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While investigating https://bugzilla.redhat.com/show_bug.cgi?id=1061827 I noticed that we pass user input unscathed for block-pull, but always pass a canonical absolute name through for block-commit. [Note that we probably _ought_ to validate that the user's request for block-pull actually matches the backing chain, the way we already do for block-commit - but that's a separate issue. Further note that the ability to pass user input through unscathed allows backdoors such as specifying a backing image that is a network URI such as a gluster disk, instead of forcing things to the local file system; which is an area still under active investigation on whether libvirt needs to behave differently for network disks.] Since qemu may write the name that the user passed in as the backing file, a user may have a reason to want a relative file name passed through to qemu, and always munging things to absolute prevents that. Put another way, if you have the backing chain: [A] <- [B(back=./A)] <- [C(back=./B)] and commit B into A (virsh blockcommit $dom vda --base A --top B), the metadata of C will have to be re-written. But should it be rewritten as [C(back=./A)] or as [C(back=/path/to/A)]? Still up in the air is whether qemu's decision should be based on whether B and/or C had relative paths, or on whether the --base and/or --top arguments to the command were relative paths; but if we always pass a canonical name, we've prevented the spelling of the command arguments from being part of the hueristics that qemu uses. I also audited the code, and verified that we never call qemuMonitorBlockCommit() with a NULL base, either before or after the change to qemu_driver.c. * src/qemu/qemu_driver.c (qemuDomainBlockCommit): Preserve user's spelling, since absolute vs. relative matters to qemu. * src/qemu/qemu_monitor.h (qemuMonitorBlockCommit): Base is never null. * src/qemu/qemu_monitor.c (qemuMonitorBlockCommit): Likewise. * src/qemu/qemu_monitor_json.h (qemuMonitorJSONBlockCommit): Likewise. * src/qemu/qemu_monitor_json.c (qemuMonitorJSONBlockCommit): Likewise. Signed-off-by: Eric Blake <eblake@redhat.com>
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>