Re: [Qemu-devel] virtio-rng and fd passing

[Anthony Liguori]

Eric Blake <address@hidden> writes:

> Stefan Berger and I discovered on IRC that virtio-rng is unable to
> support fd passing.  We attempted:
>
> qemu-system-x86_64 ... -add-fd set=4,fd=34,opaque=RDONLY:/dev/urandom
> -object rng-random,id=rng0,filename=/dev/fdset/4 -device
> virtio-rng-pci,rng=rng0,bus=pci.0,addr=0x6

Why are you using th rng-random backend instead of the rng-egd backend?

You can pass chardevs to the egd backend.  It's really not a good idea
to pass a fd via rng-rangom.

Regards,

Anthony Liguori

>
> qemu-system-x86_64: -device virtio-rng-pci,rng=rng0,bus=pci.0,addr=0x6:
> Could not open '/dev/fdset/4'
>
> Looks like this code is the culprit, in backends/rng-random.c:
>
> static void rng_random_opened(RngBackend *b, Error **errp)
> {
>     RndRandom *s = RNG_RANDOM(b);
>
>     if (s->filename == NULL) {
>         error_set(errp, QERR_INVALID_PARAMETER_VALUE,
>                   "filename", "a valid filename");
>     } else {
>         s->fd = open(s->filename, O_RDONLY | O_NONBLOCK);
>
> For fd passing to work, we have to use qemu_open() instead of raw
> open().  Is there any way to enforce that all files being opened by qemu
> go through the appropriate qemu_open() wrapper?
>
> Meanwhile, we have a quandary on the libvirt side of things: qemu 1.4
> supports fd passing in general, but does not support it for rng.  I
> guess the same is true for -blockdev - we don't (yet) have a way to do
> fd passing for backing files.  Do we need some sort of QMP command that
> will let libvirt query for a particular device whether that device is
> known to support fd passing, so that libvirt can use fd passing for all
> supported devices, while falling back to older direct open()s, and to
> know which instance of qemu can safely have open() blocked at the
> SELinux or syscall blacklist level?
>
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org