3:17 a.m.
Adding libvir-list to cc. If you come across stuff that involves both
libvirt and qemu it's best to crosspost it to libvir-list too as I've
noticed this only accidentally.
On Mon, Oct 12, 2020 at 15:03:10 -0400, Yoonho Park wrote:
I stumbled on a bug in qemu 4.2.0 (virsh 6.0.0) with a large number
of
overlays. I am using "qemu-img create" and "virsh snapshot-create-as"
to
create each overlay. When I run "virsh snapshot-create-as" for the 42nd
overlay, I get "error: No complete monitor response found in 10485760
This error comes from libvirt's DoS protection. The response from qemu
is too large. This happens when 'query-named-block-nodes' is called on
too-deep backing chains as the reply contains both nested and linear
entries. Thus for a N deep backing chain, you get N + N-1 + N-2 ...
entries in the returned data.
Libvirt stops reading after 1MiB of json to prevent buffer overflows
from a rogue qemu.
bytes: Numerical result out of range". However, I pulled down
qemu 5.1.50
(still using virsh 6.0.0), and it looks like the problem has disappeared
which is great. Does anyone know the patch set that addressed this bug?
qemu patch:
commit facda5443f5a8676fb635b82ac1046ac6b6a67ce
Author: Peter Krempa <pkrempa(a)redhat.com>
Date: Mon Jan 20 09:50:49 2020 +0100
qapi: Allow getting flat output from 'query-named-block-nodes'
When a management application manages node names there's no reason to
recurse into backing images in the output of query-named-block-nodes.
Add a parameter to the command which will return just the top level
structs.
v4.2.0-1584-gfacda5443f
libvirt patches:
commit 95080cc8b470c977d53043d4eff3e30781f472eb
Author: Peter Krempa <pkrempa(a)redhat.com>
Date: Tue Jan 21 16:51:40 2020 +0100
qemu: Don't request nested entries in qemuBlockGetNamedNodeData
Use the 'flat' flag for 'query-named-block-nodes' if qemu supports
QEMU_CAPS_QMP_QUERY_NAMED_BLOCK_NODES_FLAT in qemuBlockGetNamedNodeData.
We don't need the data so plumb in whether qemu supports the
'flat' output.
Signed-off-by: Peter Krempa <pkrempa(a)redhat.com>
Reviewed-by: Ján Tomko <jtomko(a)redhat.com>
commit 855211bbf3ed45a73159f45eba1b15f05d771b76
Author: Peter Krempa <pkrempa(a)redhat.com>
Date: Tue Jan 21 16:42:49 2020 +0100
qemu: monitor: Add 'flat' parameter for qemuMonitorJSONQueryNamedBlockNodes
Modern qemu allows to skip the nested redundant data in the output of
query-named-block-nodes. Plumb in the support for the argument that
enables it.
Signed-off-by: Peter Krempa <pkrempa(a)redhat.com>
Reviewed-by: Ján Tomko <jtomko(a)redhat.com>
commit 63610bd5fbe67ba9eb4f22f67c4bdab6eda8c041
Author: Peter Krempa <pkrempa(a)redhat.com>
Date: Wed Feb 26 12:50:53 2020 +0100
qemuCheckpointDiscardBitmaps: Use qemuBlockGetNamedNodeData
Replace qemuMonitorBlockGetNamedNodeData by qemuBlockGetNamedNodeData.
Signed-off-by: Peter Krempa <pkrempa(a)redhat.com>
Reviewed-by: Ján Tomko <jtomko(a)redhat.com>
commit f886c9f33051fc03dd6c78134c92d31e7caf0c81
Author: Peter Krempa <pkrempa(a)redhat.com>
Date: Tue Jan 21 16:33:12 2020 +0100
qemu: monitor: Refactor variable cleanup in qemuMonitorJSONQueryNamedBlockNodes
Use g_autoptr to get rid of the cleanup section.
Signed-off-by: Peter Krempa <pkrempa(a)redhat.com>
Reviewed-by: Ján Tomko <jtomko(a)redhat.com>
commit b7991c903cdd5b3c8b79a206584a4db81a6d4eaa
Author: Peter Krempa <pkrempa(a)redhat.com>
Date: Tue Jan 21 15:13:47 2020 +0100
qemu: capabilities: Add capability for the 'flat' argument of
'query-named-block-nodes'
Detect the presence of the flag and make it available internally as
QEMU_CAPS_QMP_QUERY_NAMED_BLOCK_NODES_FLAT.
Signed-off-by: Peter Krempa <pkrempa(a)redhat.com>
Reviewed-by: Ján Tomko <jtomko(a)redhat.com>
v6.1.0-29-g95080cc8b4
Also, does anyone know the "official" limit on the number
of overlays that
can be created and is there a qemu test that exercises this? I could not
Libvirt's official limit is 200 layers.
This is the documentation for the validation function explaining the
rationale:
/**
* qemuDomainStorageSourceValidateDepth:
* @src: storage source chain to validate
* @add: offsets the calculated number of images
* @diskdst: optional disk target to use in error message
*
* The XML parser limits the maximum element nesting to 256 layers. As libvirt
* reports the chain into the status and in some cases the config XML we must
* validate that any user-provided chains will not exceed the XML nesting limit
* when formatted to the XML.
*
* This function validates that the storage source chain starting @src is at
* most 200 layers deep. @add modifies the calculated value to offset the number
* to allow checking cases when new layers are going to be added to the chain.
*
* Returns 0 on success and -1 if the chain is too deep. Error is reported.
*/
int
qemuDomainStorageSourceValidateDepth(virStorageSourcePtr src,
int add,
const char *diskdst)
find an overlay limit test in tests/qemu-iotests.
I'd guess qemu doesn't really limit that, but at some point you'll get
too much of a performance penalty form traversing all the structs.