+++ This bug was initially created as a clone of Bug #2181470 +++

Description of problem:

Since the queue size (q_size) can be set to a large value, it's necessary for fapolicyd to ensure the current soft limit won't be reached, or else some undefined behavior may happen (I didn't check what would be the consequence yet).

By default, fapolicyd service use has a soft limit of 1024:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
# cat /proc/1792/limits 
Limit                     Soft Limit           Hard Limit           Units     
[...]
Max open files            1024                 262144               files     
[...]
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Potentially, if a customer sets a larger q_size than RHEL's default (640), e.g. uses the default value from manpage (1024), it's possible the limit gets reached.

Version-Release number of selected component (if applicable):

fapolicyd-1.1.3-8.el8_7.1.x86_64

How reproducible:

N/A

--- Additional comment from Steve Grubb on 2023-03-24 16:07:28 UTC ---

NOFILE gets set to infinity: https://github.com/linux-application-whitelisting/fapolicyd/blob/main/src/daemon/fapolicyd.c#L516

I don't think we have a problem here.

--- Additional comment from Steve Grubb on 2023-03-24 17:20:09 UTC ---

Actually...systemd is the problem. Anyone wanting more can add LimitNOFILE=16383  or whatever they want. I'll add this to the upstream service file. Also looks like systemd prohibits changing priority as well.

--- Additional comment from Takayuki Nagata on 2023-03-27 07:26:20 UTC ---

> Since the queue size (q_size) can be set to a large value, it's necessary for fapolicyd to ensure the current soft limit won't be reached, or else some undefined behavior may happen (I didn't check what would be the consequence yet).

I have just reviewed kernel code, and it appears to me that reading fanotify fd returns -EMFILE when fapolicyd's open files reach the limit. So the fapolicyd cannot get fanotify_event_metadata from the fd, but after closing open files, it will return the fanotify_event_metadata that cannot be returned before.