On Sun, Apr 03, 2011 at 07:06:17PM +0100, Stefan Hajnoczi wrote: > On Sun, Apr 3, 2011 at 2:12 PM, Blue Swirl <blauwir...@gmail.com> wrote: > > On Sun, Apr 3, 2011 at 2:57 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: > >> On Tue, Mar 29, 2011 at 8:04 PM, Stefan Hajnoczi > >> <stefa...@linux.vnet.ibm.com> wrote: > >>> Piggy-back on the guest CD-ROM polling to poll on the host. Open and > >>> close the host CD-ROM file descriptor to ensure we read the new size and > >>> not a stale size. > >>> > >>> Two things are going on here: > >>> > >>> 1. If hald/udisks is not already polling CD-ROMs on the host then > >>> re-opening the CD-ROM causes the host to read the new medium's size. > >>> > >>> 2. There is a bug in Linux which means the CD-ROM file descriptor must > >>> be re-opened in order for lseek(2) to see the new size. The > >>> inode size gets out of sync with the underlying device (which you can > >>> confirm by checking that /sys/block/sr0/size and lseek(2) do not > >>> match after media change). I have raised this with the > >>> maintainers but we need a workaround for the foreseeable future. > >>> > >>> Note that these changes are all in a #ifdef __linux__ section. > >>> > >>> Signed-off-by: Stefan Hajnoczi <stefa...@linux.vnet.ibm.com> > >>> --- > >>> block/raw-posix.c | 26 ++++++++++++++++++++++---- > >>> 1 files changed, 22 insertions(+), 4 deletions(-) > >>> > >>> diff --git a/block/raw-posix.c b/block/raw-posix.c > >>> index 6b72470..8b5205c 100644 > >>> --- a/block/raw-posix.c > >>> +++ b/block/raw-posix.c > >>> @@ -1238,10 +1238,28 @@ static int cdrom_is_inserted(BlockDriverState *bs) > >>> BDRVRawState *s = bs->opaque; > >>> int ret; > >>> > >>> - ret = ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT); > >>> - if (ret == CDS_DISC_OK) > >>> - return 1; > >>> - return 0; > >>> + /* > >>> + * Close the file descriptor if no medium is present and open it to > >>> poll > >>> + * again. This ensures the medium size is refreshed. If the file > >>> + * descriptor is kept open the size can become stale. This is > >>> essentially > >>> + * replicating CD-ROM polling but is driven by the guest. As the > >>> guest > >>> + * polls, we poll the host. > >>> + */ > >>> + > >>> + if (s->fd == -1) { > >>> + s->fd = qemu_open(bs->filename, s->open_flags, 0644); > >>> + if (s->fd < 0) { > >>> + return 0; > >>> + } > >>> + } > >>> + > >>> + ret = (ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT) == > >>> CDS_DISC_OK); > >>> + > >>> + if (!ret) { > >>> + close(s->fd); > >>> + s->fd = -1; > >>> + } > >>> + return ret; > >>> } > >>> > >>> static int cdrom_eject(BlockDriverState *bs, int eject_flag) > >>> -- > >>> 1.7.4.1 > >>> > >>> > >>> > >> > >> There is an issue with reopening host devices in QEMU when running > >> under libvirt. It appears that libvirt chowns image files (including > >> device nodes) so that the launched QEMU process can access them. > >> > >> Unfortunately after media change on host devices udev will reset the > >> ownership of the device node. This causes open(2) to fail with EACCES > >> since the QEMU process does not have the right uid/gid/groups and > >> libvirt is unaware that the file's ownership has changed. > >> > >> In order for media change to work with Linux host CD-ROM it is > >> necessary to reopen the file (otherwise the inode size will not > >> refresh, this is an issue with existing kernels). > >> > >> How can libvirt's security model be made to support this case? In > >> theory udev could be temporarily configured with libvirt permissions > >> for the CD-ROM device while passed through to the guest, but is that > >> feasible? > > > > How about something like this: Add an explicit reopen method to > > BlockDriver. Make a special block device for passed file descriptors. > > Pass descriptors in libvirt for CD-ROMs instead of the device paths. > > The reopen method for file descriptors should notify libvirt about > > need to pass a reopened descriptor and then block all accesses until a > > new descriptor is available. This should also solve your earlier > > problem. > > I'm hoping libvirt's behavior can be made to just work rather than > adding new features to QEMU. But perhaps passing file descriptors is > useful for more than just reopening host devices. This would > basically be a privilege separation model where the QEMU process isn't > able to open files itself but can request libvirt to open them on its > behalf.
It is rather frickin' annoying the way udev resets the ownership when the media merely changes. If it isn't possible to stop udev doing this, then i think the only practical thing is to use ACLs instead of user/group ownership. We wanted to switch to ACLs in libvirt for other reasons already, but it isn't quite as simple as it sounds[1] so we've not done it just yet. Daniel [1] Mostly due to handling upgrades from existing libvirtd while VMs are running, and coping with filesystems which don't support ACLs (or have them turned of by mount options) -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :| -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list