Samba refuses to start with ro rootfs

Question

I configured Raspbian Stretch to mount rootfs in read-only mode. Basically I did the following:

apt-get -y remove --purge triggerhappy logrotate dphys-swapfile
apt-get -y install busybox-syslogd; dpkg --purge rsyslog
rm -rf /var/lib/dhcp/ /var/run /var/spool /var/lock
ln -s /tmp /var/lib/dhcp; ln -s /tmp /var/run; ln -s /tmp /var/spool; ln -s /tmp /var/lock

and put this in etc/fstab:

proc            /proc           proc    defaults                0       0
/dev/mmcblk0p1  /boot           vfat    defaults,ro             0       2
/dev/mmcblk0p2  /               ext4    defaults,noatime,ro     0       1
/dev/mmcblk0p3  /home           ext4    defaults,noatime        0       2
tmpfs           /tmp            tmpfs   nosuid,nodev            0       0
tmpfs           /var/log        tmpfs   nosuid,nodev            0       0
tmpfs           /var/tmp        tmpfs   nosuid,nodev            0       0

all works fine expect samba that doesn't start if rootfs is mounted in ro mode. To be sure, I temporary mounter / as rw and it started as expected.

Here the relevant part of the debug output of smbd:

...
Paths:
SBINDIR: /usr/sbin
BINDIR: /usr/bin
CONFIGFILE: /etc/samba/smb.conf
LOGFILEBASE: /var/log/samba
LMHOSTSFILE: /etc/samba/lmhosts
LIBDIR: /usr/lib/arm-linux-gnueabihf
MODULESDIR: /usr/lib/arm-linux-gnueabihf/samba
SHLIBEXT: so
LOCKDIR: /var/run/samba
STATEDIR: /var/lib/samba
CACHEDIR: /var/cache/samba
PIDDIR: /var/run/samba
SMB_PASSWD_FILE: /etc/samba/smbpasswd
PRIVATE_DIR: /var/lib/samba/private
...
Processing section "[global]"
doing parameter workgroup = WORKGROUP
doing parameter netbios name = MYSAMBA
doing parameter security = user
doing parameter map to guest = Bad User
doing parameter guest account = pi
doing parameter wins support = yes
pm_process() returned Yes
lp_servicenumber: couldn't find homes
get_current_groups: user is in 1 groups: 0
unix_msg_init failed: Read-only file system
messaging_dgm_ref: messaging_dgm_init returned Read-only file system
messaging_dgm_init failed: Read-only file system
messaging_dgm_ref failed: Read-only file system

It's pretty clear it doesn't like the read-only file system, but I don't understand what prevents it from starting. And hence how to fix it!

If I didn't provide all the information to understand the scenario, please ask me in comments!

UPDATE

After adding the symlink for var/lib and var/cache I also symlinked /etc/samba to the writable partition (I don't like this, but I don't know how to symlink only /etc/samba/dhcp.conf.new when created).

Now the output is a bit different:

...
lp_servicenumber: couldn't find homes
get_current_groups: user is in 16 groups: 4, 7, 20, 24, 27, 29, 44, 46, 60, 100, 101, 108, 997, 998, 999, 1000
directory_create_or_exist_strict: invalid ownership on directory /var/run/samba/msg.lock
messaging_init: Could not create lock directory: No such file or directory

but:

$ls -l /var/run/samba
drwxr-xr-x 2 root root 40 Sep 4 13:28 msg.lock

Answer 1

These lines

LOCKDIR: /var/run/samba
STATEDIR: /var/lib/samba
CACHEDIR: /var/cache/samba
PIDDIR: /var/run/samba

point to read-only locations but the server needs to write there, I think. Read the manpage for smb.conf. The respective settings are lock directory, state directory, cache directory and pid directory. At least these four should point to writable locations.