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[libvirt] passing secrets to block devices

We're working on libvirt support for block device authentication [1]. To
authenticate, rbd needs a username and a secret. Normally, to
avoid putting the secret on the command line, you can store the secret
in a file and pass the file to qemu, but when this is automated,
there's no good way to know when the file can be removed. There are
a few ways to pass the secret to qemu that avoid this problem:

1) pass an fd to an unlinked file containing the secret

This is the simplest method, but it sounds like qemu developers don't
like fd passing from libvirt. [2]

2) start guests paused, without disks requiring authentication, then
   use the drive_add monitor command to attach them

This would make disks with authentication somewhat of a special case
in libvirt, but would be simple to implement, and require no qemu changes.

3) start guests paused, then send the secret via a new QMP/HMP
   command (block_set_conf <key> <value>?)

This is a larger change, but it would be more generally useful for
changing configuration at runtime.

What do you think is the best approach?

[1] http://permalink.gmane.org/gmane.comp.file-systems.ceph.devel/4129
[2] http://lists.gnu.org/archive/html/qemu-devel/2011-08/msg02494.html

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