Re: [libvirt PATCH v2 09/16] nodedev: add an mdevctl thread

On Tue, Aug 25, 2020 at 04:55:06PM +0200, Erik Skultety wrote: > On Tue, Aug 25, 2020 at 01:44:40PM +0100, Daniel P. Berrangé wrote: > > On Tue, Aug 18, 2020 at 09:47:59AM -0500, Jonathon Jongsma wrote: > > > We need to peridocally query mdevctl for changes to device definitions > > > since an administrator can define new devices with mdevctl outside of > > > libvirt. > > > > > > In order to do this, a new thread is created to handle the querying. In > > > the future, mdevctl may add a way to signal device add / remove via > > > events, but for now we resort to a bit of a workaround: monitoring the > > > mdevctl config directories for changes to files. When a change is > > > detected, we query mdevctl and update our device list. > > > > > > Signed-off-by: Jonathon Jongsma <jjongsma(a)redhat.com&gt; > > > --- > > > src/node_device/node_device_udev.c | 182 +++++++++++++++++++++++++++++ > > > 1 file changed, 182 insertions(+) > > > > > > diff --git a/src/node_device/node_device_udev.c b/src/node_device/node_device_udev.c > > > index e06584a3dc..a85418e549 100644 > > > --- a/src/node_device/node_device_udev.c > > > +++ b/src/node_device/node_device_udev.c > > > @@ -41,6 +41,7 @@ > > > #include "virnetdev.h" > > > #include "virmdev.h" > > > #include "virutil.h" > > > +#include <gio/gio.h> > > > > > > #include "configmake.h" > > > > > > @@ -66,6 +67,11 @@ struct _udevEventData { > > > virCond threadCond; > > > bool threadQuit; > > > bool dataReady; > > > + > > > + virThread mdevctlThread; > > > + virCond mdevctlCond; > > > + bool mdevctlReady; > > > + GList *mdevctl_monitors; > > > }; > > > > > > static virClassPtr udevEventDataClass; > > > @@ -85,8 +91,10 @@ udevEventDataDispose(void *obj) > > > udev = udev_monitor_get_udev(priv->udev_monitor); > > > udev_monitor_unref(priv->udev_monitor); > > > udev_unref(udev); > > > + g_list_free_full(priv->mdevctl_monitors, g_object_unref); > > > > > > virCondDestroy(&priv->threadCond); > > > + virCondDestroy(&priv->mdevctlCond); > > > } > > > > > > > > > @@ -117,6 +125,11 @@ udevEventDataNew(void) > > > return NULL; > > > } > > > > > > + if (virCondInit(&ret->mdevctlCond) < 0) { > > > + virObjectUnref(ret); > > > + return NULL; > > > + } > > > + > > > ret->watch = -1; > > > return ret; > > > } > > > @@ -1520,6 +1533,7 @@ nodeStateCleanup(void) > > > virObjectLock(priv); > > > priv->threadQuit = true; > > > virCondSignal(&priv->threadCond); > > > + virCondSignal(&priv->mdevctlCond); > > > virObjectUnlock(priv); > > > virThreadJoin(&priv->th); > > > } > > > @@ -1601,6 +1615,40 @@ udevEventMonitorSanityCheck(udevEventDataPtr priv, > > > } > > > > > > > > > +/* Thread to query mdevctl for the current state of persistent mediated device > > > + * defintions when any changes are detected */ > > > +static > > > +void mdevctlThread(void *opaque G_GNUC_UNUSED) > > > +{ > > > + udevEventDataPtr priv = driver->privateData; > > > + > > > + while (1) { > > > + virObjectLock(priv); > > > + while (!priv->mdevctlReady && !priv->threadQuit) { > > > + if (virCondWait(&priv->mdevctlCond, &priv->parent.lock)) { > > > + virReportSystemError(errno, "%s", > > > + _("handler failed to wait on condition")); > > > + virObjectUnlock(priv); > > > + return; > > > + } > > > + } > > > + > > > + if (priv->threadQuit) { > > > + virObjectUnlock(priv); > > > + return; > > > + } > > > + > > > + virObjectUnlock(priv); > > > + > > > + mdevctlEnumerateDevices(); > > > + > > > + virObjectLock(priv); > > > + priv->mdevctlReady = false; > > > + virObjectUnlock(priv); > > > + } > > > +} > > > + > > > + > > > /** > > > * udevEventHandleThread > > > * @opaque: unused > > > @@ -1708,6 +1756,69 @@ udevEventHandleCallback(int watch G_GNUC_UNUSED, > > > } > > > > > > > > > +static int timeout_id; > > > > Please keep driver state in the driver state struct. > > > > > + > > > +static gboolean > > > +signalMdevctlThread(void *user_data) > > > +{ > > > + udevEventDataPtr priv = user_data; > > > + > > > + if (timeout_id) { > > > + g_source_remove(timeout_id); > > > + timeout_id = 0; > > > + } > > > + > > > + virObjectLock(priv); > > > + priv->mdevctlReady = true; > > > + virCondSignal(&priv->mdevctlCond); > > > + virObjectUnlock(priv); > > > + > > > + return G_SOURCE_REMOVE; > > > +} > > > > I don't see why we need to have a timer that then signals a persistent > > thread. Why cant this just spawn a throw-away thread to do the work > > and avoid all the conditional signaling complexity. > > We sure don't - in that case we'd have to introduce another lock in > mdevctlEnumerateDevices to serialize the threads so that the updates to the > devices are guaranteed to be performed in the order the events come. > Just a question, if we spawn a thread for each event, can't we by any chance > theoretically DOS the host system if there's a malicious process > creating/removing/stating files? Yes, it can but the code is already delaying and merging processing of the events. Also the only person able to inflict such a DoS is root themselves, so I don't think there's risk from malicious attacks here, just broken apps.