Amir Goldstein [off-list ref] wrote:

Well I am sure that ring buffer for fanotify events would be useful, so
seeing that David is proposing a generic notification mechanism, I wanted
to know how that mechanism could best share infrastructure with fsnotify.

But apart from that I foresee the questions from users about why the
mount notification API and filesystem events API do not have better
integration.

The way I see it, the notification queue can serve several classes
of notifications and fsnotify could be one of those classes
(at least FAN_CLASS_NOTIF fits nicely to the model).

It could be done; the main thing that concerns me is that the buffer is of
limited capacity.

However, I could take this:

	struct fanotify_event_metadata {
		__u32 event_len;
		__u8 vers;
		__u8 reserved;
		__u16 metadata_len;
		__aligned_u64 mask;
		__s32 fd;
		__s32 pid;
	};

and map it to:

	struct fanotify_notification {
		struct watch_notification watch; /* WATCH_TYPE_FANOTIFY */
		__aligned_u64	mask;
		__u16		metadata_len;
		__u8		vers;
		__u8		reserved;
		__u32		reserved2;
		__s32		fd;
		__s32		pid;
	};

and some of the watch::info bit could be used:

	n->watch.info & WATCH_INFO_OVERRUN	watch queue overran
	n->watch.info & WATCH_INFO_LENGTH	event_len
	n->watch.info & WATCH_INFO_RECURSIVE	FAN_EVENT_ON_CHILD
	n->watch.info & WATCH_INFO_FLAG_0	FAN_*_PERM
	n->watch.info & WATCH_INFO_FLAG_1	FAN_Q_OVERFLOW
	n->watch.info & WATCH_INFO_FLAG_2	FAN_ON_DIR
	n->subtype				ffs(n->mask)

Ideally, I'd dispense with metadata_len, vers, reserved* and set the version
when setting the watch.

	fanotify_watch(int watchfd, unsigned int flags, u64 *mask,
		       int dirfd, const char *pathname, unsigned int at_flags);

We might also want to extend the watch_filter to allow you to, say, filter on
the first __u64 after the watch member so that you could filter on specific
events:

	struct watch_notification_type_filter {
		__u32	type;
		__u32	info_filter;
		__u32	info_mask;
		__u32	subtype_filter[8];
		__u64	payload_mask[1];
		__u64	payload_set[1];
	};

So, in this case, it would require:

	n->mask & wf->payload_mask[0] == wf->payload_set[0]

to be true to record the message.

David