[Bug 1826294] Re: os-prober exits prematurely with "logger: socket /dev/log: Protocol wrong type for socket"

[Dimitri John Ledkov]

Steps i can reproduce:

wget https://bugs.launchpad.net/ubuntu/+source/os-
prober/+bug/1826294/+attachment/5652492/+files/reproducer.sh

chmod +x reproducer.sh

sudo journalctl -f -e &

clear

sudo ./reproducer.sh

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Title:
  os-prober exits prematurely with "logger: socket /dev/log: Protocol
  wrong type for socket"

Status in AppArmor Profiles:
  New
Status in os-prober package in Ubuntu:
  New
Status in rsyslog package in Ubuntu:
  New

Bug description:
  Failure occurs on Ubuntu 16.04 with the apparmor-
  profiles-2.10.95-0ubuntu2.10 package installed.

  Running update-grub will run /usr/bin/os-prober, which spews about a
  dozen of the following line to stderr:

      logger: socket /dev/log: Protocol wrong type for socket

  … but fails to report the existence of some installed operating
  systems as expected.

  Furthermore, /var/log/messages contains:

      kernel: audit: type=1400 audit(1556043066.679:11460):
  apparmor="ALLOWED" operation="sendmsg" info="Failed name lookup -
  disconnected path" error=-13 profile="syslog-ng" name="dev/log"
  pid=28566 comm="logger" requested_mask="r" denied_mask="r" fsuid=0
  ouid=0

  
  Here is a stripped-down skeleton of the /usr/bin/os-prober script, which demonstrates the problem:

      #!/bin/sh
      set -e -x
      
      newns () {
        [ "$OS_PROBER_NEWNS" ] || exec /usr/lib/os-prober/newns "$0" "$@"
      }
      
      log() {
        logger -t "$(basename "$0")" "$@"
      }
      
      debug() {
        log "debug: $@"
      }
      
      ls -l /dev/log
      debug "Hello world"
      newns "$@"

  The expected behavior is that it should write "debug: os-prober-
  testcase Hello world" to /var/log/messages twice.  However, it only
  succeeds in writing "Hello world" once.  After the script respawns
  itself with /usr/lib/os-prober/newns (which is like `unshare -m`), the
  second attempt to write to /dev/log fails as described above.

  Since the os-prober Bash script runs with the -e flag, any error, even
  just a logging error, causes the script to terminate prematurely.
  (Arguably, the log() function should call `logger -t "$(basename
  "$0")" "$@" || :` so that logging failures aren't fatal.)

  
  The fix, for me, is to edit /etc/apparmor.d/sbin.syslog-ng, and change

      profile syslog-ng /{usr/,}sbin/syslog-ng flags=(complain) {\
        …
      }

  to

      profile syslog-ng /{usr/,}sbin/syslog-ng flags=(complain,attach_disconnected) {
        …
      }

  … then run `aa-complain sbin.syslog-ng` and `service syslog-ng
  restart`, before running update-grub again.  I assume that similar
  fixes would be required for the other logging daemons.

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