Re: [PATCH 2/2] fanotify: Add pidfd support to the fanotify API

On Tue, Apr 27, 2021 at 6:35 AM Matthew Bobrowski [off-list ref] wrote:

On Mon, Apr 26, 2021 at 02:11:30PM +0300, Amir Goldstein wrote:

quoted

quoted

Amir, I was just thinking about this a little over the weekend and I
don't think we discussed how to handle the FAN_REPORT_PIDFD |
FAN_REPORT_FID and friends case? My immediate thought is to make
FAN_REPORT_PIDFD mutually exclusive with FAN_REPORT_FID and friends,
but then again receiving a pidfd along with FID events may be also
useful for some? What are your thoughts on this? If we don't go ahead
with mutual exclusion, then this multiple event types alongside struct
fanotify_event_metadata starts getting a little clunky, don't you
think?

The current format of an fanotify event already supports multiple info records:

[fanotify_event_metadata]
[[fanotify_event_info_header][event record #1]]
[[fanotify_event_info_header][event record #2]]...

(meta)->event_len is the total event length including all info records.

For example, FAN_REPORT_FID | FAN_REPORT_DFID_MAME produces
(for some events) two info records, one FAN_EVENT_INFO_TYPE_FID
record and one FAN_EVENT_INFO_TYPE_DFID_NAME record.

Ah, that's right! I now remember reviewing some patches associated
with the FID change series which mentioned the possibility of
receiving multiple FID info records. As the implementation currently
stands, AFAIK there's not possibility for fanotify to ever return more
than two info records, right?

Right.
Record types FAN_EVENT_INFO_TYPE_DFID_NAME and
FAN_EVENT_INFO_TYPE_DFID are mutually exclusive.

quoted

So I see no problem with combination of FAN_REPORT_FID
and FAN_REPORT_PIDFD.

OK.

Is there any preference in terms of whether the new FAN_REPORT_PIDFD
info records precede or come after FAN_REPORT_FID/FAN_REPORT_DFID_NAME
info records in FAN_REPORT_FID or FAN_REPORT_FID |
FAN_REPORT_DFID_NAME configurations?

Doesn't matter.
Your typical application would first filter by pid/pidfd and only if process
matches the filters would it care to examine the event fid info, correct?
So you go first :)

Thanks,
Amir.