fanotify and LSM path hooks

[Miklos Szeredi]

On Wed, Apr 17, 2019 at 4:06 PM Jan Kara <jack at suse.cz> wrote:
>
> On Wed 17-04-19 14:14:58, Miklos Szeredi wrote:
> > On Wed, Apr 17, 2019 at 1:30 PM Jan Kara <jack at suse.cz> wrote:
> > >
> > > On Tue 16-04-19 21:24:44, Amir Goldstein wrote:
> > > > > I'm not so sure about directory pre-modification hooks. Given the amount of
> > > > > problems we face with applications using fanotify permission events and
> > > > > deadlocking the system, I'm not very fond of expanding that API... AFAIU
> > > > > you want to use such hooks for recording (and persisting) that some change
> > > > > is going to happen and provide crash-consistency guarantees for such
> > > > > journal?
> > > > >
> > > >
> > > > That's the general idea.
> > > > I have two use cases for pre-modification hooks:
> > > > 1. VFS level snapshots
> > > > 2. persistent change tracking
> > > >
> > > > TBH, I did not consider implementing any of the above in userspace,
> > > > so I do not have a specific interest in extending the fanotify API.
> > > > I am actually interested in pre-modify fsnotify hooks (not fanotify),
> > > > that a snapshot or change tracking subsystem can register with.
> > > > An in-kernel fsnotify event handler can set a flag in current task
> > > > struct to circumvent system deadlocks on nested filesystem access.
> > >
> > > OK, I'm not opposed to fsnotify pre-modify hooks as such. As long as
> > > handlers stay within the kernel, I'm fine with that. After all this is what
> > > LSMs are already doing. Just exposing this to userspace for arbitration is
> > > what I have a problem with.
> >
> > There's one more usecase that I'd like to explore: providing coherent
> > view of host filesystem in virtualized environments.  This requires
> > that guest is synchronously notified when the host filesystem changes.
> >   I do agree, however, that adding sync hooks to userspace is
> > problematic.
> >
> > One idea would be to use shared memory instead of a procedural
> > notification.  I.e. application (hypervisor) registers a pointer to a
> > version number that the kernel associates with the given inode.  When
> > the inode is changed, then the version number is incremented.  The
> > guest kernel can then look at the version number when verifying cache
> > validity.   That way perfect coherency is guaranteed between host and
> > guest filesystems without allowing a broken guest or even a broken
> > hypervisor to DoS the host.
>
> Well, statx() and looking at i_version can do this for you. So I guess
> that's too slow for your purposes?

Okay, missing piece of information: we want to make use of the dcache
and icache in the guest kernel, otherwise lookup/stat will be
painfully slow.  That would preclude doing statx() or anything else
that requires a synchronous round trip to the host for the likely case
of a valid cache.

> Also how many inodes do you want to
> monitor like this?

Everything that's in the guest caches.  Which means: a lot.

Thanks,
Miklos