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For /tmp in /etc/fstab, I have mode=1777, but after a reboot, the permissions on /tmp are 0755. Another directory /var/tmp is configured in exactly the same way but does not have this problem (see below). This is a Raspberry Pi running Ubuntu 18.04 Server. The root filesystem is a microSD card mounted read-only.

What is the proper way to make the 1777 permissions permanent?

Here are some additional details (after a fresh boot):

$ touch /tmp/test
touch: cannot touch '/tmp/test': Permission denied

$ whoami
ubuntu

$ ls -ld /tmp /var/tmp
drwxr-xr-x 9 root root 180 Dec 26 13:54 /tmp
drwxrwxrwt 4 root root 80 Dec 26 13:54 /var/tmp

$ mount |grep /tmp
tmpfs on /var/tmp type tmpfs (rw,nosuid,nodev,noexec,noatime,size=65536k)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noexec,noatime,size=131072k)

$ grep /tmp /etc/fstab
tmpfs /var/tmp tmpfs defaults,noatime,nosuid,nodev,noexec,mode=1777,size=64M 0 0
tmpfs /tmp tmpfs defaults,noatime,nosuid,nodev,noexec,mode=1777,size=128M 0 0

$ sudo systemctl status tmp.mount
● tmp.mount - /tmp
 Loaded: loaded (/etc/fstab; generated)
 Active: active (mounted) since Sun 2018-01-28 15:58:18 UTC; 10 months 27 days ago
 Where: /tmp
 What: tmpfs
 Docs: man:fstab(5)
 man:systemd-fstab-generator(8)
 Process: 642 ExecMount=/bin/mount tmpfs /tmp -t tmpfs -o defaults,noatime,nosuid,nodev,noexec,mode=1777,size=128M (code=exited, status=0/SUCCESS)
 Tasks: 0 (limit: 2146)
 CGroup: /system.slice/tmp.mount

Jan 28 15:58:18 testsystem systemd[1]: Mounting /tmp...
Jan 28 15:58:18 testsystem systemd[1]: Mounted /tmp.

$ grep -R '/tmp' /etc/tmpfiles.d /usr/lib/tmpfiles.d
/usr/lib/tmpfiles.d/x11.conf:D! /tmp/.X11-unix 1777 root root 10d
/usr/lib/tmpfiles.d/x11.conf:D! /tmp/.ICE-unix 1777 root root 10d
/usr/lib/tmpfiles.d/x11.conf:D! /tmp/.XIM-unix 1777 root root 10d
/usr/lib/tmpfiles.d/x11.conf:D! /tmp/.font-unix 1777 root root 10d
/usr/lib/tmpfiles.d/x11.conf:D! /tmp/.Test-unix 1777 root root 10d
/usr/lib/tmpfiles.d/x11.conf:r! /tmp/.X[0-9]*-lock
/usr/lib/tmpfiles.d/tmp.conf:D /tmp 1777 root root -
/usr/lib/tmpfiles.d/tmp.conf:#q /var/tmp 1777 root root 30d
/usr/lib/tmpfiles.d/tmp.conf:x /tmp/systemd-private-%b-*
/usr/lib/tmpfiles.d/tmp.conf:X /tmp/systemd-private-%b-*/tmp
/usr/lib/tmpfiles.d/tmp.conf:x /var/tmp/systemd-private-%b-*
/usr/lib/tmpfiles.d/tmp.conf:X /var/tmp/systemd-private-%b-*/tmp
/usr/lib/tmpfiles.d/tmp.conf:R! /tmp/systemd-private-*
/usr/lib/tmpfiles.d/tmp.conf:R! /var/tmp/systemd-private-*

$ sudo chmod 1777 /tmp

$ ls -ld /tmp /var/tmp
drwxrwxrwt 9 root root 180 Dec 26 13:55 /tmp
drwxrwxrwt 4 root root 80 Dec 26 13:55 /var/tmp

$ cat /etc/rc.local
#!/bin/bash
service ntp start
exit 0

$ uname -a
Linux testsystem 4.15.0-1030-raspi2 #32-Ubuntu SMP PREEMPT Fri Dec 7 09:15:28 UTC 2018 armv7l armv7l armv7l GNU/Linux

Related, unanswered questions:

• /tmp tmpfs permissions problem


• How can I mount a tmpfs to /tmp via fstab, writable to anyone?

This was part of my initial configuration (because / is mounted read-only):

sudo rm -rf /var/spool && sudo ln -s /tmp /var/spool

Apparently at boot, the system does chmod 755 /var/spool, which changed /tmp in my case.

The fix was to replace the symlink with a normal directory and add a third tmpfs mount:

sudo rm -rf /var/spool && sudo mkdir /var/spool && sudo chmod 755 /var/spool
echo 'tmpfs /var/spool tmpfs defaults,noatime,nosuid,nodev,noexec,mode=0755,size=64M 0 0' |sudo tee -a /etc/fstab

Thanks to everyone for the comments which directed me in the correct direction, especially Filipe Brandenburger's "Do you have any other scripts or units messing with /tmp on startup?"