With World IPv6 Day last week there have been a few people trying to setup IPv6 connectivity for their virtual guests with libvirt and KVM. For those whose guests are using bridged networking to their LAN, there is really not much to say on the topic. If your LAN has IPv6 enabled and your virtualization host is getting a IPv6 address, then your guests can get IPv6 addresses in exactly the same manner, since they appear directly on the LAN. For those who are using routed / NATed networking with KVM, via the “default” virtual network libvirt creates out of the box, there is a little more work todo. That is what this blog posting will attempt to illustrate.
Network architecture
Before continuing further, it probably helps to have a simple network architecture diagram
- LAN Router A: This is the machine which provides IPv6 connectivity to machines on the wired LAN. In my case this is a LinkSys WRT 54GL, running OpenWRT, with an IPv6 tunnel + subnet provided by SIXXS
- Virt Host A/B: These are the 2 hosts which run virtual machines. They are initially getting their IPv6 addresses via stateless autoconf, thanks to the RADVD program on LAN Router A sending out route advertisements.
- Virtual Network A/B: These are the libvirt default virtual networks on Virt Hosts A/B respectively. Out of the box, they both come with a IPv4 only, NAT setup under 192.168.122.0/24 using bridge virbr0.
- Virt Guest A/B: These are the virtual machines running on Virt Host A
- Virt Guest C/D: These are the virtual machines running on Virt Host B
The goal is to provide IPv6 connectivity to Virt Guests A, B, C and D, such that every guest can talk to every other guest, every other LAN host and the internet. Every LAN host, should also be able to talk to every Virt Guest, and (firewall permitting) any machines on the entire IPv6 internet should be able to connect to any guest.
Initial network configuration
The initial configuration of the LAN is as follows (NB: my real network addresses have been substituted for fake ones). Details on this kind of setup can be found in countless howtos on the web, so I won’t get into details on how to configure your LAN/tunnel. All that’s important here is the network addressing in use
LAN Router A
The network interface for the AICCU tunnel to my SIXXS POP is called ‘sixxs’, and was assigned the address 2000:beef:cafe::1/64. The ISP also assigned a nice large subnet for use on the site 2000:dead:beef::/48. The interface for the Wired LAN is called ‘br-lan’. Individual networks are usually assigned a /64, so with the /48 assigned, I have enough addresses to setup 65336 different networks within my site. Skipping the ‘0’ network, for sake of clarity, we give the Wired LAN the second network, 2000:dead:beef:1::/64, and configure the interface ‘br-lan’ with the global IPv6 address 2000:dead:beef:1::1/64. It also has a link local address of fe80::200:ff:fe00:0/64. To allow hosts on the Wired LAN to get Ipv6 addresses, the router is also running the radvd daemon, advertising the network prefix. The radvd config on the router looks like
interface br-lan
{
AdvSendAdvert on;
AdvManagedFlag off;
AdvOtherConfigFlag off;
prefix 2000:dead:beef:1::/64
{
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr off;
};
};
The address/route configuration looks like this
# ip -6 addr
1: lo: <LOOPBACK,UP>
inet6 ::1/128 scope host
2: br-lan: <BROADCAST,MULTICAST,UP>
inet6 fe80::200:ff:fe00:0/64 scope link
inet6 2000:dead:beef:1::1/64 scope global
# ip -6 route
2000:beef:cafe::/64 dev sixxs metric 256 mtu 1280 advmss 1220
2000:dead:beef:1::/64 dev br-lan metric 256 mtu 1500 advmss 1440
fe80::/64 dev br-lan metric 256 mtu 1500 advmss 1440
ff00::/8 dev br-lan metric 256 mtu 1500 advmss 1440
default via 2000:beef:cafe:1 dev sixxs metric 1024 mtu 1280 advmss 1220
Virt Host A
The network interface for the wired LAN is called ‘eth0’ and is getting an address via stateless autoconf. The NIC has a MAC address 00:11:22:33:44:0a, so it has a link-local IPv6 address of fe80::211:22ff:fe33:440a/64, and via auto-config gets a global address of 2000:dead:beef:1:211:22ff:fe33:440a/64. The address/route configuration looks like
# ip -6 addr
1: lo: mtu 16436
inet6 ::1/128 scope host
3: wlan0: mtu 1500 qlen 1000
inet6 2000:dead:beaf:1:211:22ff:fe33:440a/64 scope global dynamic
inet6 fe80::211:22ff:fe33:440a/64 scope link
# ip -6 route
2000:dead:beaf:1::/64 dev wlan0 proto kernel metric 256 mtu 1500 advmss 1440
fe80::/64 dev wlan0 proto kernel metric 256 mtu 1500 advmss 1440
default via fe80::200:ff:fe00:0 dev wlan0 proto static metric 1024 mtu 1500 advmss 1440
Virt Host B
The network interface for the wired LAN is called ‘eth0’ and is getting an address via stateless autoconf. The NIC has a MAC address 00:11:22:33:44:0b, so it has a link-local IPv6 address of fe80::211:22ff:fe33:440b/64, and via auto-config gets a global address of 2000:dead:beef:1:211:22ff:fe33:440b/64.
