Re: [RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications

David Howells
2019-06-04T12:33:20Z

Amir Goldstein <amir73il@gmail.com> 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