[Libguestfs] [libguestfs BUG] ISO images can't be inspected properly
I discovered that when a VM has an ISO attached via CD-ROM, libguestfs
fails to read its partition table and therefore fails to perform basic
operations on a VM, e.g. listing a directory or editing a file:
# virsh domblklist rocky9-vm
Target Source
---------------------------------------------------------------------------
sda /vz/vmprivate/e827caad-b9e4-4858-b3dc-53ae6b8a5145/harddisk.hdd
sdb /vzt/images/Rocky-9.3-x86_64-minimal.iso
# virt-ls -d rocky9-vm /
libguestfs: error: inspect_os: sgdisk: Invalid partition data!
# virt-edit -d rocky9-vm /etc/fstab
libguestfs: error: inspect_os: sgdisk: Invalid partition data!
Even virt-inspector can't give us proper output on an ISO9660 image:
# virt-inspector /vzt/images/Rocky-9.3-x86_64-minimal.iso
libguestfs: error: inspect_os: sgdisk: Invalid partition data!
virt-inspector: no operating system could be detected inside this disk
image.
...
First I found a 3 years old topic where you suggested that sgdisk itself
might be too old:
https://listman.redhat.com/archives/libguestfs/2021-March/025939.html.
However this doesn't seem to be it in this case:
I've compiled the latest version from
https://sourceforge.net/projects/gptfdisk -- no luck as well. Seems
that sgdisk isn't capable of recognizing ISO9660 images.
Then I noticed 2 things:
1. libguestfs appliance is being created (with backend=libvirt) with ISO
attached as device='disk' (not 'cdrom'):
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2' cache='unsafe'/>
<source file='/tmp/libguestfsD7IHvF/overlay2.qcow2'
index='2'/>
<backingStore type='file' index='5'>
<format type='raw'/>
<source file='/vzt/images/Rocky-9.3-x86_64-minimal.iso'/>
<backingStore/>
</backingStore>
<target dev='sdb' bus='scsi'/>
<alias name='scsi0-0-1-0'/>
<address type='drive' controller='0' bus='0'
target='1' unit='0'/>
</disk>
That forces this disk to be supplied with "scsi-hd" driver in use, which
in turn leads to the device being visible inside the guest as a regular
SCSI hdd /dev/sdb. Shouldn't we, for instance, make sure that for such
images device='cdrom' and "scsi-cd" driver are being used? In this
case
the guest will see it as /dev/sr0, and I suspect srX devices are being
skipped by guestfs_inspect_os.
2. When trying to perform the same operation via guestfish, everything
appears to be working:
# guestfish -a /vzt/images/Rocky-9.3-x86_64-minimal.iso -m /dev/sda ls /
/dev/null && echo $?
0
File edit works as well. From debug output I see that sgdisk isn't
being invoked at all. That means there's a code path for the same
operations where sgdisk can be omitted. Maybe we should follow that
same code path when invoking other tools like virt-ls or virt-edit?
Overall, what would be an appropriate fix in your opinion? We should at
least make sure that working with domains (with the "-d" option) works
no matter what.
Andrey