# ip -6 addr
1: lo: mtu 16436
inet6 ::1/128 scope host
3: wlan0: mtu 1500 qlen 1000
inet6 2000:dead:beaf:1:211:22ff:fe33:440b/64 scope global dynamic
inet6 fe80::211:22ff:fe33:440b/64 scope link
# ip -6 route
2000:dead:beaf:1::/64 dev wlan0 proto kernel metric 256 mtu 1500 advmss 1440
fe80::/64 dev wlan0 proto kernel metric 256 mtu 1500 advmss 1440
default via fe80::200:ff:fe00:0 dev wlan0 proto static metric 1024 mtu 1500 advmss 1440
Adjusted network configuration
Both Virt Host A and B have a virtual network, so the first thing that needs to be done is to assign a IPv6 subnet for each of them. The /48 subnet has enough space to create 65336 /64 networks, so we just need to pick a couple more. In this we’ll assign 2000:dead:beef:a::/64 to the default virtual network on Virt Host A, and 2000:dead:beef:b::/64 to Virt Host B.
LAN Router A
The first configuration task is to tell the LAN router how each of these new networks can be reached. This requires adding a static route for each subnet, directing traffic to the link local address of the respective Virt Host. You might think you can use the global address of the virt host, rather than its link local address. I had tried that at first, but strange things happened, so sticking to the link local addresses seems best
2000:dead:beef:a::/64 via fe80::211:22ff:fe33:440a dev br-lan metric 256 mtu 1500 advmss 1440
2000:dead:beef:b::/64 via fe80::211:22ff:fe33:440b dev br-lan metric 256 mtu 1500 advmss 1440
One other thing to beware of is that the ‘ip6tables’ FORWARD chain may have a rule which prevents forwarding of traffic. Make sure traffic can flow to/from the 2 new subnets. In my case I added a generic rule that allows any traffic that originates on br-lan, to be sent back on br-lan. Traffic from ‘sixxs’ (the public internet) is only allowed in if it is related to an existing connection, or a whitelisted host I own.
Virt Host A
Remember how all hosts on the wired LAN can automatically configure their own IPv6 addresses / routes using stateless autoconf, thanks to radvd. Well, unfortunately, now that the virt host is going to start routing traffic to/from the guest you can’t use autoconf anymore :-( So the first job is to alter the host eth0 configuration, so that it uses a statically configured IPv6 address for eth0, or gets an address from DHCPv6. If you skip this bit, things may appear to work fine at first, but next time you reconnect to the LAN, autoconf will fail.
Now that the host is not using autoconf, it is time to reconfigure libvirt to add IPv6 to the virtual network. To do this, we stop the virtual network, and then edit its XML
# net-destroy default
# net-edit default
....vi launches..
In the editor, we need to insert a snippet of XML giving details of the subnet assigned to this host, 2000:dead:beef:a::/64. Again we pick address ‘1’ for the host interface, virbr0.
<ip family='ipv6' address='2000:dead:beef:a::1' prefix='64'/>
After exiting the editor, simply start the network again
# net-start default
If all went to plan, virbr0 will now have an IPv6 address, and a route will be present. There will also be an radvd process running on the host advertising the network.
/usr/sbin/radvd --debug 1 --config /var/lib/libvirt/radvd/default-radvd.conf --pidfile /var/run/libvirt/network/default-radvd.pid-bin
If you look at the auto-generated configuration file, it should contain something like
interface virbr0
{
AdvSendAdvert on;
AdvManagedFlag off;
AdvOtherConfigFlag off;
prefix 2a01:348:157:1::1/64
{
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr off;
};
};
Virt Host B
As with the previous Virt Host A, the first configuration task is to switch the host eth0 from using autoconf, to a static IPv6 address configuration, or DHCPv6. Then it is simply a case of running the same virsh commands, but using this host’s subnet, 2000:dead:beef:b::/64
<ip family='ipv6' address='2000:dead:beef:b::1' prefix='64'/>
The guest setup
At this stage, it should be possible to ping & ssh to the address of the ‘virbr0’ interface of each virt host, from anywhere on the wired LAN. If this isn’t working, or if ping works, but ssh fails, then most likely the LAN router has missing/incorrect routes for the new subnets, or there is a firewall blocking traffic on the LAN router, or Virt Host A/B. In particular check the FORWARD chain in ip6tables.
