Read/write access for passthrough (9p) filesystems with libvirt/qemu

9pfilesystemslibvirtqemuvirtualization

I have recently started experimenting with the support for passthrough filesystems in recent versions of KVM/QEMU/libvirt. Making the filesystems available for read access has "just worked", but I'm a little puzzled about how write access is supposed to work. After mounting the filesystem in the guest:

# mount -t 9p -o trans=virtio /fs/data /mnt

I can modify an existing file if it has mode o+w:

# cd /mnt/work
# ls -l foo
-rw-rw-rw-. 1 root root 17 Dec 20 11:16 foo
# cat foo
this is line one
# echo this is line two >> foo
# cat foo
this is line one
this is line two

But regardless of the directory permissions, I cannot create a new file or directory:

# ls -ld /mnt/work
drwxrwxrwx. 2 root root 4096 Dec 20 11:16 /mnt/work
# cd /mnt/work
# touch bar
touch: setting times of `bar': No such file or directory
# mkdir bar
# mkdir: cannot create directory `bar': Operation not permitted

The documentation I've been able to find (e.g., this) does not explicitly address this issue. I'm hoping someone here at Serverfault can help me set up a passthrough filesystem that will let me grant write access to a user in the guest to a host filesystem.

Best Answer

This seems to me like a permission issue on the host:

By default, the qemu/kvm process is started as a non-privileged user (libvirt-qemu in Debian Wheezy). So only files accessible (or writable) by that user are accessible (writable) by the VM guests.

You might try setting ownership of the directory to the user qemu is run as (see user= and group= in /etc/libvirt/qemu.conf)

Maybe this helps in your case?

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