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Re: [libvirt] [Qemu-devel] [PATCH v2 3/3] raw-posix: Re-open host CD-ROM after media change



On Sun, Apr 3, 2011 at 2:57 PM, Stefan Hajnoczi <stefanha gmail com> wrote:
> On Tue, Mar 29, 2011 at 8:04 PM, Stefan Hajnoczi
> <stefanha 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 <stefanha 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.


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