Messages in this thread | | | From | Andreas Gruenbacher <> | Subject | Re: [GIT PULL] notification tree - try 37! | Date | Tue, 17 Aug 2010 10:09:50 +0200 |
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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 happens when a process watching for FAN_OPEN_PERM or > > FAN_ACCESS_PERM events exits or dies while events are in flight? I > > can't see anything in the code that would wake sleeping processes up > > when the fsnotify_group of the listener is torn down. > > We can get stuck. There was code which cleaned that up, but it got > accidentally removed long ago when, upon review on list, I was told to > remove all timeout code. It's easy enough to fix up. I'll post a patch > this week.
This needs to be fixed then. Not such a big deal, but it shows that the tree wasn't ready for being merged yet and needs further review.
> > 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.
Thanks, Andreas
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