Assuming, this all works though, it should now be possible to start some guests on Virt Host A / B. In this case the guest will of course have a NIC configuration that uses the ‘default’ network:
<interface type='network'>
<mac address='52:54:00:e6:1f:01'/>
<source network='default'/>
</interface>
Starting up this guest, autoconfiguration should take place, resulting in it getting an address based on the virtual network prefix and the MAC address
# ip -6 addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436
inet6 ::1/128 scope host
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
inet6 2000:dead:beef:a:5054:ff:fee6:1f01/64 scope global dynamic
inet6 fe80::5054:ff:fee6:1f01/64 scope link
# ip -6 route
2000:dead:beef:a::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440
fe80::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440
default via fe80::211:22ff:fe33:440a dev eth0 proto kernel metric 1024 mtu 1500 advmss 1440
A guest running on Host A ought to be able to connect to a guest on Host B, and vica-verca, and any host on the LAN should be able to connect to any guest.
A common question from developers/users new to lbvirt, is to wonder what the benefit of using libvirt is, over directly scripting/developing against QEMU/KVM. This blog posting attempts to answer that, by outlining features exposed in the libvirt QEMU/KVM driver that would not be automatically available to users of the lower level QEMU command line/monitor.
“All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us?”
Insurance against QEMU/KVM being replaced by new $SHINY virt thing
Linux virtualization technology has been through many iterations. In the beginning there was User Mode Linux, which was widely used by many ISPs offering Linux virtual hosting. Then along came Xen, which was shipped by many enterprise distributions and replaced much usage of UML. QEMU has been around for a long time, but when KVM was added to the Linux kernel and integrated with QEMU, it became the new standard for Linux host virtualization, often replacing the usage of Xen. Application developers may think
“QEMU/KVM is the Linux virtualization standard for the future, so why do we need a portable API?”
To think this way is ignoring the lessons of history, which are that every virtualization technology that has existed in Linux thus far has been replaced or rewritten or obsoleted. The current QEMU/KVM userspace implementation may turn out to be the exception to the rule, but do you want to bet on that? There is already a new experimental KVM userspace application being developed by a group of LKML developers which may one day replace the current QEMU based KVM userspace. libvirt exists to provide application developers an insurance policy should this come to pass, by isolating application code from the specific implementation of the low level userspace infrastructure.
Long term stable API and XML format
The libvirt API is an append-only API, so once added an existing API wil never be removed or changed. This ensures that applications can expect to operate unchanged across multiple major RHEL releases, even if the underlying virtualization technology changes the way it operates. Likewise the XML format is append-only, so even if the underlying virt technology changes its configuration format, apps will continue to work unchanged.
Automatic compliance with changes in KVM command line syntax “best practice”
As new releases of KVM come out, the “best practice” recommendations for invoking KVM processes evolve. The libvirt QEMU driver attempts to always follow the latest “best practice” for invoking the KVM version it finds
Example: Disk specification changes
- v1: Original QEMU syntax:
-hda filename
- v2: RHEL5 era KVM syntax
-drive file=filename,if=ide,index=0
- v3: RHEL6 era KVM syntax
-drive file=filename,id=drive-ide0-0-0,if=none \
-device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide-0-0-0
- v4: (possible) RHEL-7 era KVM syntax:
-blockdev ...someargs... \
-device ide-drive,bus=ide.0,unit=0,blockdev=bdev-ide0-0-0,id=ide-0-0-0
Isolation against breakage in KVM command line syntax
Although QEMU attempts to maintain compatibility of command line flags across releases, there may been cases where options have been removed / changed. This has typically occurred as a result of changes which were in the KVM branch, and then done differently when the code was merged back into the main QEMU GIT repository. libvirt detects what is required for the QEMU binary and uses the appropriate supported syntax:
Example: Boot order changes for virtio.
- v1: Original QEMU syntax (didn’t support virtio)
-boot c
- v2: RHEL5 era KVM syntax
-drive file=filename,if=virtio,index=0,boot=on
- v3: RHEL-6.0 KVM syntax
-drive file=filename,id=drive-virtio0-0-0,if=none,boot=on \
-device virtio-blk-pci,bus=pci0.0,addr=1,drive=drive-virtio0-0-0,id=virtio-0-0-0
- v4: RHEL-6.1 KVM syntax (boot=on was removed in the update, changing syntax wrt RHEL-6.0)
-drive file=filename,id=drive-virtio0-0-0,if=none,bootindex=1 \
-device virtio-blk-pci,bus=pci0.0,addr=1,drive=drive-virtio0-0-0,id=virtio-0-0-0
Transparently take advantage of new features in KVM
Some new features introduced in KVM can be automatically enabled / used by the libvirt QEMU driver without requiring changes to applications using libvirt. Applications thus benefit from these new features without expending any further development effort
Example: Stable machine ABI in RHEL-6.0
The application provided XML requests a generic ‘pc’ machine type. libvirt automatically queries KVM to determine the supported machines types and canonicalizes ‘pc’ to the stable versioned machine type ‘pc-0.12’ and preserves this in the XML configuration. Thus even if host is updated to KVM 0.13, existing guests will continue to run with the ‘pc-0.12’ ABI, and thus avoid potential driver breakage or guest OS re-activation. It also makes it is possible to migrate the guest between hosts running different versions of KVM.
