On Tue, May 25, 2021 at 05:39:49PM -0700, Andy Lutomirski wrote: > On 5/21/21 9:23 AM, Sargun Dhillon wrote: > > Andy pointed out that we need a mechanism to determine whether or > > notifications are preempted. He suggested we use EPOLLPRI to indicate > > whether or not notifications are preempted. My outstanding question is > > whether or not we need to be able to get insight of what caused the > > preemption, and to which notification. > > > > In the past, Christian has suggested just background polling > > notification IDs for validity, which is a fine mechanism to determine > > that preemption has occurred. We could raise EPOLLPRI whenever a > > notification has changed into the preempted state, but that would > > require an O(n) operations across all outstanding notifications to > > determine which one was preempted, and in addition, it doesn't give a > > lot of information as to why the preemption occurred (fatal signal, > > preemption?). > > > > In order to try to break this into small parts, I suggest: > > 1. We make it so EPOLLPRI is raised (always) on preempted notifications > > 2. We allow the user to set a flag to "track" notifications. If they > > specify this flag, they can then run a "stronger" ioctl -- let's say > > SECCOMP_IOCTL_NOTIF_STATUS, which, if the flag was specified upon > > receiving the notification will return the current state of the > > notification and if a signal preempted it, it will always do that. > > > > --- > > Alternatively (and this is my preference), we add another filter flag, In general this sounds good to me. > > like SECCOMP_FILTER_FLAG_NOTIF_PREEMPT, which changes the behaviour And make it combinable with SECCOMP_FILTER_FLAG_NEW_LISTENER, I like that. > > to: > > 1. Raise EPOLLPRI on preempted notifications > > 2. All preemption notifications must be cleared via > > SECCOMP_IOCTL_NOTIF_RECV_STATUS. > > This seems sensible, except I don't think "preempted" is the right word. > The state machine is pretty simple: > > live -> signaled -> killed > > (and we can go straight from live to killed, too.) So EPOLLPRI could be > signaled if any notification changes state, and a new ioctl could read > the list of notifications that have changed state. A common case is will likely be to just rely the status to the supervised task and not even perform some complicated action in the supervisor. So I think a way to optionally combine recv+send at the same time might be a good idea. Either another ioctl which is a combined recv+send or a flag on the SECCOMP_IOCTL_NOTIF_RECV_STATUS. Christian
