pstore: introduce tmpfiles.d/systemd-pstore.conf

The systemd pstore service archives the contents of /sys/fs/pstore
upon boot so that there is room for a subsequent dump.  The issue is
that while the service is present, the kernel still needs to be
configured to write data into the pstore. The kernel has two
parameters, crash_kexec_post_notifiers and printk.always_kmsg_dump,
that control writes into pstore.

The crash_kexec_post_notifiers parameter enables the kernel to write
dmesg (including stack trace) into pstore upon a panic, and
printk.always_kmsg_dump parameter enables the kernel to write dmesg
upon a shutdown (shutdown, reboot, halt).

As it stands today, these parameters are not managed/manipulated by
the systemd pstore service, and are solely reliant upon the user [to
have the foresight] to set them on the kernel command line at boot, or
post boot via sysfs. Furthermore, the user would need to set these
parameters in a persistent fashion so that that they are enabled on
subsequent reboots.

This patch introduces the setting of these two kernel parameters via
the systemd tmpfiles technique.

man/systemd-pstore.service.xml

@@ -81,6 +81,24 @@
       <citerefentry><refentrytitle>pstore.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
       </para>
     </refsect2>
+
+    <refsect2>
+      <title>Controlling kernel parameters</title>
+
+      <para> The kernel has two parameters,
+      <filename>/sys/module/kernel/parameters/crash_kexec_post_notifiers</filename> and
+      <filename>/sys/module/printk/parameters/always_kmsg_dump</filename>,
+      that control writes into pstore.
+      The crash_kexec_post_notifiers parameter enables the kernel to write
+      dmesg (including stack trace) into pstore upon a panic or crash, and
+      printk.always_kmsg_dump parameter enables the kernel to write dmesg
+      upon a normal shutdown (shutdown, reboot, halt). These kernel
+      parameters are managed via the
+      <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      mechanism, specifically the file <filename>/usr/lib/tmpfiles/systemd-pstore.conf</filename>.
+      </para>
+    </refsect2>
+
   </refsect1>
 
   <refsect1>

tmpfiles.d/meson.build

@@ -8,6 +8,7 @@ tmpfiles = [['home.conf',            ''],
             ['systemd-nspawn.conf',  'ENABLE_MACHINED'],
             ['systemd-tmp.conf',     ''],
             ['portables.conf',       'ENABLE_PORTABLED'],
+            ['systemd-pstore.conf',  'ENABLE_PSTORE'],
             ['tmp.conf',             ''],
             ['x11.conf',             ''],
             ['legacy.conf',          'HAVE_SYSV_COMPAT'],

tmpfiles.d/systemd-pstore.conf

@@ -0,0 +1,29 @@
+# SPDX-License-Identifier: LGPL-2.1+
+#
+# The systemd-pstore.service(1) archives the contents of /sys/fs/pstore
+# upon boot so that there is room for a subsequent dump. This service
+# is enabled with:
+#  systemctl enable systemd-pstore
+#
+# With the service enabled, the kernel still needs to be configured
+# to write data into the pstore. The kernel has two parameters,
+# crash_kexec_post_notifiers and printk.always_kmsg_dump, that
+# control writes into pstore.
+#
+# The crash_kexec_post_notifiers parameter enables the kernel to write
+# dmesg (including stack trace) into pstore upon a panic, and
+# printk.always_kmsg_dump parameter enables the kernel to write dmesg
+# upon a normal shutdown (shutdown, reboot, halt).
+#
+# To configure the kernel parameters, uncomment the appropriate
+# line(s) below. The value written is either 'Y' to enable the
+# kernel parameter, or 'N' to disable the kernel parameter.
+#
+# After making a change to this file, do:
+#  systemd-tmpfiles --create path/to/tmpfiles.d/systemd-pstore.conf
+#
+# These changes are automatically applied on future re-boots.
+
+d /var/lib/systemd/pstore 0755 root root 14d
+#w /sys/module/printk/parameters/always_kmsg_dump - - - - Y
+w /sys/module/kernel/parameters/crash_kexec_post_notifiers - - - - Y