Example: Stable PCI device addressing in RHEL-6.0
The application provided XML requests 4 PCI devices (balloon, 2 disks & a NIC). libvirt automatically assigns PCI addresses to these devices and ensures that every time the guest is launched the exact same PCI addresses are used for each device. It will also manage PCI addresses when doing PCI device hotplug and unplug, so the sequence boot; unplug nic; migrate will result in stable PCI addresses on the target host.
Automatic compliance with changes in KVM monitor syntax best practice
In the same way that KVM command line argument “best practice” changes between releases, so does the monitor “best practice”. The libvirt QEMU drivers aims to always be in compliance with the “best practice” recommendations for the version of KVM being used.
Example: monitor protocol changes
- v1: Human parsable monitor in RHEL5 era or earlier KVM
- v2: JSON monitor in RHEL-6.0
- v3: JSON monitor, with HMP passthrough in RHEL-6.1
Isolation against breakage in KVM monitor commands
Upstream QEMU releases have generally avoided changing monitor command syntax, but the KVM fork of QEMU has not been so lucky when merging changes back into mainline QEMU. Likewise, OS distros sometimes introduce custom commands in their branches.
Example: Method for setting VNC/SPICE passwords
- v1: RHEL-5 using the ‘change’ command only for VNC, or ‘set_ticket’ for SPICE
'change vnc 123456'
'set_ticket 123456'
- v2: RHEL-6.0 using the (non-upstream) ‘__com.redhat__set_password’ for SPICE or VNC
{ "execute": "__com.redhat__set_password", "arguments": { "protocol": "spice", "password": "123456", "expiration": "60" } }
- v4: RHEL-6.1 using the upstream ‘set_password’ and ‘expire_password’ commands
{ "execute": "set_password", "arguments": { "protocol": "spice", "password": "123456" } }
{ "execute": "expire_password", "arguments": { "protocol": "spice", "time": "+60" } }
Security isolation with SELinux / AppArmour
When all guests run under the same user ID, any single KVM process which is exploited by a malicious guest, will result in compromise of all guests running on the host. If the management software runs as the same user ID as the KVM processes, the management software can also be exploited.
sVirt integration in libvirt, ensures that every guest is assigned a dedicated security context, which strictly confines what resources it can access. Thus even if a guest can read/write files based on matching UID/GID, the sVirt layer will block access, unless the file / resource was explicitly listed in the guest configuration file.
Security isolation using POSIX DAC
A additional security driver uses traditional POSIX DAC capabilities to isolate guests from the host, by running as an unprivileged UID and GID pair. Future enhancements will run each individual guest under a dedicated UID.
Security isolation using Linux container namespaces
Future will take advantage of recent advances in Linux container namespace capabilities. Every QEMU process will be placed into a dedicate PID namespace, preventing QEMU seeing any processes on the system, should it be exploited. A dedicated network namespace will block access to all host network devices, only allow access to TAP device FDs, preventing an exploited guest from making arbitrary network connections to outside world. When QEMU gains support for a ‘fd:’ disk protocol, a dedicated filesystem namespace will provide a very secure chrooted environment where it can only use file descriptors passed in from libvirt. UID/GID namespaces will allow separation of QEMU UID/GID for each process, even though from the host POV all processes will be under the same UID/GID.
Avoidance of shell code for most operations
Use of the shell for invoking management commands is susceptible to exploit by users by providing data which gets interpreted as shell metadata characters. It is very hard to get escaping correctly applied, so libvirt has an advanced set of APIs for invoking commands which avoid all use of the shell. They are also able to guarantee that no file descriptors from the management layer leak down into the QEMU process where they could be exploited.
Secure remote access to management APIs
The libvirt local library API can be exposed over TCP using TLS, x509 certificates and/or Kerberos. This provides secure remote access to all KVM management APIs, with parity of functionality vs local API usage. The secure remote access is a validated part of the common criteria certifications
Operation audit trails
All operations where there is an association between a virtual machine and a host resource will result in one or more audit records being generated via the Linux auditing subsystem. This provides a clear record of key changes in the virtualization state. This functionality is typically a mandatory requirement for deployment into government / defence / financial organizations.
Security certification for Common Criteria
libvirt’s QEMU/KVM driver has gone through Common Criteria certification. This provides assurance for the host management virtualization stack, which again, is typically a mandatory requirement for deployment into government / defense related organizations. The core components in this certification are the sVirt security model and the audit subsystem integration
Integration with cgroups for resource control
All guests are automatically placed into cgroups. This allows control of CPU schedular priority between guests, block I/O bandwidth tunables, memory utilization tunables. The devices cgroups controller, provides a further line of defence blocking guest access to char & block devices on the host in the unlikely event that another layer of protection fails. The cpu accounting group will also enable querying of the per-physical CPU utilization for guests, not available via any traditional /proc file.
