Description of problem:

"dnf needs-restarting -r" work incorrectly inside systemd-nspawn containers

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

binary: python3-dnf-plugins-core-4.0.17-5.el8.noarch.rpm
source: dnf-plugins-core-4.0.17-5.el8.src.rpm

How reproducible:

always

Steps to Reproduce:

1. Install RHEL 8.3 / CentOS 8.3 on host, install systemd-container package on host:

dnf install systemd-container

2. Install RHEL 8.3 / CentOS 8.3 inside systemd-nspawn container:

yum --installroot=/var/lib/machines/test --releasever=8 install dnf python3-dnf-plugins-core systemd

3. start container test:

machinectl start test

4. enter inside container test:

machinectl shell test

5. check if container needs restarting:

dnf needs-restarting -r

Actual results:

# dnf needs-restarting -r
Core libraries or services have been updated since boot-up:
  * dbus
  * dbus-daemon
  * glibc
  * systemd

Reboot is required to fully utilize these updates.

Expected results:

# dnf needs-restarting -r
No core libraries or services have been updated since boot-up.
Reboot should not be necessary.

Additional info:

Cause of bug located in file:

/usr/lib/python3.6/site-packages/dnf-plugins/needs_restarting.py

Linus Torvalds say: "Talk is cheap. Show me the code".

Ok.

Patch to solve this bug:

After this patch - command "dnf needs-restarting -r" will work correctly
inside and outside systemd-nspawn containers.

The first process start time could be used as container/host start time:

 int(os.stat('/proc/1/cmdline').st_mtime)

/proc/1/cmdline is a virtual file in the procfs, whose creation date is the same 
as the container creation date or linux host boot time, if no containers is used.

--- needs_restarting.py.orig    2020-08-12 00:06:14.000000000 +0300
+++ needs_restarting.py 2021-01-07 22:27:20.779006509 +0200
@@ -174,11 +174,7 @@

     @staticmethod
     def get_boot_time():
-        with open('/proc/stat') as stat_file:
-            for line in stat_file.readlines():
-                if not line.startswith('btime '):
-                    continue
-                return int(line[len('btime '):].strip())
+        return int(os.stat('/proc/1/cmdline').st_mtime)

     @staticmethod
     def get_sc_clk_tck():