Re: robinhood, fanotify name info events and lustre changelog

[Amir Goldstein <amir73il@gmail.com>]

> > > > With fanotify FAN_CREATE event, for example, the parent fid + name
> > > > information should be used by the rbh adapter code to call
> > > > name_to_handle_at(2) and get the created object's file handle.
> > > >
> > > > The reason we made this API choice is because fanotify events should
> > > > not be perceived as a sequence of changes that can be followed to
> > > > describe the current state of the filesystem.
> > > > fanotify events should be perceived as a "poll" on the namespace.
> > > > Whenever notified of a change, application should read the current state
> > > > for the filesystem. fanotify events provide "just enough" information, so
> > > > reading the current state of the filesystem is not too expensive.
>
> I am a little worried about objects that would move around constantly and thus
> "evade" name_to_handle_at(). A bad actor could try to hide a setuid binary this
> way... Of course they could also just copy/delete the file repeatedly and in
> this case having the fid becomes useless, but it seems harder to do, and it is
> likely it would take more time than a simple rename.
>

I am not following. This threat model sounds bogus, but I am not a security
expert, and fanotify async events shouldn't have anything to do with security.

If you can write a concrete use case and explain how your application
wants to handle it and why it cannot without the missing object fid information
I get give a serious answer.

> > > > When fanotify event FAN_MODIFY reports a change of file size,
> > > > along with parent fid + name, that do not match the parent/name robinhood
> > > > knows about (i.e. because the event is received out of order with rename),
> > > > you may use that information to create rbh_fsevent_ns_xattr event to
> > > > update the path or you may wait for the FAN_MOVE_SELF event that
> > > > should arrive later.
> > > > Up to you.
>
> This is making me think: if I receive such a FAN_MODIFY event, and an object
> is moved at parent_fid + name before I query the FS, how can I know which file
> the event was originally meant for?
>

FAN_MODIFY/FAN_ACCESS/FAN_ATTRIB events do have the object_fid in
addition to parent_fid + name.
FAN_CREATE/FAN_DELETE/FAN_MOVE do NOT have the object_fid,
FAN_DELETE_SELF/FAN_MOVE_SELF do have the object_fid
FAN_DELETE_SELF does NOT have parent_fid + name
FAN_MOVE_SELF does have parent_fid + name (new parent/name)

Is there anything missing in your opnion for robinhood to be able to
perform any of its missions?

Thanks,
Amir.