fanotify and LSM path hooks

[Jan Kara]

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

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