On Sun, Apr 3, 2011 at 2:57 PM, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote:
> On Tue, Mar 29, 2011 at 8:04 PM, Stefan Hajnoczi
> <stefanha@xxxxxxxxxxxxxxxxxx> wrote:
>> Piggy-back on the guest CD-ROM polling to poll on the host. ÂOpen 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
>> Â 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
>> Â be re-opened in order for lseek(2) to see the new size. ÂThe
>> Â inode size gets out of sync with the underlying device (which you can
>> Â confirm by checking that /sys/block/sr0/size and lseek(2) do not
>> Â match after media change). ÂI have raised this with the
>> Â 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 <stefanha@xxxxxxxxxxxxxxxxxx>
>> ---
>> Âblock/raw-posix.c | Â 26 ++++++++++++++++++++++----
>> Â1 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)
>> Â Â BDRVRawState *s = bs->opaque;
>> Â Â int ret;
>>
>> - Â Âret = ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT);
>> - Â Âif (ret == CDS_DISC_OK)
>> - Â Â Â Âreturn 1;
>> - Â Âreturn 0;
>> + Â Â/*
>> + Â Â * Close the file descriptor if no medium is present and open it to poll
>> + Â Â * again. ÂThis ensures the medium size is refreshed. ÂIf the file
>> + Â Â * descriptor is kept open the size can become stale. ÂThis is essentially
>> + Â Â * replicating CD-ROM polling but is driven by the guest. ÂAs the guest
>> + Â Â * polls, we poll the host.
>> + Â Â */
>> +
>> + Â Âif (s->fd == -1) {
>> + Â Â Â Âs->fd = qemu_open(bs->filename, s->open_flags, 0644);
>> + Â Â Â Âif (s->fd < 0) {
>> + Â Â Â Â Â Âreturn 0;
>> + Â Â Â Â}
>> + Â Â}
>> +
>> + Â Âret = (ioctl(s->fd, CDROM_DRIVE_STATUS, CDSL_CURRENT) == CDS_DISC_OK);
>> +
>> + Â Âif (!ret) {
>> + Â Â Â Âclose(s->fd);
>> + Â Â Â Âs->fd = -1;
>> + Â Â}
>> + Â Âreturn ret;
>> Â}
>>
>> Âstatic 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. ÂIt 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. ÂThis 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? ÂIn
> 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.
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