Integration with libnuma for memory/cpu affinity
QEMU does not provide any native support for controlling NUMA affinity via its command line. libvirt integrates with libnuma and/or sched_setaffinity for memory and CPU pinning respectively
CPU compatibility guarantees upon migration
libvirt directly models the host and guest CPU feature sets. A variety of policies are available to control what features become visible to the guest, and strictly validate compatibility of host CPUs across migration. Further APIs are provided to allow apps to query CPU compatibility ahead of time, and to enable computation of common feature sets between CPUs. No comparable API exists elsewhere.
Host network filtering of guest traffic
The libvirt network filter APIs allow definition of a flexible set of policies to control network traffic seen by guests. The filter configurations are automatically translated into a set of rules in ebtables, iptables and
ip6tables. The rules are automatically created when the guest starts and torn down when it stops. The rules are able to automatically learn the guest IP address through ARP traffic, or DHCP responses from a (trusted) DHCP server. The data model is closely aligned with the DMTF industry spec for virtual machine network filtering
Libvirt ships with a default set of filters which can be turned on to provide ‘clean’ traffic from the guest. ie it blocks ARP spoofing and IP spoofing by the guest which would otherwise DOS other guests on the network
Secure migration data transport
KVM does not provide any secure migration capability. The libvirt KVM driver adds support for tunnelling migration data over a secure libvirtd managed data channel
Management of encrypted virtual disks for guests
An API set for managing secret keys enables a secure model for deploying guests to untrusted shared storage. By separating management of the secret keys from the guest configuration, it is possible for the management app to ensure a guest can only be started by an authorized user on an authorized host, and prevent restarts once it shuts down. This leverages the Qcow2 data encryption capabilities and is the only method to securely deploy guest if the storage admins / network is not trusted.
Seamless migration of SPICE clients
The libvirt migration protocol automatically takes invokes the KVM graphics clients migration commands at the right point in the migration process to relocate connected clients
Integration with arbitrary lock managers
The lock manager API allows strict guarantees that a virtual machine’s disks can only be in use by once process in the cluster. Pluggable implementations enable integration with different deployment scenarios. Implementations available can use Sanlock, POSIX locks (fcntl), and in the future DLM (clustersuite). Leases can be explicitly provided by the management application, or zero-conf based on the host disks assigned to the guests
Host device passthrough
Integration between host device management APIs and KVM device assignment allows for safe passthrough of PCI devices from the host. libvirt automatically ensures that the device is detached from the host OS before guest startup, and re-attached after shutdown (or equivalent for hotplug/unplug). PCI topology validation ensures that the devices can be safely reset, with FLR, Power management reset, or secondary bus reset.
Host device API enumeration
APIs for querying what devices are present on the host. This enables applications wishing to perform device assignment, to determine which PCI devices are available on the host and how they are used by the host OS. eg to detect case where user asked to assign a device that is currently used as the host eth0 interface.
NPIV SAN management
The host device APIs allow creation and deletion of NPIV SCSI interfaces. This in turns provides access to sets of LUNs, whcih can be associated with individual guests. This allows the virtual SCSI HBA to “follow” the guest where ever it may be running
Core dump integration with “crash”
The libvirt core dump APIs allows a guest memory dump to be obtained and saved into a format which can then be analysed by the Linux crash tool.
Access to guest dmesg logs even for crashed guests
The libvirt API for peeking into guest memory regions allows the ‘virt-dmesg’ tool to extract recent kernel log messages from a guest. This is possible even when the guest kernel has crashed and thus no guest agent is otherwise available.
Cross-language accessibility of APIs.
Provides an stable API which allows KVM to be managed from C, Perl, Python, Java, C#, OCaml and PHP. Components of the management stack are not tied into all using the same language
Zero-conf host network access
Out of the box libvirt provides a NAT based network which allows guests to have outbound network access on a host, whether running wired, or wireless, or VPN, without needing any administrator setup ahead of time. While QEMU provides the ‘SLIRP’ networking mode, this has very low performance and does not integrate with the networking filters capabilities libvirt also provides, nor does it allow guests to communicate with each other
Storage APIs for managing common host storage configurations
Few APIs existing for managing storage on Linux hosts. libvirt provides a general purpose storage API which allows use of NFS shares; formatting & use of local filesystems; partitioning of block devices; creation of LVM volume groups and creation of logical volumes within them; enumeration of LUNs on SCSI HBAs; login/out of ISCSI servers & access to their LUNs; automatic discovery of NFS exports, LVM volume groups and iSCSI targets; creation and cloning of non-raw disks formats, optionally including encryption; enumeration of multipath devices.
