[systemd-devel] Child of daemon sending SIGCHLD to systemd

[Ian Pilcher]

I originally posted a variation of the question on the SELinux mailing
list, but the more I look at this the more I realize that it really
isn't a SELinux questions.  I'm not really sure that it's a systemd
question either, but it definitely falls into the area of Linux process
management, so I'm hopeful that someone here at least has an idea what
is going on ...

I'm in the (hopefully) final stages of creating the policy module for a
daemon that I've written to monitor my home NAS.

The daemon is started by systemd (init_t) and runs as its own type
(freecusd_t).  In order to read the SMART attributes of the NAS drives,
the daemon runs a helper application, which has its own type
(freecusd_smart_t).  So:

   systemd (init_t) --> freecusd (freecusd_t)
                            --> freecusd_smart_helper (freecusd_smart_t)

I've got my policy basically working, but I'm getting this SELinux
denial, which I just don't understand:

type=AVC msg=audit(1593392372.230:9215): avc:  denied  { sigchld } for 
pid=1 comm="systemd" scontext=system_u:system_r:freecusd_smart_t:s0 
tcontext=system_u:system_r:init_t:s0 tclass=process permissive=0

This seems to be saying that the helper is trying to send SIGCHLD to
systemd.  I'm seeing this message repeated 4 times when the freecusd
daemon starts and then sporadically afterwards.  (freecusd repeatedly
spawns the helper to read the drive states.)

Is there a circumstance in which the grandchild (freecusd_smart_helper)
would send SIGCHLD to systemd while its parent is still running?

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