Re: [libvirt] [PATCH 4/4] storage: don't follow backing chain symlinks too eagerly

If you have a qcow2 file /path1/to/file pointed to by symlink /path2/symlink, and pass qemu /path2/symlink, then qemu treats a relative backing file in the qcow2 metadata as being relative to /path2, not /path1/to. Yes, this means that it is possible to create a qcow2 file where the choice of WHICH directory and symlink you access its contents from will then determine WHICH backing file (if any) you actually find; the results can be rather screwy, but we have to match what qemu does. Libvirt and qemu default to creating absolute backing file names, so most users don't hit this. But at least VDSM uses symlinks and relative backing names alongside the --reuse-external flags to libvirt snapshot operations, with the result that libvirt was failing to follow the intended chain of backing files, and then backing files were not granted the necessary sVirt permissions to be opened by qemu. See https://bugzilla.redhat.com/show_bug.cgi?id=903248 for more gory details. This fixes a regression introduced in commit 8250783. I tested this patch by creating the following chain: ls /home/eblake/Downloads/Fedora.iso # raw file for base cd /var/lib/libvirt/images qemu-img create -f qcow2 \ -obacking_file=/home/eblake/Downloads/Fedora.iso,backing_fmt=raw one mkdir sub cd sub ln -s ../one onelink qemu-img create -f qcow2 \ -obacking_file=../sub/onelink,backing_fmt=qcow2 two mv two .. ln -s ../two twolink qemu-img create -f qcow2 \ -obacking_file=../sub/twolink,backing_fmt=qcow2 three mv three .. ln -s ../three threelink then pointing my domain at /var/lib/libvirt/images/sub/threelink. Prior to this patch, I got complaints about missing backing files; afterwards, I was able to verify that the backing chain (and hence DAC and SELinux relabels) of the entire chain worked.