Integration with common monitoring tools
Plugins are available for munin, nagios and collectd to enable performance monitoring of all guests running on a host / in a network. Additional 3rd party monitoring tools can easily integrate by using a readonly libvirt connection to ensure they don’t interfere with the functional operation of the system.
Expose information about virtual machines via SNMP
The libvirt SNMP agent allows information about guests on a host to be exposed via SNMP. This allows industry standard tools to report on what guests are running on each host. Optionally SNMP can also be used to control guests and make changes
Expose information & control of host via CIM
The libvirt CIM agent exposes information about, and allows control of, virtual machines, virtual networks and storage pools via the industry standard DMTF CIM schema for virtualization.
Expose information & control of host via AMQP/QMF
The libvirt AMQP agent (libvirt-qpid) exposes information about, and allows control of, virtual machines, virtual networks and storage pools via the QMF object modelling system, running on top of AMQP. This allows data center wide view & control of the state of virtualization hosts.
The libvirt library uses a number of approaches to versioning to cover various usage scenarios:
- Software release versions
- Package manager versions (RPM)
- pkgconfig version
- libtool version
- ELF library soname/version
- ELF symbol version
The goal of libvirt is to never change the library ABI, in other words, once an API is added, struct declared, macro declared, etc it can never be changed. If an API was found flawed, the existing API may be annotated as deprecated, but it will never be removed. Instead a new alternative API is added.
Each new software release has an associated 3 component version number, eg 0.6.3, 0.8.0, 0.8.1. There is no strict policy on the significance of each component. At time of writing, the major component has never been changed. The minor component is changed when there is significant new functionality added to the library (usually a major collection of new APIs). The macro component is changed in each release and reset to zero when the minor component changes. So from an application developer’s point of view, if they want to use a particular API they look for the software release version that introduced the new API.
The text that follows often uses libvirt as an example. The principals / effects described apply to any ELF library and are not specific to libvirt.
Simple compile time version checking
Applications building against libvirt will make use of the pkgconfig tool to probe for existance of libvirt. The software release version is used, unchanged, as the pkgconfig version number. Thus if an application knows the API it wants was introduced in version 0.8.3, it will typically perform a compile time check like
$ pkg-config --atleast-version=0.8.3 libvirt \
&& echo "Found" || echo "Missing"
Or in autoconf macros
PKG_CHECK_MODULES([LIBVIRT], [libvirt >= 0.8.3])
This will result in compiler/linker flags that look something like
-lvirt
Or
-I/home/berrange/usr/include -L/home/berrange/usr/lib -lvirt
When the compiler links the application to libvirt, it will take the libvirt ELF soname and embed it in the application. The libvirt ELF soname is simply “libvirt.so.0” and since libvirt policy is to maintain ABI, it will never change.
libtool also includes library versioning magic that is intended to allow for version checks that are independent of the software release version. Few libraries ever fully make use of this capability since it is poorly understood, easy to get wrong and doesn’t integrate with the rest of the toolchain / OS distro. libvirt sets libtool version based on the package release version. It won’t be discussed further.
Simple runtime version checking
When an application is launched the ELF loader will attempt to resolve the linked libraries and verify their current soname matches that embedded in the application. Since libvirt will never break ABI, the soname check will always succeed. It should be clear that the soname check is a very weak versioning scheme. The application may have been linked against version 0.8.3, but the installed version it is run against could be 0.6.0 and this mismatch would not be caught by the soname check.
Installation version checking
The software release version is also used unchanged for the package manager version field. In RPM packages there is an extra ‘release’ component appended to the version for extra fine checking.
Thus the package manager can be used to provide stronger runtime version guarantee, by checking the full version number at installation time. The aforementioned example application would typically want to include a dependency
Requires: libvirt >= 0.8.3
The RPM package manager, however, also has support for automatic dependency extraction. For this it will extract the soname from any ELF libraries the application links against and secretly add statements of the form
Requires: libvirt.so.0()(64bit)
The libvirt RPM itself also gains secret statements of the form
Provides: libvirt.so.0()(64bit)
Since this is based on the soname though, these automatic dependencies are a very weak versioning scheme.
Symbol versioning
Most linkers will provide library developers with a way to control which symbols are exported for use by applications. This may be done via annotations in the source file, or more often via a standalone “linker script” which simply whitelists symbols to be exported.
The GNU ELF linker has support for doing more than simply whitelisting symbols. It allows symbols to be grouped together and a version number attached to each group. A group can also be annotated as being a child of another group. In this manner you can create trees of versioned symbols, although most common libraries will only use one path without branching.
