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Re: [libvirt] [PATCH v2 7/7] qemu_security: Lock metadata while relabelling



On 08/23/2018 05:51 PM, Daniel P. Berrangé wrote:
> On Thu, Aug 23, 2018 at 05:45:42PM +0200, Michal Privoznik wrote:
>> On 08/23/2018 05:01 PM, Daniel P. Berrangé wrote:
>>> On Thu, Aug 23, 2018 at 04:54:01PM +0200, Michal Privoznik wrote:
>>>> On 08/20/2018 07:17 PM, Michal Prívozník wrote:
>>>>> On 08/20/2018 05:16 PM, Daniel P. Berrangé wrote:
>>>>>> On Tue, Aug 14, 2018 at 01:19:43PM +0200, Michal Privoznik wrote:
>>>>>>> Fortunately, we have qemu wrappers so it's sufficient to put
>>>>>>> lock/unlock call only there.
>>>>>>>
>>>>>>> Signed-off-by: Michal Privoznik <mprivozn redhat com>
>>>>>>> ---
>>>>>>>  src/qemu/qemu_security.c | 107 +++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>  1 file changed, 107 insertions(+)
>>>>>>>
>>>>
>>>>


> 
> Oh right, yes, that kills the idea of using a WAIT flag for lockspaces,
> unless we want to introduce threads ingto virtlockd, but we can't do
> that because Linux has totally fubard  locking across execve() :-(

But we need WAIT. I guess we do not want to do:

lockForMetadata(const char *path) {
  int retries;

  while (retries) {
    rc = try_lock(path, wait=false);

    if (!rc) break;
    if (errno = EAGAIN && retries--) {sleep(.1); continue;}

    errorOut();
  }
}

However, if we make sure that virtlockd never forks() nor execs() we
have nothing to fear about. Or am I missing something? And to be 100%
sure we can always provide dummy impl for fork() + execve() in
src/locking/lock_daemon.c which fails everytime.

Michal


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