On Friday 20 August 2010 05:38:17 Eric Paris wrote: > So the actual bugs you have reported, I see two. > > The (nearly) unbounded number of potential outstanding notifications > events is a known situation, pointed out in previous discussions well > before this commit and is one of the (numerous) reasons why fanotify is > at this time CAP_SYS_ADMIN only. It is something that is difficult to > address while still making fanotify useful for permissions gating. But > the issue is clearly noted. Clearly noting and blissfully ignoring the problem is not enough unfortunately; this needs to be addressed now. I have already pointed out (quoted below) that permission gating is not the worst problem here; the worst problem are listeners whose fanotify event queue just grows and grows. There is no throttling, and no guarantee that even a listener which simply reads and completely ignores all events will manage to keep up. The system will run out of memory eventually. Here is a quote from a previous message about this problem that only went to linux-kernel: On Tuesday 17 August 2010 05:39:47 Eric Paris wrote: > On Mon, 2010-08-16 at 22:32 +0200, Andreas Gruenbacher wrote: > > Q: What prevents the system from going out of memory when a listener > > decides to stop reading events or simply can't keep up? There doesn't > > seem to be a limit on the queue depth. Listeners currently need > > CAP_SYS_ADMIN, but somehow limiting the queue depth and throttling when > > things start to go bad still sounds like a reasonable thing to do, > > right? > > It's an interesting question and obviously one that I've thought about. > You remember when we talked previously I said the hardest part left was > allowing non-root users to use the interface. It gets especially > difficult when thinking about perm-events. I was specifically told not > to timeout or drop those. But when dealing with non-root users using > perm events? As for pure notification we can do something like inotify > does quite easily. > > I'm not certain exactly what the best semantics are for non trusted > users, so I didn't push any patches that way. Suggestions welcome :) The system will happily go OOM for trusted users and non-perm events if the listener doesn't keep up, so some throttling, dropping, or both needs to happen for non-perm events. This is the critical case. Doing what inotify does (queue an overflow event and drop further events) seems to make sense here. The situation with perm-events is less severe because the number of outstanding perm events is bounded by the number of running processes. This may be enough of a limit. I don't think we need to worry about perm-events for untrusted users. We can start supporting some kinds of non-perm-events for untrusted users later; this won't change the existing interface. Andreas -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