The libvirt library uses the software release version to group symbols which were added in that release. The top levels of the libvirt tree are logically arranged like:
+------------------------+
| LIBVIRT_0.0.3 |
+------------------------+
| virConnectClose |
| virConnectOpen |
| virConnectListDomains |
| .... |
+------------------------+
|
V
+------------------------+
| LIBVIRT_0.0.5 |
+------------------------+
| virDomainGetUUID |
| virConnectLookupByUUID |
+------------------------+
|
V
+------------------------+
| LIBVIRT_0.1.0 |
+------------------------+
| virInitialize |
| virNodeGetInfo |
| virDomainReboot |
| .... |
+------------------------+
|
V
....
Runtime version checking with version symbols
When a library is built normally, the ELF headers will contain a list of plain symbol names:
$ eu-readelf -s /usr/lib64/libvirt.so | grep 'GLOBAL DEFAULT' | awk '{print $8}'
virConnectOpen
virConnectClose
virDomainGetUUID
....
When a library is built with versioned symbols though, the name is mangled to include a version string:
$ eu-readelf -s /usr/lib64/libvirt.so | grep 'GLOBAL DEFAULT' | awk '{print $8}'
virConnectOpen@@LIBVIRT_0.0.3
virConnectClose@@LIBVIRT_0.0.3
virDomainGetUUID@@LIBVIRT_0.0.5
....
Similarly when an application is built against a normal library, the ELF headers will contain a list of plain symbol names that it uses:
$ eu-readelf -s /usr/bin/virt-viewer | grep 'GLOBAL DEFAULT' | awk '{print $8}'
virConnectOpenAuth
virDomainFree
virDomainGetID
And as expected, when built against an library with versioned symbols, the application ELF headers will contained symbols mangled to include a version string:
$ eu-readelf -s /usr/bin/virt-viewer | grep 'GLOBAL DEFAULT' | awk '{print $8}'
virConnectOpenAuth@LIBVIRT_0.4.0
virDomainFree@LIBVIRT_0.0.3
virDomainGetID@LIBVIRT_0.0.3
When the application is launched, the linker is now able to perform strict version checking. For each symbol ‘FOO’
- If application listed an unversioned 'FOO'
- If library has an unversioned 'FOO'
=> Success
- Else library has a versioned 'FOO'
=> Success
- Else application listed a versioned 'FOO'
- If library has an unversioned 'FOO'
=> Fail
- Else library has a versioned 'FOO'
- If versions of 'FOO' match
=> Success
- Else versions don't match
=> Fail
Notice that an application with unversioned symbols will work with a versioned libary. A versioned application will not work with an unversioned library. This allows for versioned symbols to be added to be introduced to an existing unversioned library without causing an ABI compatibility problem.
With an application and library both using versioned symbols, the compile time requirement the developer declared to pkgconfig with “libvirt >= 0.8.3” is now effectively encoded in the binary via symbol versioning. Thus a fairly strict check can be performed at runtime to ensure the correct library is installed.
Installation version checking with versioned symbols
Remember that the RPM automatic dependencies extractor will find the ELF soname in libraries/applications. When using versioned symbols, it can go one step further and find the symbol version strings too.
The libvirt RPM will thus automatically get extra versioned dependencies
$ rpm -q libvirt-client --provides | grep libvirt
libvirt.so.0()(64bit)
libvirt.so.0(LIBVIRT_0.0.3)(64bit)
libvirt.so.0(LIBVIRT_0.0.5)(64bit)
libvirt.so.0(LIBVIRT_0.1.0)(64bit)
libvirt.so.0(LIBVIRT_0.1.1)(64bit)
libvirt.so.0(LIBVIRT_0.1.4)(64bit)
libvirt.so.0(LIBVIRT_0.1.5)(64bit)
An application will also automatically get extra data based on the versions of the symbols it links against
$ rpm -q virt-viewer --requires | grep libvirt
libvirt.so.0()(64bit)
libvirt.so.0(LIBVIRT_0.0.3)(64bit)
libvirt.so.0(LIBVIRT_0.0.5)(64bit)
libvirt.so.0(LIBVIRT_0.1.0)(64bit)
libvirt.so.0(LIBVIRT_0.4.0)(64bit)
libvirt.so.0(LIBVIRT_0.5.0)(64bit)
This example shows that this virt-viewer cannot run against this libvirt, since the libvirt library is missing versions 0.4.0 and 0.5.0
Thus, there is now rarely any need for an application developer to manually list dependencies against the libvirt library. ie they can remove any line like
Requires: libvirt >= 0.8.3
RPM will determine the minimum version automatically. This is good, because application developers will often forget to update these manually stated deps. The only possible exception to this, is where the
application needs to avoid a implementation bug in a particular version of libvirt and so request a version that is newer than that indicated by the symbol versions. In practice this isn’t very useful for libvirt, since most libvirt drivers have a client server model and the RPM dependency only validates the client, not the server.
The perils of backporting APIs with versioned symbols
It is fairly common when maintaining software in Linux distributions to want to backport code from a newer release, rather than rebase the entire package. Most maintainers will only attempt this for bug fixes or security fixes, but sometimes there is demand to backport new features, and even new APIs. From the descriptions shown above it should be clear that there are a number of serious downsides associated with backporting new APIs.
