Re: [RFC PATCH] fanotify: notify on mount attach and detach

[Jan Kara <jack@suse.cz>]

On Tue 03-12-24 14:03:24, Amir Goldstein wrote:
> On Tue, Dec 3, 2024 at 12:40 PM Karel Zak <kzak@redhat.com> wrote:
> > Thank you for working on this.
> >
> > On Thu, Nov 28, 2024 at 03:39:59PM GMT, Miklos Szeredi wrote:
> > > To monitor an entire mount namespace with this new interface, watches need
> > > to be added to all existing mounts.  This can be done by performing
> > > listmount()/statmount() recursively at startup and when a new mount is
> > > added.
> >
> > It seems that maintaining a complete tree of nodes on large systems
> > with thousands of mountpoints is quite costly for userspace. It also
> > appears to be fragile, as any missed new node (due to a race or other
> > reason) would result in the loss of the ability to monitor that part
> > of the hierarchy. Let's imagine that there are new mount nodes added
> > between the listmount() and fanotify_mark() calls. These nodes
> > will be invisible.
> 
> That should not happen if the monitor does:
> 1. set fanotify_mark() on parent mount to get notified on new child mounts
> 2. listmount() on parent mount to list existing children mounts

Right, that works in principle. But it will have all those headaches as
trying to do recursive subtree watching with inotify directory watches
(mounts can also be moved, added, removed, etc. while we are trying to
capture them). It is possible to do but properly handling all the possible
races was challenging to say the least. That's why I have my doubts whether
this is really the interface we want to offer to userspace...

> > It would be beneficial to have a "recursive" flag that would allow for
> > opening only one mount node and receiving notifications for the entire
> > hierarchy. (I have no knowledge about fanotify, so it is possible that
> > this may not be feasible due to the internal design of fanotify.)
> 
> This can be challenging, but if it is acceptable to hold the namespace
> mutex while setting all the marks (?) then maybe.

So for mounts, given the relative rarity of mount / umount events and depth
of a mount tree (compared to the situation with ordinary inodes and
standard fanotify events), I think it might be even acceptable to walk up
the mount tree and notify everybody along that path.

> What should be possible is to set a mark on the mount namespace
> to get all the mount attach/detach events in the mount namespace
> and let userspace filter out the events that are not relevant to the
> subtree of interest.

Or this.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR