and v2 qcow2 images,
Alberto Garcia <=