Impact on compile time pkgconfig checks
Application developers look at the version where a symbol was introduced and encode that in a pkgconfig check at compile time. If an OS distribution backports a symbol to an earlier version, the application developer has to now create OS-distro specific checks for the presence of the backported API. This is a retrograde step for the application developer, because the primary reason for using pkgconfig is that it is OS-distro *independent*
Next consider what happens to the libvirt.so symbols when an API is backported. There are three possible approaches. Keep the original symbol version, or adopt the symbol version of the release being backported to, or invent a new version. All approaches ultimately fail.
Impact of maintaining the new version with a backport
Consider backporting an API from 0.5.0, to a 0.4.0 release. If the original symbol version is maintained the runtime checks by the ELF library loader will still succeed normally. There is an impact on the install time package manager checks though. Backporting a single API from 0.5.0 release will result in the libvirt-client library gaining
Provides: libvirt.so.0(LIBVIRT_0.5.0)(64bit)
Even if there are many other APIs in 0.5.0 that are not backported. An application which uses an API from the 0.5.0 relase will gain
Requires: libvirt.so.0(LIBVIRT_0.5.0)(64bit)
This application can now be installed against either 0.4.0 + backported API, or 0.5.0. RPM is unable to correctly check requirements at application install time, because it cannot distinguish the two scenarios. The API backport has broken the RPM versioning.
Impact of adopting the older version with a backport
The other option for backporting is thus to adopt the version of the release being backported to. ie change the backported API’s symbol version string from
virEventRegisterImpl@LIBVIRT_0.5.0
To
virEventRegisterImpl@LIBVIRT_0.4.0
The problem here is that an application which was built against a normal libvirt binary will have a requirement for
virEventRegisterImpl@LIBVIRT_0.5.0
Attempting to run this application against the libvirt.so with the backported API will fail, because the linker will see that “LIBVIRT_0.4.0” is not the same as “LIBVIRT_0.5.0”.
All libvirt applications now have to be specially compiled for this OS distro’s libvirt to pick up correct symbol versions. This doesn’t solve the core problem, it merely reverses it. The application binary is now incompatible with a normal libvirt, making it impossible to test it against upstream libvirt releases without rebuilding the application too.
In addition the RPM version checking has once again been broken. An application that needs the backported symbol will have
Requires: libvirt.so.0(LIBVIRT_0.4.0)(64bit)
There is no way for RPM to validate whether this means the plain ‘0.4.0’ release, or the 0.4.0 + backport release.
Changing the symbol version when doing the backport has in effect change the ABI of the library in this particular OS distro.
What is worse, is that this changed symbol version has to be preserved in every single future release of the OS distro. ie applications from RHEL-5 expect to be able to run against and libvirt from RHEL-6, RHEL-7, etc without recompilation.
So changing the symbol version has all the downsides of not changing the symbol version, with an additional permanent maintenance burden of patching versions for every future OS release.
Impact of inventing a new version with a backport
For libraries which only target the Linux ELF loader and no other OS platform there is one other possible trick. The ELF linker script does not require a single linear progression of versioned symbol groups. The tree can have arbitrary branches, provided it remains a DAG. A symbol can also appear in multiple places at once.
This could hypothetically be leveraged during a backport of an API, by introducing a branch in the version tree at which the symbol appears. So instead of the symbol being either
virEventRegisterImpl@LIBVIRT_0.5.0
virEventRegisterImpl@LIBVIRT_0.4.0
It could be (eg based off the RPM release where it was backported).
virEventRegisterImpl@LIBVIRT_0.4.0_1
The RPM will thus gain
Provides: libvirt.so.0(LIBVIRT_0.4.0_1)(64bit)
And applications built against this backport will gain
Requires: libvirt.so.0(LIBVIRT_0.4.0_1)(64bit)
As compared to the first option of maintaining the original symbol version LIBVIRT_0.5.0, the RPM version checking will continue to operate correctly.
This approach, however, still has all the permanent package maintenance problems of the second approach. ie the patch which adds a ‘LIBVIRT_0.4.0_1’ version has to be maintained for every single future release of the OS distro, even once the backport of the actual code has gone. It also means that applications built against the backported library won’t work with an upstream build and vica-versa.
Conclusion for backporting symbols
It has been demonstrated that whatever approach is taken, backporting a API in a library with version symbols, will break either RPM version checking or cause incompatibilities in ELF loader version checking or both. It also requires applications to write OS distro specific checks. For a library that intends to maintain permanent API/ABI stability for application developers this is unacceptable.
The conclusion is clear: never backport APIs from *any* library, especially not one that makes any use of symbol versioning.
If a new API is required, then the entire package should be rebased to the version which includes the desired API.