Re: [libvirt] [PATCH] Updated to deal with specifying user IDs to that do not map to usernames
On 06/07/2016 11:15 AM, Peter Krempa wrote:
On Tue, Jun 07, 2016 at 15:50:38 +0100, Daniel Berrange wrote:
> On Tue, Jun 07, 2016 at 08:24:14AM +0200, Peter Krempa wrote:
>> On Mon, Jun 06, 2016 at 14:25:23 -0500, Roy Keene wrote:
>>> Patch to libvirt master to avoid failing when a user ID is specified,
>>> e.g. for <seclabel type='dac'>, that does not map to a user
name.
>>>
>>> This is useful if you want to run each VM as a separate user and not
>>> bother creating an /etc/passwd entry for each UID.
>> For this use case you shall prefix the name with a +. Please refer to
>> the documentation on seclabels.
> Empirically that does not currently work:
>
> # virsh dumpxml serial | grep --after 2 '<seclabel'
> <seclabel type='static' model='dac' relabel='no'>
> <label>+21421:+12421421</label>
> </seclabel>
>
> # virsh start serial
> error: Failed to start domain serial
> error: Cannot access storage file '/var/lib/libvirt/images/demo.qcow2' (as
uid:21421, gid:12421421): Success
>
> # ls -al /var/lib/libvirt/images/demo.qcow2
> -rw-r--r--. 1 21421 12421421 197120 Apr 30 2015 /var/lib/libvirt/images/demo.qcow2
>
>
> Looking at the libvirtd logs we see
>
> 2016-06-07 14:49:13.724+0000: 13490: debug : qemuDomainCheckDiskPresence:3954 :
Checking for disk presence
> 2016-06-07 14:49:16.551+0000: 13490: error : virGetUserEnt:801 : Failed to find user
record for uid '21421'
> 2016-06-07 14:49:16.551+0000: 13490: error : virStorageFileGetMetadataRecurse:3114 :
Cannot access storage file '/var/lib/libvirt/images/demo.qcow2' (as uid:21421,
gid:12421421): Success
>
>
> So even though the QEMU driver has honoured the '+' syntax, some of the
> things QEMU is calling appears to be trying to resolve the UID back into
> a user password record and failing.
Indeed. This is caused by calling virFileAccessibleAs which calls
virGetGroupList so that it can add all groups for the given UID which is
very strange. The group list is then set after forking at attempting to
check file presence. I hate root squashed NFS.
I guess we could just skip reporting the error if we can't get the
group list in that case and just set the provided numerical UID/GID and
try it that way.
Without taking the time to wind through the code - are you saying that
we're attempting to get the grouplist for a different uid while running
as root, then doing a fork/seteuid, and then adding the groups that we
learned pre-fork?
If that's the case, then I think the same results can be achieved by
getting the grouplist *after* seteuid(). It looks like getgroups() gets
the groups for the current uid, while mgetgroups() gets the groups for a
specified username, so getgroups() would avoid the need for a username.