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@xxxxxxxxxx> > >>>>> --- > >>>>> src/qemu/qemu_security.c | 107 +++++++++++++++++++++++++++++++++++++++++++++++ > >>>>> 1 file changed, 107 insertions(+) > >>>>> > >> > >> > >>>> I'm wondering if this is really the right level to plug in the metadata > >>>> locking ? What about if we just pass a virLockManagerPtr to the security > >>>> drivers and let them lock each resource at the time they need to modify > >>>> its metadata. This will trivially guarantee that we always lock the exact > >>>> set of files that we'll be changing metadata on. > >>>> > >>>> eg in SELinux driver the virSecuritySELinuxSetFileconHelper method > >>>> would directly call virLockManagerAcquire & virLockManagerRelease, > >>>> avoiding the entire virDomainLock abstraction which was really > >>>> focused around managing the content locks around lifecycle state > >>>> changes. > >>>> > >>> > >>> Yeah, I vaguely recall writing code like this (when I was trying to > >>> solve this some time ago). Okay, let me see if that's feasible. > >>> > >>> But boy, this is getting hairy. > >> > >> So as I'm writing these patches I came to realize couple of things: > >> > >> a) instead of domain PID we should pass libvirtd PID because we want to > >> release the locks if libvirtd dies not domain. > >> > >> b) that, however, leads to a problem because > >> virLockManagerLockDaemonAcquire() closes the socket to virtlockd causing > >> it to kill the owner of the lock, i.e. it kills libvirtd immediately. > > > > This is fine ;-P > > > >> c) even if I hack around b) so that we connect only once for each > >> lock+unlock pair call, we would still connect dozens of times when > >> starting a domain (all the paths we label times all active secdrivers). > >> So we should share connection here too. > > > > Yeah, makes sense. > > > >> Now question is how do do this effectively and cleanly (code-wise). For > >> solving b) we can have a new flag, say VIR_LOCK_MANAGER_KEEP_CONNECT > >> that would cause virLockManagerLockDaemonAcquire() to save the > >> (connection, program, counter) tuple somewhere into lock driver private > >> data so that virLockManagerLockDaemonRelease() called with the same flag > >> can re-use the data. > >> > >> For c) I guess we will need to open the connection in > >> virLockManagerLockDaemonNew(), put the socket FD into event loop so that > >> EOF is caught and initiate reopen in that case. However, I'm not sure if > >> this is desirable - to be constantly connected to virtlockd. > > > > Can we use a model similar to what I did for the shared secondary > > driver connections. > > > > By default a call to virGetConnectNetwork() will open a new connection. > > > > To avoid opening & closing 100's of connections though, some places > > will call virSetConnectNetwork() to store a pre-opened connection in > > a thread local. That stays open until virSetConnectNetwork is called > > again passing in a NULL. > > > > We would put such a cache around any bits of code that triggers > > many connection calls to virlockd. > > Actually, would sharing connection for case c) work? > > Consider the following scenario: two threads A and B starting two > different domains, both of them which want to relabel /dev/blah. > > Now, say thread A gets to lock the path first. It does so, and proceed > to chown(). > > Then thread B wants to lock the same path. It tries to do so, but has to > wait until A unlocks it. However, at this point virtlockd is stuck in > virFileLock() call (it waits for the lock to be released). > > Because virtlockd runs single threaded it doesn't matter anymore that A > will try to unlock the path eventually. virtlockd has deadlocked. > > > I don't see any way around this :( 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() :-( Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
