journald: introduce new "systemd-journal" group and make it own the journal files

Previously all journal files were owned by "adm". In order to allow
specific users to read the journal files without granting it access to
the full "adm" powers, introduce a new specific group for this.

"systemd-journal" has to be created by the packaging scripts manually at
installation time. It's a good idea to assign a static UID/GID to this
group, since /var/log/journal might be shared across machines via NFS.

This commit also grants read access to the journal files by default to
members of the "wheel" and "adm" groups via file system ACLs, since
these "almost-root" groups should be able to see what's going on on the
system. These ACLs are created by "make install". Packagers probably
need to duplicate this logic in their postinst scripts.

This also adds documentation how to grant access to the journal to
additional users or groups via fs ACLs.

1 files changed, 13 insertions, 0 deletions

diff --git a/README b/README
index d8b1b1296a..300a4cf8bc 100644
--- a/README
+++ b/README
@@ -100,6 +100,19 @@ REQUIREMENTS:
         being 'html' or 'latexpdf'. If using DESTDIR for installation,
         pass the same DESTDIR to 'make sphinx-html' invocation.
 
+USERS AND GROUPS:
+        During runtime the journal daemon requires the
+        "system-journal" system group to exist. New journal files will
+        be readable by this group (but not writable) which may be used
+        to grant specific users read access.
+
+        It is also recommended to grant read access to all journal
+        files to the system groups "wheel" and "adm" with a command
+        like the following in the post installation script of the
+        package:
+
+        # setfacl -nm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/
+
 WARNINGS:
         systemd will warn you during boot if /etc/mtab is not a
         symlink to /proc/mounts. Please ensure that /etc/mtab is a